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Hello
I have recently noticed a change in behavior of the fuel pressure during the pre-start sequence in the families Pa-24-250 and am looking for some ideas of what might be happening.
During pre-start the electrically driven boost pumps are engaged, I listen for the phase shift and rate of pumping decrease audibly once pressure is built, after which the electric boost pumps are shut off and fuel pressure is monitored - checking that it remains slightly above the red line (1.5-2psi perhaps). I inherited this technique from my Grandfather, who got it from Maurice Taylor.
Recently I noticed one of the two electrically driven pumps had seemingly failed. During pre-start the rate of pumping never decreased after building fuel pressure, there was no noticeable phase shift between the pumps, and fuel pressure would drop to zero after shutting off the pumps.
I had the airplane grounded and both electric fuel pumps replaced, and the aircraft was released from maintenance. Now when I engage the electrically driven fuel pumps I can hear the pumps go in and out of phase, fuel pressure rises normally, and the decrease in the rate of pumping is noticeable but less than before. (The new pumps are from a different manufacturer, PMA Products Inc out of Liberty NC so the sound may be a bit different than I am used to for that reason). When I shut off the electric boost pumps, the fuel pressure goes to zero within about 10 seconds.
I spoke with maintenance personnel and some peers (only one is a fellow Comanche owner) and they all felt it was normal, that there is nothing they can think of that holds the fuel pressure in the line after disengaging the boost pumps. However from experience I disagree, though perhaps I am wrong.
Maintenance suggested running the electric driven boost pumps for 45-60 seconds to see if perhaps the carburetor float was overflowing and spilling onto the tarmac. It was not.
I have given the engine a ground run and cannot see any fuel leaks appearing, nor is fuel being dumped onto the tarmac. Engine driven pump works normally. I don't suspect a fuel leak at this time.
I am wondering what I should expect the fuel pressure to go to after engaging the electric boost pumps during the pre-start sequence - should I expect pressure to maintain approx 1.5-2psi?
If so, do you folks have any ideas of where this fuel pressure may be going and what should be investigated next?
I would suggest you go looking for leaks in your rubber fuel hoses. These degrade over time and they are rarely replaced on a regular schedule. I have had several "mysterious" fuel issues related to degraded rubber hoses. The outer braid looks find but the inner hose has lost its ability to seal.
Generally this shows up with air being sucked into the system (if on the suction side) or leaking during operation (engine running.)
Could also be a cracked fitting especially near the engine driven pump.
I would turn on the pumps on the ground with the engine off and then go have a very good look at the entire fuel line system on the pressure side looking for signs of leaks. Wiggle the hoses and see if you can notice anything. I know you mentioned you did a ground run but this can be quite subtle - you wouldn't see a bunch of fuel on the ground. Do it with the engine off and have a good look.
Needles in the carb float bowl might be leaking but I doubt it. More likely it is a 50 year old hose or fitting.
William
Also, there are check valves in the electric pumps and they are "ahead" of the fuel pump. The carburetor float bowl needle also seals the pressure in. So it is possible that this could hold some pressure after the pumps stop if everything was in really good condition.
But I'd say it is more normal for this pressure to bleed down quickly since it doesn't take much relaxation for the relatively incompressible liquid fuel to drop pressure.
If, under pressure, fuel is not leaking out anywhere you are good.
