M

MPB

Member
Looking for a little insight. Maybe one of you has dealt with something like this before. I am getting ready for the first flight and in ground run ups the mechanical fuel pump is not delivering enough pressure to feed the fuel injection system and the engine is quitting. It seems to run OK at low RPM, but when brought up to medium power it quits. We finally got it anchored down today and did a full power run up and shut off the boost pump once it was there and we were getting 27 psi of fuel pressure with only the mechanical pump. When the throttle was reduced the pressure quickly dropped to about 15 and the engine again quit. Any ideas?

Thanks,
Mike
RV4, CA
 
I had the same problem. Turned out to be a loose b-nut on the suction side of the boost pump. I had torque sealed it so when I was checking for loose nuts I didn't Check the ones I'd marked. Duh. It was letting air bubbles in.

Tightened it up and the engine runs fine.
 
Thanks. I will check that today. I hope that is what it is because you have to pull the engine on the 4 to change the fuel pump.
 
Ditto

But my problem was a poor fuel line flare which caused the air leak.

-John

Veetail88 said:
I had the same problem. Turned out to be a loose b-nut on the suction side of the boost pump. I had torque sealed it so when I was checking for loose nuts I didn't Check the ones I'd marked. Duh. It was letting air bubbles in.

Tightened it up and the engine runs fine.
Click to expand...
 
Just to be sure you dont have any leaks, you can cap the line into the spider. Turn on elec pump.
If no leaks found, Id suspect you in fact do have a bad mechanical pump.:(
 
No leaks

One point, in my case I had no fuel leaks. When I called Lycoming tech support, they said to check all fuel line connections even if there was not fuel leaking.

-John

Kahuna said:
Just to be sure you dont have any leaks, you can cap the line into the spider. Turn on elec pump.
If no leaks found, Id suspect you in fact do have a bad mechanical pump.:(
Click to expand...
 
THANK YOU

Thanks for the advice, everyone! 474AM is officially an airplane as of today. It was as many of you suggested, just a loose fitting (not loose enough to leak fuel). First flight was uneventful flown by my partner, father and 3 time repeat offender. Only about 39.5 more hours for phase one completion!

Mike
RV4, CA
 
Glad you found your leak so quickly and resolved it! Took me quite a while longer. :-(

For the sake of future readers and following up on Kahuna's comment, MY loose nut was on the SUCTION side of the electric pump. It would never see positive head there and that's why it didn't show up with the electric pump on. So future "leakers", check them all, from the fuel tank connections to the carb or fuel injectors.
 
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