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cardinal flier
02-04-2012, 11:25 PM
Today I tested my KF 4, 912S by climbing to 15,500'. OAT was -3C at this altitude. Everything performed great but I quit climbing because I was cold. I then started my descent and 30 mile return to my home airport when my Dynon flashed a red "low fuel pressure" warning. Needless to say, I forget that I was cold. I think I was down to about 8-10,000'. I turned on the electric fuel pump and the warning disappeared. I then tested by turning off the elec. pump and the warning returned immediately. After turning on the elec pump, I toggled the Dynon into the page showing the actual fuel pressure, and repeated the test. The fuel pressure immediately started dropping when I turned off the elec. pump. I suspected that the mech. pump had failed. I returned home on the electric pump, landed, but then could not replicate the low fuel pressure warning when on the mech pump only. Full power run up on the ground, on the mech pump only, ran great. Any suggestions? Did the cold temps cause a problem with my 100ll av gas? I sumped the tanks and gascolator but did not find any water. Bruce Estes

Dorsal
02-05-2012, 05:07 AM
I have had issues twice with low fuel pressure, in both cases for me it was the sender. What pressure did you get with the electric and was it lower than normal? This all may not be related but I had my sender tapped into my fuel system over the engine, moving it back to the firewall with a restrictor tube has worked great. Also what have you set for your low pressure limit?

GDN
02-05-2012, 09:39 AM
If it's a pressure sensor failure, why is it OK on electrical pump ?

Dave S
02-05-2012, 11:06 AM
Bruce,

A couple thoughts if it may help:

1) As has already been commented - sorting out what is a sensor issue and what is a pump issue has to be determined. For a low pressure indication - if it is simply a light and not an actual pressure reading it can be a bit of a red herring - i.e. the difference between light coming on and not coming on may only be a fraction of a PSI - the conditions at altitude which can affect the sensor (temperature for instance) may be small but sufficient to cause the sensor to trigger under the conditions at altitude but not on the ground or lower altitudes. If this is the first time the plane has been operated under this specific set of conditions - it is entirely possible the low pressure indication would not have shown up any other time.

2) Was the engine running just fine when the pressure indicator came on? If it was - apparently the engine was getting adequate fuel. On the other hand if the power started to drop with the low pressure indication - a person pretty well would know there is a pressure/fuel supply problem. There is a tolerance/range for fuel pressure from low to high which is deemed acceptable by Rotax - Consider that the "set point" for the light may be at a different pressure than the low tolerance as defined by Rotax. Another point to consider - the mechanical pump and the electric pump probably have different pressure values - then there is the issue that in series with both pumps on it would be likely the total pressure may be a bit higher than with either pump operating separately. Theoretically no low pressure light should be coming on with either or both pumps working.

3) There was an SB on certain Rotax Fuel Pumps and pump eccentrics back in 2006 - here is where to find it to see if this applies or doesn't apply to your engine:

http://legacy.rotax-owner.com/si_tb_info/ulserviceb/sb-912-049ul.pdf

Basically the SB applies to a reduction in pump stroke due to wear caused by a specific out of spec part - reduced pump stroke results in reduced fuel flow - at high flow demand - the next thing that happens is reduced fuel pressure - it's sort of like an accelerated fuel pump wear out.

4) THE ONLY REAL STANDARD for determining the actual fuel operating pressure is to plumb in an accurate fuel pressure gauge and operate the engine - a full throttle ground run should be telling.

It's not a bad idea to have fuel pressure determined and logged so a person knows if any changes have occurred over time.

5) Beware of the possibility of a fuel line restriction somewhere - filters? line swelling? line kinking? - some of this can add up slowly till a sensor is finally tripped.

Not a bad idea to do periodic gravity fuel flow tests and log for so a person knows if any changes have occurred over time.

