Replacing engine fuel pump - how to?

Paul Eastham · #1 08-14-2007, 11:42 AM

My carburated O-320 has been showing high fuel pressures since birth, and now at 25 hours, pressures are occasionally leaping up as high as 20 PSI (max 8 per Lycoming) with the engine pump only. Gauge has been calibrated and tests ok.

Aerosport has been kind enough to rush out a replacement pump -- are there any tricks to replacing the pump (I've never really opened up an engine in any way before)? Is this something I should have the FBO lend a hand with?

Thanks!
Paul

Harvey L. Sorensen · #2 08-14-2007, 11:48 AM

Imho with your experience you should have an aircraft mech do it. It is a simple job and should not cost much.
RV-9A flying with Phase one just compleated.

AlexPeterson · #3 08-14-2007, 12:04 PM

See:
http://www.vansairforce.com/communit...ad.php?t=15148

They are a real PITA to replace, but there is a sense of satisfaction that comes with getting it done.

Paul Eastham · #4 08-15-2007, 10:18 PM

Thanks guys,
I had the local shop lend a hand swapping the pump and it was a good learning experience (though I bet I could have done it myself ... would have taken much longer).

Anyway, after all that, the fuel pressure got really high again after 10-20 minutes of flight. Later, I put compressed air on the sender and it showed the correct values through the whole range. Perhaps when it gets hot it reads wrong? I put the heat gun on it for a while and could not reproduce the problem.

I have just collected some other ideas from searching the forums: check breather tube open, clean/redo faston connectors, purge air from the line. I may also try repointing my mag's blast tube at the sender (temporarily.) Gotta love those intermittent problems...

robertahegy · #5 08-16-2007, 05:03 PM

An intermittent problem could be a poor ground somewhere. Or perhaps the gage or sender is bad.

Roberta

az_gila · #6 08-16-2007, 07:36 PM

If you are measuring pressure after the pump, it could be a restriction or sticking float at the carb...

gil A

Paul Eastham · #7 08-16-2007, 08:05 PM

Quote:

Originally Posted by az_gila

If you are measuring pressure after the pump, it could be a restriction or sticking float at the carb...

gil A

Thanks Gil,
That's a theory I haven't heard yet. (I have checked grounding on the sensor, etc) Is there any easy way to diagnose such things? Fuel flow and engine operation seem normal to me.

az_gila · #8 08-16-2007, 08:17 PM

Quote:

Originally Posted by Paul Eastham

Thanks Gil,
That's a theory I haven't heard yet. (I have checked grounding on the sensor, etc) Is there any easy way to diagnose such things? Fuel flow and engine operation seem normal to me.

The first easy thing to do is to empty the carb bowl and clean the inlet screen of the carb and look for **** there... check that the hose from the pump to carb is clear, no little internal "flaps" if you assembled it yourself....

Then I guess it would be time to remove the carb and have an old-time A&P look at it....

gil A

Paul Eastham · #9 08-16-2007, 08:28 PM

Will do, thanks.
I thought of one other possible carb anomaly: I don't see any rpm rise when leaning. I never saw that in spam cans either; they always had tiny analog meters. Now I've got digital and it sure doesn't rise at all.

AlexPeterson · #10 08-17-2007, 06:41 AM

Maybe I'm missing something, but the carb should not be able to increase the engine pump's pressure. The fuel pump has a relief valve integral to it. You should be able to block all pump output, and the pressure should still be only a few psi.

Do you have a restriction in the hose going from the engine to your sender? If that restriction is too small, you could be seeing pressures due to boiling in this hose. Heat causes bubbles to form in the hose, but the restriction is so small that the pressure can't quickly be relieved back into the fuel system.

Just a thought.