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Fuel Pressure Drop at Runup

f1rocket

Well Known Member
I just started having this issue. Cold start-up is normal. The engine oil is warming as I slowly taxi out to the end our runway. Once oil temp is green, I do a mag check and the fuel pressure drops down into the yellow and I get a warning. Reduce throttle and the fuel pressure goes back up. On takeoff, I sometimes get an alarm. All other times, there's no indication of a problem.

The only difference is that I am still running 100LL in the tank, left over from my last trip. This only happens before the first flight and only when I advance the throttle to do the run up. Both pumps are running as I pulled the breaker to check and the pressure dropped, but not into the yellow at idle.

Any ideas?
 
Yes, and I've replaced mine already but it seems to go up and down accurately with pulling the fuse of the electric fuel pump and with the engine RPM. I would expect it to do crazy jumping around at unexpected times, but it's not doing that.

I might try cleaning out the fuel pressure line and the sending unit and see if that makes a difference. It bugs me that it just started doing that. If it's a prelude to a failure, I'd love to find it now.
 
I am annoyed and worried about fuel pressure fluctuations for a long time and cannot find the cause.
I don't like fuel alarms and do all I can to find it, but no luck after 220 hours of flying. The drops come up very irratic and may stay away for hours and suddenly return for half an hour or do. I have the feeling it happens more frequent after a long flight and slow descent.

We have two identical RV12's and on the oldest one with 580 hours it has never been a problem. Rock stable and just a slight drop at full throttle which seems normal.

The new plane has exactly the same components and had problems all the time, but the old Rotax never missed for a second.
I cleaned the tank, filters and fuel lines with no effect.
Replaced the pressure sensor with the exact same result.
Now i even have two identical pressure sensors installed at different locations of the fuel system. The additional one has an analogue gauge connected.
The two sensirs read exactly the same all the time so I am convinced it is not related to the sensor.
It is also not related to the electrical fuel pump as the problem persists with the pump disconnected.
I can only conclude the actual pressure is unstable and I suspect the pump as a last option however both planes have the same type (vented) cast pump.
On the other hand how can a pump vary the pressure? It works or it does'nt.
A new pump is expensive so I want to make sure that is the probable cause before ordering it.
Anyway it worries me as well.
 
With about 60 hours on the plane, mine also started to randomly drop to 1.5-2.5 at high power. It was also leaking a small amount of oil from the drain tube. Most of my fuel has been mogas with alcohol.

At the recommendation of Lockwood, I replaced the pump and all is fine now. In my case, they are replacing it under warranty.

Mitch
 
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Fuel press drop

The other day for first time I had fuel pressure drop from normal 4.5 to 2.2 and yellow warning...I turned back to airport just in case but after a short time the pressure returned to 4.5...kinda unnerving..suggestions..thanks
 
That started happening to me. Eventually my fuel pressure went to zero and stayed there. Replacing the red cube fixed it.
 
That started happening to me. Eventually my fuel pressure went to zero and stayed there. Replacing the red cube fixed it.

That's curious. I've replaced the red cube over 100 hours ago. Not sure how that would affect the fuel pressure unless it was completely blocked. In my case the fuel flow went to zero but the pressure was fine.
 
Check the Dynon data log to see if the fuel flow went up higher than normal at the same time that the pressure dropped. If so, I would suspect the engine driven fuel pump.
 
Not sure on which aircraft your fuel pressure problem was on but i have experienced similar fuel pressure changes on my rv9 carb 160HP with mechanical and back up electric pump. The suction line on mine has individual filters one on each side tank. That's the way I built it. After about 300 hours the pressure side of both pumps was obviously low at high engine speed. occasionally i would get the low alarm turn, on the boost and pressure recovered. I suspected pluggage or blockage in the tank side strainers. On removal of the strainer found small amounts of what looked like filter element yard / material. Changed out and all was good. Lesson learned, A slight amount of pluggage on the suction side of a pump makes a lot of difference on the pressure side. And 100LL out of a hose may not be as clean as you think.
 
I've had rare strange sensor readings: hi fuel flow, low fuel pressure, low oil pressure and low EGT. Never more than one at a time, and most did not persist. The ones that did were hard failures of a sensor or connector. Is there something in the design that makes the sensor circuits prone to this? Maybe the grounding?
 