Let us know what you find out.
Sincerely,

Dave S
KF7 trigear
912ULS - warp drive

cardinal flier
02-05-2012, 12:51 PM
Yesterday after the flight, I checked everything and found nothing amiss. Today, I test flew, but did not climb to the same altitude and temperature as yesterday, and today, everything went perfect, as it has in the past. Yesterday when I got the low fuel pressure reading, I immediately started the electric pump so the engine never had a chance to stumble. Today, with full throttle, at 3,000', I got pressure readings on the Dynon of 3.7 with the mech. pump only, and 4.0 with the elect. back up pump. jAt 1800 rpm, I got 4.6 with the mech. and 5.3 with the elect back up. I think that yesterday, I must have had some moisture that froze in the small, brass screen in my gascolator that caused a partial blockage. I'm open to any thoughts anyone might have. thanks,Bruce

Dorsal
02-05-2012, 01:04 PM
Do you recall what you got for numbers during the flight with the warnings?

cardinal flier
02-05-2012, 01:16 PM
4.0 with the electrical pump, less with the mechanical, but that red "LOW FUEL PRESSURE" warning, when running only on the mechanical pump had me pretty concerned so I never let the fuel pressure decline very much, once I saw that the pressure was indeed declining when only under the mechanical pump. As I had to reduce my altitude to get under the SFO Class B airspace that my home airport is located under, I quit fooling around with testing as I did not want to risk killing the engine. Bruce

Dave S
02-05-2012, 01:54 PM
Bruce,

Quick question - what kind of pressure reading do you get with the Dynon under the following condition - on the ground with the engine not running but the electric fuel pump running?

Dave S

cardinal flier
02-05-2012, 02:43 PM
Not sure but I'll check tomorrow. bruce

cardinal flier
02-06-2012, 04:55 PM
O/K Answers to questions above. Without the engine running, but with the elect pump on, pressure max's at 4.9. The alarm (LOW FUEL PRESSURE) warning comes on at 2.2#, which is set by myself per the Rotax's min. fuel pressure. I think what happened is that I got some ice in the gascolator filter screen. The gascolator is mounted on the firewall down low, and my engine temps were declining because of the OAT at 0deg.C The elect pump forced fuel past the gascolator. The filter screen is a fairly small cylinder, approx 1" high x 3/4" diameter, with a small brass screen. The warning came on after my descent, which was faster airspeed than my climb, which combined with the wind chill factor, would have really chilled the gascolator. Thoughts?

Bruce Estes

Woodennickle
02-06-2012, 06:53 PM
Skin knows wind chill, airplanes don't.

DBVZ
02-07-2012, 08:45 AM
Skin knows wind chill, airplanes don't.Of course an aircraft can't feel wind chill, but this may still be an issue here. Under the cowl, the engine generated heat would tend to warm all attached equipment. If the relative wind is cooling that equipment faster than the engine is heating it, the equipment gets colder. The coldest it can be is the OAT, but it would not get as cold as the OAT without wind blowing the engine heat away.

I think the wind chill description of "feels like" is a bit misleading too. The reason no wind temps feel warmer than the actual temps, is similar. In no wind, your warm blood is heating a little air at the surface of your skin. If you are not moving, and the wind is totally still, you "feel like" it is warmer because the air at the surface of your skin IS warmer. Coats and blankets just capture body heat. Not very effective if there is no heat source. They describe it the other way, as a lower wind chill, because otherwise people may not take the effects of wind seriously enough.

Same thing happens in cold water, which is why a wet suit works. Warm a little water next to the skin, and do not let the surrounding cold water carry that little bit of warm water away. Some people carry some large trash bags when flying over water for this reason. Getting inside a bag will have a similar effect, keeping the water in the bag a little warmer than the surrounding cold water. And you can catch air in one or two bags, tie them off, and use them for flotation.

The reverse is also true. You can boil water in a paper cup with a gas flame, because the water on the inside carries away the heat and keeps the paper at below its ignition temp even though the gas flame is well above. To the paper, the gas flame "feels like" 212* with the water boiling.

Dave S
02-07-2012, 09:46 AM
Bruce,

Sorry for not getting back right away - had another deal to do.

I have to qualify anything I suggest because it is next to impossible to diagnose a problem without having one's hands on the airplane. Take my suggestions with that understanding.

1) Based on what you have said - the low pressure warning was real - not likely caused by the sensor.

