Engine Fuel Pump

[romair]

While flying today I noticed a strange behavior on my fuel pump pressure gauge. Intermittently, the needle would wander toward the low side of the green arc, but always stayed in the green arc. When turning the boost pump on the pressure would go back to the the usual high end of the green arc. Turning the boost pump off would then make the needle go towards the low side, then back to normal, only to intermittently start wavering again. I was at 8500, running full throttle, 2400 RPMs, slightly lean of peak; engine ran well, fuel flow stayed constant. Is this a sign that the pump is trying to quit on me? This is new behavior for it to do that, and made me somewhat uncomfortable...my hand was always close to the boost pump switch, just in case.

Thanks

Stefan

1989 M20J

[jfdez]

How many hours on that pump?

[aaronk25]

Exactly the same thing with me also in my j. Was cruising under exact same perceived conditions only in flight I turned on my pump for 1 sec and pressure went from 24 to 26 then when pump went of it fell to 16 (still in the green) barley then it immediately came back to 24psi.

It seems what gets the biggest dip in pressure is just tapping the pump on so pressure builds then let off.

Maybe someone will know more about it but I'm guessing whatever mechanism regulates the pressure takes just a second or so to adjust to the flow fluctuation or turbulence in the fuel from the electric pump? Who knows how these work?

[Guillaume]

I experienced more or less the same phenomenon 2 years ago : the fuel gage started to fluctuate (arround 1 oscilation per sec and the rate of oscilation was more or less related to engine RPM). It happened suddenly. Turning ON the Boost pump would fix the problem too.

In my case, I think the fuel pressure would sometimes dropped in the yellow arc.

I installed a new engine fuel pump and that fixed the problem definitely.

[N201MKTurbo]

I had the same problem, but it only happened on one tank. I rebuilt the fuel selector and the problem went away. In my case I beleve it was drawing air into the fuel stream at the fuel selector.

I did notice that the markings on the fuel pressure gage are very accurate, as soon as the pressure drops below the green, the engine starts to run rough.

[Bill_Pyles]

My original analog fuel pressure gage always lurks about a needle width into the green and depoying the boost pump has no effect on the fuel pressure needle position. My fuel flow reading from my EI engine analyzer reacts immediately when I change the mixture, so I assume a) my fuel delivery system works properly and since my engine runs smoothly I assume it is getting enough fuel. I had Top Gun in Stockton check both the fuel pressure and oil pressure gages and they said they both read low by 2-3 needle widths.

[romair]

I ended up changing the fuel pump. It solved the problem, no more fluctuations. But damn it was hard to get to it.

Stefan

1989 M20J