Check the Dynon data log to see if the fuel flow went up higher than normal at the same time that the pressure dropped. If so, I would suspect the engine driven fuel pump.

I'm going to fill up with Mogas this morning and go flying to see if I can narrow this down a bit. Thanks to all for your ideas. Today is the last 70 degree day before Winter comes tomorrow to the Midwest.
 
I would check your fuel tank venting system. A slight blockage could cause a change like this to occur.

That's an excellent suggestion. It would cause a problem like mine. However my -12 uses the JD Air vent fitting which has a secondary vent AND my fuel cap is also vented so I don't think that's it.

I filled the tank today and the fuel pressure was rock steady at 4.5 during run up. Somehow I think the pressure created by the weight of the fuel in the tank is either causing the problem or masking an issue, perhaps with the mechanical fuel pump. I will keep on eye on it closely.
 
Hi Randy,

I had an experience similar to yours about a month ago. My run up was normal, but about five seconds of going full throttle on my take off, I received a low fuel pressure alarm. I aborted and pulled off the runway. By then, fuel pressure had returned to normal. I ran it up to around 4500 RPM, and fuel pressure went into the red again. I was able to repeat that several more times. I then taxied back up to the run up area, and applied full throttle. All was well this time. Whatever it was has not reoccurred since.

I had 91 octane in the tank when this happened. I don't think it was vapor lock since it was early in the morning with cool air temperatures. In fact, my oil temp was still in the yellow when it happened, so the engine and cowl were fairly cool.

Please keep us advised if you figure anything out.

Thanks,
Alex
 
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Fuel pressure drop

Back in September, I had the same thing happen. I was leaving from Jekyll island around 09:00 with about 11 gallons of 93 octane ethanol fuel onboard. During run-up everything was in the normal range( 4.5-5.5 PSI fuel pressure). I advanced the throttle quickly and rolled about 200 feet and got a low fuel pressure warning, 1.9 PSI. I aborted quickly and turned off to the taxiway. Checked the fuel shut-off, made sure the fuse for the electric pump was OK and noted that the fuel pressure was again in the normal range. Of course I had confirmed all those items during my previous check list/run-up. Slightly unnerving to say the least, I had never had this happen before. So I taxied back to try again, another full throttle run-up watching fuel pressure all the while. Noticed a slight drop in pressure but still normal. This time I eased the throttle to full and watched the pressure closely. It dropped to 2.4 PSI then started rising back to 4.5 like always. It stayed in the normal range for the rest of the flight and all subsequent flights, about 10 hours. I wondered if it was because of the less than full fuel state, the departure from sea level (home field is 1240 msl) or just a random occurrence. My fuel cap is vented and I have the latest version tank vent as well. I pay more attention to the fuel pressure now. Just wanted to add another Data point to the discussion. If anyone can nail down the cause I'd like to hear it. Thanks
 
fuel press drop

I would guess like most the real concern is loss of power, especially on takeoff...none of the posts ever say they had loss of power which I am very glad to hear...with two fuel pumps it would surprise me if both gave up the ghost at same time?? thanks for all the posts and help...
 
Olderthandirt Gets It

Four RV-12s, all E-LSA, and a Piper Sport fly out of my home airpark. The Sport and one RV-12 had frequent fuel pressure alarms on departure. They occurred after rotation and continued for 30 to 60 seconds. For reasons unknown, the low pressure instances gradually became infrequent.

During Phase I testing of my wife's RV-12 I experienced its one and so far fuel pressure alarm after a power off glide from 5000 to 1000'. During power up and climb the pressure dropped similarly to others' departure experiences.

Importantly, in all of these instances no engine roughness or power loss occurred! Yes, the alarms are disconcerting, but seemingly inconsequential.

I have almost 400 hrs on my E-AB SeaRey with a 912ULS. I did not see any reason to install fuel pressure or flow sensors on a carbureted engine with a return line. Never had roughness or power loss. Sometimes ignorance is bliss.
 
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I agree with Bruce. Fuel flow, fuel pressure and EGT/CHT indications are very helpful in diagnosing problems, but spurious alarms at critical phases of a flight can be dangerously distracting.
 
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