2) The pressure readings you reported under the various conditions point to either a restriction of some sort; or a mechanical pump which is not quite up to snuff - or a combination of the two - and something transitory when the low pressure light tripped..

a) The static pressure 4.9 psi (Engine off electric pump on) indicates the electric pump is fine; and, pressure will be normal as long as fuel is not being consumed by the engine ; however, this says nothing about the condition of the mechanical pump and nothing about any restrictions in the line.

b) The Pressures you observed at Idle (1800 rpm) of 4.6 with the mechanical pump alone and 5.3 with with the electric pump added still indicates the electric pump is fine (note - when a mechanical and electric pump are operated in series it is normal that the pressure will be a bit higher than what the electric pump would provide alone). This Indicates the mechanical pump can provide adequate fuel flow at the low flow rate at idle. Still doesn't rule out a line restriction or a weak but moderately functional mechanical pump.

c) The pressures you observed of 3.7 psi with the mechanical only and 4.0 psi with the electric under full throttle is sufficient and within spec to operate the engine. The fact that both are reduced from the pressured observed at idle may indicate a developing restriction in the fuel system or a weakening pump(s) - just something to think about. The fact that adding the electric doesn't push the pressure back up to 4.9 or better may indicate the problem is associated with developing line restriction rather than a weak fuel pump - not for certain - I said MAY. FYI - I don't have a fuel pressure gauge on our plane but I did do ground run testing with a new engine and a test gauge there was no drop in pressure under full throttle compared to idle.

d) Your conjecture on the possible formation of ice crystals on the gascolator screen is certainly worthy of consideration - that is a possible explanation for the drop below the low limit under the specific conditions conditions. FYI - I don't know what your normal flying temperatures are in the part of California where you fly - however - Minnesota has about 5 months of sub freezing weather in the winter and it is done where some folks running 100 LL will add isopropyl to the 100 LL to insure against the formation of screen clogging ice needles in the fuel - I have personally observed the little ice needles suspended in 100 LL sampled from aircraft sitting overnight in sub freezing weather. I would not rule out the ice crystal problem.

e) Now - what to think about the difference between the pressures you observed of 3.7 psi with the mechanical only and 4.0 psi with the electric under full throttle vs the instance of the low pressure light tripping.... Something was obviously different when the light tripped; however, the other information you developed still indicates there may be some sort of developing restriction in addition to the low pressure indication which could certainly exacerbated the low pressure deal which appears to be transitory.

I think you are very fortunate having the plane set up with the fuel pressure - I am going to speculate without that you may have never known anything was amiss if you didn't have the dynon.

For what it worth - I had a PA 28 fuel pressure gauge go to near zero on climb out at full throttle - that one, as it turns out had a pump near the end of its life - reducing the throttle allowed the pump to catch up with the engine and I didn't have to go for "whatever runway you want" as cleared by the tower.....oh well.

Sincerely,

Dave S
KF7 Trigear
912ULS Warp Drive

kitfox2009
05-01-2012, 08:14 PM
Hello All
This is a bit of an old thread but it seems to relate to my story today.
I finally got around to installing a fuel pressure gauge in my KF Vixen 912UL. I purchased a 2" Holley electrical fuel pressure gauge (P/N 26-503) from Side Street Auto and mounted the sender about 3 inches down stream from the fuel pump. The gauge is graduated from 0 to15 psi.
With 2/3 tanks of fuel the gauge reads slightly on the low side of the 5 psi mark with a engine SHUT OFF. With engine at normal idle there is very little difference in the gauge reading. On full throttle climb out (5500rpm) or cruise at roughly 5000 the gauge seems to read right in the middle of the 5 psi mark.
The conclusion I am drawing from this is in the event of a fuel pump failure there appears to be adequate fuel pressure to run a 912UL at full throttle!!
I find this very reassuring. My engine is a mid 90`s with 330 hours and the original pump with no return line.
Thought you may find this interesting.
Cheers
Don:)

kitfox2009
05-01-2012, 09:43 PM
Hello all
I just received a private email from Lowell that seems to blow a hole in my "reassurance" about 5 psi at engine OFF. Seems that I have forgotten my high school physics.
It will be very interesting to read everyone`s comments about this one!!!
Oh well, another useless gauge to look at!
Have a great day.
Don:confused::confused:

TahoeTim
05-02-2012, 05:02 AM
The problem here is some of you equate good pressure with good flow. Think of a garden hose. When you hold your thumb over the end the pressure goes up and flow is zero. When you take your thumb off the flow is high and the pressure is only the resistance of the hose.

Of course, at the extremes no pressure is no flow as well as high pressure can be no flow.

