Intermittent low fuel pressure on 912ULS

[3kdscf]

I have put 200 hours on my 912ULS since my first flight 1.5 years ago. Although the engine only has 200hrs on it, the engine was manufactured almost 5 years ago. I started getting low fuel pressure (1.4psi) sporadically on climb outs at full power. I figured it was either 1) clogged fuel filter 2) bad sender or 3) bad fuel pump. Because the low pressure only happened at full power on climb out, a bad sender didn't seem likely. I checked the gravity flow rate and it was above 12gph so it wasn't a clogged filter. Replaced the fuel pump and the problem was solved. Over 20 hours on the engine since replacing the pump and the pressure always stays right around 3.8psi under all power settings.

I took the old fuel pump apart and the rubber diaphragm seemed to be in excellent shape so I found nothing that could explain the low pressure. I would have felt better had I found something wrong with it. The fuel pump is on 5 year rubber replacement list. I guess the moral is that replacing the fuel pump before the 5 years might be a good idea and also that it's 5 years from the date of manufacture, not 5 years from the date of first use.

As an aside, the time and weight of my electric boost fuel pump was well worth it. Even though I experimented and the engine kept full power with low fuel pressure, it was very re-assuring to know that I had my finger on the boost pump for "plan B".

[jiott]

I had the exact same issue with my 912 uls manufactured in 2012. The incidents never once caused the engine to lose power or even hiccup, so I eventually quit worrying about it. I do not have any backup electric fuel pump. I finally replaced the fuel pump at the 5 year time, but it made no difference; maybe just slightly more intermittent. I do believe it is fuel pump related, especially after hearing your success after pump replacement. I don't think it is anything "wrong" or failed in the pump, but is likely a design issue. Over on the Rotax-Owner forum this issue has come up many times with no apparent definitive resolution. The factory said they were looking into it, but so far no answers that I am aware of. It is very strange because it only seems to happen to a certain few people on an intermittent basis-always on high power climb-out. It always recovers after a short while. I did all the things you mentioned: checked filter, gravity flow rate with nose level and nose high, hose routing, etc.

[3kdscf]

Thank you! I feel better knowing that others have seen this odd behavior as well. In my case though, the new fuel pump definitely fixed it because with the old pump I was getting a low pressure warning about once a flight and now with the new pump I have 50hrs on it without a single warning.

As you well put this in perspective, that's why we measure gravity flow fuel rate - to know without a pump we can still get full power. The Rotax mechanical fuel pump is a backup.

[Backpacker]

I have a Rotax 912 UL and when i rigged in my fuel pressure dial guage I installed the Rotax return flow orifice(I never had it when i first got my Chinook) and I installed a filter before the orifice to prevent stuff getting to the orifice...been a long time but i i had some low fuel pressure sometimes and remember one time finding something partially blocking the return orifice(I think it was after that i installed a filter before the orifice...the orifice hole is small...so if a chunk of something was sitting near the orifice it may not block flow until full power when it could get swept up and partially block the orifice...something worth checking...but depends where the supply for the pressure reading is from. When i first got my Chinook it never even had a return orifice, only a return line and never had a fuel pressure guage then either but had no issues.

[Backpacker]

I got thinking about this more I think I had some crap in / near the fuel pressure guage inlet hole.

[jiott]

I agree that partial plugging of the fuel pressure sensor damping orifice could account for your intermittent low pressure warnings. Whether the return (bypass) orifice is plugged or not should not make any difference on fuel pressure readings; many folks operate without a bypass at all.

I hope clearing the fuel gauge orifice solves your issue, but don't get your hopes too high (see my post #2 on this thread). There are many, many folks living with this mysterious issue, just search thru the Rotax-Owner.com website forum. In almost every case it has not caused an actual problem, but can be scary. Rotax factory seems to have no good answer for it.

[Slyfox]

let me get this straight, you said the return line with the restrictor( or .030 hole or there abouts), the line that actually goes back to the header tank. You are stating that if that hole restricts will cause low pressure. I say no. What that system will actually do is lower pressure, usually about .5 to 1.0 pounds of pressure. So if that hole restricts it will actually raise the fuel system pressure just a tad.

now I read similar over on the rv list with the rv12. It seems it's a normal thing to happen and vans actually has the system designed so the facet pump works anytime the motor is running to combat this issue. At least that's how I read it. take care

[jiott]

Sorry to disagree Steve, but the bypass system orifice size recommended by Rotax is 0.014". This orifice is not anywhere nearly large enough to bypass enough fuel flow to affect the pressure in any way. In order for the bypass system to reduce fuel pressure even 0.5 psi the orifice would have to be so large that it is bypassing so much fuel that the fuel pump is on the verge of not being able to keep up. You would never run a fuel system this way; its too close to disaster if the fuel pump were to deteriorate just a little bit. The Rotax fuel pump puts out more flow than is ever needed (including engine demand and a proper bypass system) and the excess is spilled over a pressure relief valve, which is what controls fuel system pressure.

[Slyfox]

sorry to burst your bubble. But I just took the apparatus that I have from my rotax motor when I bought it years ago and the piece that actually meters the fuel looks like a long tube that screws into the return section that, is about 1/2 long with 6 holes around the neck and with a final hole on the end where the screw driver goes is about .030 opening. I have read over on the rotax page from their expert I believe that it will lower the fuel pressure a tad. blocking this at best will keep pressure the same or slightly higher. This is also something I haven't put in my plane because I don't think I need it. I do have the new fuel pump, but puts out 4.5 lbs. I've never had a problem in the past with fuel getting hot and causing an issue so it sits in my desk at the shop.

[jiott]

Steve I can't speak to your older 912 installation manual, but the one for my 2011 engine and the latest installation manual online today shows that return orifice size to be a pilot jet #35, which is 0.35 mm, which is 0.014" diameter (see manual section 73-00-00 page 11). I'm certainly not saying your 0.030" orifice doesn't work because it obviously does; but it is bypassing about 4 times more fuel than the Rotax recommended size. This only proves that the fuel pump is capable of producing a lot more fuel than is needed by the engine. I also been monitoring the Rotax-Owner forums for 10 years and have yet to hear one of their factory experts say that the bypass flow reduces fuel pressure even a little bit-would be interested to hear who you heard it from? These minor details are not really worth arguing about, so forgive me for starting it. I just personally am interested in all details of the systems on my Kitfox.