Add a fuel cube flow sensor to your Dynon.

kitfox2009
05-02-2012, 08:03 AM
G`morning All
My original idea with the gauge was to be able to monitor fuel pressure in flight. A drop in pressure should indicate a possible pump failure. As Lowell pointed out there should be very little pressure (less than 1 psi) with the engine stopped. Obviously some of these senders do not appear to be very accurate!
Well at least I now have a 1/8"NTP outlet in order to connect a test gauge to and find out what is actually happening. Hopefully I will actually be able to find a practical use for this instrument. After testing I plan on replacing the sender.
Cheers
Don

jamesmil
05-02-2012, 08:23 AM
hi guys, just saw this thread and want to share my fuel pressure adventure. while flying off time on our ss7 with a 912uls.
after climbing to 7500' and setting power to 5300 rpm just set back and flew the box to build time, had never ran the engine at a high power setting for a very long time in the earlier testing so at about 2.5 hr, i saw a fuel pressure fluctuation on the garmin g3x. at first i thought it might be anouther sender problem as with the oil pressure sender after only 3.0.hr" a story for later". after watching the pressure it was obvious it was trending lower. reducing the power to about 4900. rpm stoped the droping pressure at about 2.5 psi.
i made it back home at that power setting still not sure i realiy had a fuel pressure problem.
after landing i did a full power run up, the pressure fluctuated as before allways trending down. after about 2 min. at full power the pressure reached 0.2 psi and the engine started shuting down.
after looking at all hoses, filters, fittings i found a tywrap i had used to support the vent line on the right tank pulled to tite on the #1 rib:o it took a long time for this restricted vent to show up just glad it wasnt at night on my first long cross country. hope this helps someone

DanB
05-02-2012, 03:13 PM
The problem here is some of you equate good pressure with good flow. Think of a garden hose. When you hold your thumb over the end the pressure goes up and flow is zero. When you take your thumb off the flow is high and the pressure is only the resistance of the hose.

Of course, at the extremes no pressure is no flow as well as high pressure can be no flow.

Add a fuel cube flow sensor to your Dynon.

This has been a good thread as it promotes a little thought and (what if) scenarios as we should all be doing. What Tim has mentioned about Flow vs. Pressure has been my thought process as i built my plane. I currently have a Grand Rapids EIS to monitor my sensors and I went with the "Red Cube" fuel flow sensor (which by the way will also work with a Dynon engine monitor). Certainly, there is reason to want to know your fuel presure (which I do not have to date), however, I can and do monitor my fuel flow. Reasoning (and I know I will be shot down if there are holes in it ;) and please do as I am still testing and evaluating :
1. I have done a static fuel flow (no engine running) to the fuel pump and found to have 15 GPH with a head pressure in the wings loaded to only 3 gallons on each side (no, I didn't check pressure). This is more than enough considering my 912s will ask for no more than 6 or so on climb out and I will have considerably more head pressure (more than 3 gal per side).
2. During my testing phase ( that would be now)...I am beginning to develop standards (or primary benchmarks) for what my fuel flows are at given RPM's / altitudes, speeds, etc. May make up a little chart to keep handy in my knee pad.
3. If my "standard" flow begins slowing considerably, I could start thinking blockage building up somewhere? If it stays the same and my airspeed and or RMP begins droping (given no head or tailwind), that would be a possible leak to look for.

I am not yet sold on the need for a fuel pressure sensor (give me a list of reasons of what more it will do for me), but I am now re-thinking about installing an electric fuel pump. I know many have them and does make sence as a back-up pump or I think even more importantly to force that fuel up (or from behind) through a possible choking fuel line or filter. Could be enough to help hang on long enough to get on the ground as in the case of Cardinal Flyer's initial post. Go ahead...start throwing flaming arrows ;). Good stuff here

jiott
05-02-2012, 03:56 PM
Just one thing-you say that when you have the wing tanks nearly full you will have "considerably" more head pressure than with only 3 gallons. This is only true to a very small extent since head pressure depends only on height above the measuring point, not volume. Because the wing tanks are fairly thin heightwise, a full vs. empty tank only gives you a couple more inches of head pressure. I believe a rule of thumb is about 1/2 psi per foot of height!

Jim

Esser
05-02-2012, 04:34 PM
Just one thing-you say that when you have the wing tanks nearly full you will have "considerably" more head pressure than with only 3 gallons. This is only true to a very small extent since head pressure depends only on height above the measuring point, not volume. Because the wing tanks are fairly thin heightwise, a full vs. empty tank only gives you a couple more inches of head pressure. I believe a rule of thumb is about 1/2 psi per foot of height!

Jim

0.43 to be exact. Haha don't mean to be a smart ass but I do this stuff for work all the time and someone might be interested to know.

jiott
05-02-2012, 04:55 PM
Esser, no you arn't a smartass and you are correct. I also did this stuff for a living as a hydraulics engineer. Since we are not dealing with more than a few feet when we talk Kitfox fuel systems, the rule of thumb is easier to remember. Its good we keep each other honest.

Jim

DanB
05-02-2012, 05:35 PM
0.43 to be exact. Haha don't mean to be a smart ass but I do this stuff for work all the time and someone might be interested to know.

Love it Esser :D

Dorsal
05-02-2012, 06:29 PM
I think if your going to be a smart ass you will have to multiply that by the specific gravity of the fuel which is around 0.7 :D

Esser
05-03-2012, 02:52 AM
I think if your going to be a smart ass you will have to multiply that by the specific gravity of the fuel which is around 0.7 :D

Nicely done Dorsal

HighWing
05-04-2012, 09:43 AM
My dad once said - PhD chemistry Columbia University - that the reason we worry about chemicals in the environment is because now we have the technology to measure them. EGT - when they first came out in the 70s they had four lines on the gauge and were designed to indicate relative temps. Now they have °F settings and folks worry about absolute temps. Airplanes have flown - Kitfoxes included - for generations with fuel level gauges and now we can monitor fuel flow and fuel pressure. Are all these numbers really that significant? I used the aux pump once and diagnosing the problem proved a pinched fuel line forward of the firewall. The big indicator was a poorly running engine. I guess a gauge would have shown the problem moments before the engine did, but... Round gauges throughout - I like the looks.

One other thought, years ago a guy posted a note telling of his rough running engine problem and emergency landing. He had a habit of tightening his hose clamps a bit each annual. you can probably guess the rest.
Lowell

kitfox2009
05-05-2012, 09:24 PM
Hello again All
I have some more very interesting observations regarding my new fuel pressure gauge!! ( it is a ROUND gauge to match all the others). Anyway last week I said the gauge read 5 psi with the engine stopped.(Lowell kindly suggested I refresh high school physics)
I would like to clarify that. Today I went flying and the gauge indicated 5 psi throughout the entire flight. I shut off the fuel as I was taxing in and ran the engine until the fuel pressure bled off to zero. Then restart and immediately got 5 psi again. Shut down and waited about 15 minutes, still 5 psi (engine stopped). BUT after about 30 minutes the reading increased to 10psi (engine stopped)!!
My take on what is happening is that the fuel cannot escape into the carb (float valve stops this) cannot escape into the fuel pump (check valve) therefore it maintains the 5 psi even though the engine is not running. But after it sits for 30 minutes or so the fuel in the line between the carbs and the pump heats up enough from dissipated heat off the engine that it expands and registers approximately 10 psi. This in turn over powers the float valves and concequently "floods" the engine on a restart about 30 minutes after shutdown!!
I guess that is one of the reasons why I understand that Rotax is now recommending installation of a return line back to the tank or gascolator.
Occasionly I have had restart problems after a flight and I always thought it was vapour lock. Now I believe it is flooding due to high pressure hot fuel in the line from the pump.
Next week. Install a return line!!!
Comments please.
Cheers:rolleyes::rolleyes:
Don

Dave S
05-06-2012, 06:02 AM
Don,

Precisely!:) Your take provides the exact explanation and reason why Rotax makes this recommendation. So often, manufacturer's instructions say what to do, or what they recommend, but not the details of why.

Your logic is spot on.:)

There are a few (very few) of the older carbureted automobiles that had designed in three spigots on the in line fuel filter. One in, one out and one return via a bleed on the other side of the fuel pump.

Appreciate you sharing what you discovered on the plane. I can see a return line in my future too - having suffered the same occasional hot soak situation with a flooded start on a summer day. ( FWIW - won't happen when the outside air temp is 5 degrees F:o).

Dave S

Dave Holl
05-06-2012, 02:15 PM
I thought the reason for the return fuel line was to reduce the chances of fuel vaporization?
I guess it also stops fuel pressure build up with heat when the engine is stopped!

Dave S
05-06-2012, 06:40 PM
Dave H,

Sorta both or either - in a hot soak soak shutdown - the bleed line relieves the pressure between the pump and carb so flooding does not occur via the carb needle valve - what starts out as warm fuel can certainly progress to fuel vapor if there is enough heat buildup. It's a matter of degree.

In any case - without a bleed line excess fuel or vapor or both can be dumped to the carb past the float valve and cause flooding - the pressure buildup will be released through the float valve because it can't go the other way.

Sincerely,

Dave S

Dave Holl
05-06-2012, 10:54 PM
Should a non return valve be fitted in tis line at the firewall?
Dave

Dave S
05-07-2012, 07:35 AM
Dave H

In my opinion, a person should follow the recommendations of Rotax for the specific engine regarding how to hook this up. If you happen to have the engine installation manual; or search their site - you should be able out find what their recommendation is.

Sincerely,

DS

kitfox2009
05-18-2012, 07:22 AM
Hello All
Yesterday I installed my new fuel return line. I obtained the "pilot jet" from Rotax. This is a threaded jet designed to fit in the Rotax fuel distribution block. Probably a similar unit could be obtained from a motorcycle shop. Anyway, with a slight modification I installed it in the fitting going into the top of the gascolator. The fuel pressure stays at 5 PSI through all power settings and drops to zero within seconds after shutdown. I have not had the opportunity to test it for hot starts yet but hopefully this will eliminate the problem.
Cheers
Don

Dave S
05-18-2012, 07:52 AM
Don,

Thanks so much for posting your results so far.:) Will certainly be interested in how the hot starts work out. Sounds like your fuel pressure is right where it should be now.

Do you happen to have any photos you could post which show the completed installation layout including the gascolator hookup?

I have been mulling this over in my mind for a while but haven't taken the step to do it yet - but do want to to. I have the occasional hot start deal going on too; and, I don't have a gascolator (another likely improvement).

Sincerely,

Dave S

cardinal flier
06-13-2012, 03:34 PM
After three low pressure warnings, on three different flights, and in totally different flight conditions, and a LOT of attempted fixes to the fuel system, I think I finally found the reason I got a "low fuel pressure" warning on my Dynon. I installed a mechanical guage to test the accuraracy of the Dynon system and got approximately .5lb. lower readings with the Dynon, and variations of the spread between the Dynon and the mechanical. So, yesterday I asked Dynon if they had any history of problems with the fuel pressure sender. Dynon stated that they no longer sell the unit I had because of all the problems with the unit. The unit was prone to reading low at lower pressure from the pump. (Yes John, I can hear you all the way from Idaho, laughing, and saying "I told you so") I tested the mechanical and the Dynon guage today and the Dynon reads lower than it should. So, I've got the new, latest design, unit on order from Dynon and am hopeful of the solution. I'm also going to install a 1" fuel pressure guage in the panel as a back up to the Dynon.
Bruce Estes

kitfox2009
04-06-2013, 11:00 PM
Hello All
I know this is an old thread but I would like to reactivate it. I was overseas for 4 months so I drained all the fuel from the Vixen and parked it in the garage. On startup last week all was normal on taxi and runup but about 2 or 3 minutes into climb out the fuel pressure started dropping (normal is 5 psi)and fluxuating in the 1 to 2 psi range. Throttle back on downwind with full 5 psi.
Suspecting a weak pump or restriction I did a fuel flow test today and get 20 lites in 12 minutes or 100 lites an hour. Lots of fuel!
The engine is mid 90`s 912UL with 400 hours and the original fuel pump. I also installed a return line back to the gascolator last year.
Is this the type of behavior that a worn pump would exibit? At rpms up to 4000 or so the fuel pressure remains at 5psi. at closer to 5000rpm the pressure gradually reduces.
Anyone have this experience or should I just go buy a new pump. It is probably due anyway.
Your comments/experience appreciated
Don

kitfox2009
04-06-2013, 11:08 PM
Also different topic but my Westac Quad gauge oil pressure jumps up to 100psi (at times, sometimes even after shut down). I suspect the sending unit but Westac has now changed to a different style.
Would anyone have a spare 378-MM sender in their parts tray. Preferably new or with very little use.
If necessary I may have bite the bullet and upgrade to more modern stuff. Suggestions????
Cheers
Don

Dave S
04-07-2013, 09:51 AM
Hi Don,

I'll take a stab at your question; but, since I am not on site with you looking at the issue first hand, I could easily overlook something so take that grain of salt with my comments.

First off, the scenario you describe, the fuel pressure dropping with higher power output when it is apprent the system has adequate flow generally is what you see with a weak fuel pump. Your analysis is spot on. Anybody's guess if the cause was the due to the draining of the system; however, fuel pump poppet valves can stick, diaphrams can break down with drying out.

Second, gauges sometimes lie.....it is entirely possible for an electronic gauge system to display low fuel pressure even if the pressure is OK. The way this is normally sorted out is to plug in a direct reading gauge; or, a known good pressure monitor to see if the gauge displaying the low reading is lying or not. Easy to do on the ground.....not so easy to do in flight without a lot of messing around :(.

Another consideration. If the fuel pump is original and goes back to the 90's, I think Rotax has aa recommended replacement schedule for the fuel pump at a much shorter interval.

On your other gauge deal, I am not so sure; but, these types of anomalies can certainly be caused by less than perfect electrical connections or the gauge in addittion to the sensor....sorting that out is good to do. If you are lucky enough to have the original documentation for the quad gauge, I believe they include a basic troubleshooting guide.

Sincerely,

Dave S
KF7 Trigear
912ULS Warp
St Paul, MN

thilokind
08-27-2017, 08:09 AM
Hello everyone,

I seem to have the exact same problem than Kitfox 2009:

after a few minutes in flight the fuel pressure is slowly decreasing. Normally, I see 5 to 6 psi, but it slowly keeps dropping to 20 psi. The engine never misses a beat, even at WOT. When coming in for landing and reducing throttle, the fuel pressure climbs up to 5 to 6 psi.
My setup: two leading edge wing tanks, each with the own electrical fuel pump, connected in series with the engine driven fuel pump.
Switching the electrical fuel pumps on or off does not change anything.
So far I have:
- replaced all fuel hoses in the engine compartment
- replaced one of the electrical fuel pumps
- temporarily re-installed an old Pierburg mechanical fuel pump
- replaced the fuel pressure sensor
- cleaned the fuel filters multiple times
- checked the fuel flow with the electrical pumps on - there is plenty of fuel flowing to the carburetors.

Anybody with an idea:
@ Kitfox 2009: did you ever found the reason for this behavior? How did you fix it?

Thanks a lot

kitfox2009
08-27-2017, 02:19 PM
Hi Guys
Back in 2013 at 401 hours I replaced the original fuel pump with the updated one with the "over flow/pressure" nipple that must be directed overboard.
No issues since. Now have 750 hours. Runs 4 or 5 pounds steady.
Seems like it`s just like being in the oilfield. Keep throwing money at it until it works!!!
Have fun Don

fastfred
01-25-2018, 01:37 PM
I have a 2003 uls with a fuel pump not on the bad list. My 7 does not have the return fuel line either. 860 hours on it . What are the opinions on the return line and how many hours are the pumps good for?

Dave S
01-25-2018, 06:28 PM
Fastfred,

I have an early S7 with the 912ULS also; and, no return fuel line.

Agreed the newest pump design has a better patent on the bleed hole so if a leak develops the stuff is piped away from the engine rather than dripping on the exhaust manifold. This has nothing to with the fuel return line.

What the return fuel line does is help prevent carb flooding in hot weather following engine shutdown. Say if a person is out cruising, its 90 degrees F you land and park the plane for 10 minutes come back and find you have a hot start problem (vapor/flooded). What can happen under this condition is the fuel in the lines between the fuel pump and the carburetors picks up enough engine heat to expand the fuel in these lines, increase the pressure and overpowering the carburetor needles - pushing enough fuel past the needle/seat to flood the engine when you go to restart. What the return line does is provide a small orifice in the line between the pump and carbs to bleed off the pressure after the engine is shut down and return the excess fuel back to the supply side of the fuel system.

I considered installing the return line; but have not because the flooding issue seldom arises - either re-start the engine a very short time after shutdown before the issue can develop, or let the plane sit long enough before restart that the fuel expansion/vapor deal goes away on its own and that is only under high ambient temperatures. If you are happy with the way the plane operates now - I wouldn't bother with the expense and installation time - just my opinion.

On the fuel pump. Rotax has what they call a 5 year rubber parts replacement requirement which includes replacing the fuel pump every 5 years. If you have the same pump on the engine that has been running since 2002 - i'd say it's time to replace it based on age. I don't see that Rotax has any hour replacement requirement - just time. Fuel pumps, at least the automotive ones of the same design (diaphragm type) also seem to suffer from a half live more so than mileage. I don't know that anyone can say exactly what the life expectancy of a fuel pump is, it also depends on how the plane is stored and flown; but, both the poppet valves and the diaphragm become less reliable with age.

jiott
01-25-2018, 08:13 PM
I feel stronger than Dave on the necessity of the fuel return line. First, Rotax says to do it in the installation manual; I don't believe it is an option. Also I believe it is more important as a vapor-lock preventative than as a pressure relief. Vapor lock usually occurs on hot day shutdowns as Dave said and the return line will quickly recirculate cool fuel from the header tank. If you burn Mogas like many of us do, it is quite a bit more prone to vapor lock than 100LL, in fact there are cases where it has occurred in flight on hot days and/or high altitude. I personally feel much better following the recommendations and installing the return line.

Bud Davidson
01-26-2018, 08:19 AM
Does this fuel heating problem exist when the engine is shut down by shutting off the fuel supply, ie, fuel pressure drop ? I always shut down engines in the past with mixture shutoff. Should I not be shutting down the 912 by shutting off the fuel supply ?

jiott
01-26-2018, 10:45 AM
Not sure about the fuel heating effect, but that method of shut down will empty the carb bowls, so the next restart will be more difficult as the carb bowls have to refill.

Esser
01-26-2018, 10:50 AM
I don’t think it will make it harder to start. As soon as you open your fuel valve, the bowls will fill. I like emptying the bowls so if you do get fuel vapour it won’t flood as the bowls can take fuel if there is an expansion

kmach
01-26-2018, 11:04 AM
If you have all your engine compartment fuel lines in fire sleeve (recommended), your chance of vapor lock decreases, allot.

In summer, I usually open the oil cap access door after shutdown, it lets allot of heat out.

I did install the fuel return line when installing the new engine ( as per Rotax's instructions), but I never ever had vapor lock with the old engine with no fuel return line.

jiott
01-26-2018, 11:43 AM
I also believe firesleeve helps a lot, and have started opening the oil access door after shutdown. However, I do this mainly to avoid cooking the expensive ignition modules.

Vapor lock is a funny thing; I have the fuel return line and never had a vapor lock incident for about 3 years. Then one hot summer day (about 95F) I shut down for maybe 10 minutes on a gravel bar. I restarted OK and immediately started my T/O roll. Suddenly the engine lost most power and started coughing but didn't die. I aborted the T/O and shortly the engine was running fine again. I am sure it was vapor lock due to the short, hot shutdown (had not opened the oil door), but the fuel return line soon circulated cool fuel to kill the vapor lock. Lesson learned: after a short hot shutdown I need to let the engine idle for a while after restart before attempting an immediate T/O. This has never happened again, but that one time it could have had a very bad outcome.

fastfred
01-27-2018, 09:17 PM
Thank you for that information. The plane is new to me so I am unlocking its secrets all the time. Trucked it to the airport today and hope to get it up soon but I can't find anyone to check me out in it yet.

Bud Davidson
01-31-2018, 08:40 AM
OK. I am now able to run my engine. Shutting down the engine by shutting off fuel does not work well for normal shutdown. Engine stops so quickly washing oil off cylinder walls by turning off the ignition is likely no issue. I stand corrected and will shut down with ignition switch.
I have a lot to learn about the Rotax.

efwd
01-31-2018, 08:50 AM
That is a very interesting statement. Remove one of the three elements needed for combustion and the speed of the mechanics slowing down is different depending on which one you remove? Cylinder walls don't move, did you mean piston? Im just critical thinking here, not a challenge or an argument. Is this subject found in the Rotax Manual?
Respectfully
Eddie

fastfred
01-31-2018, 09:10 AM
Thank you for that information . it is a sweet running engine and I guess I will change the pump soon which will require a different T in the lines to pressure test it. Also probably fuel lines.

I need another opinion on the throttle. We were taxi testing Sunday and when we pull throttle all the way back it took the RPM so low that is stalled. ( I saw that below 200 there is no spark) . What RPM do you have the throttle stop at and do you adjust the idle screws or the cable stops?