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Car runs great, but loses pressure under boost


usafdarkhorse

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^^^^^^ FUEL pressure^^^^^^ ( oops! )

Hi all! I know this is a forum with purist intentions, but I have a highly modified Z car with a problem that could plague any car, purist or not.

Alright, so I've posted a thread or two before about the problems I had been having with my '75 Z. So about 5 months later, I've come full circle having fixed alot of nagging problems, and I've got one left to solve.

My Z has an L28ET from an '82 and runs MSII, and somehow between reflashing the firmware or my alternator troubles, the tune was either corrupted or lost. So after getting retuned ( MS thought my car was running a V8....it is not ha ), the car runs beautifully. Idles as smooth as can be after the warmup enrichments.

However, after taking her for a few test drives, I've still got a naggy problem of shaky fuel pressure at idle and losing fuel pressure while under boost.

The specifics of the fuel system is I've got the OEM fuel tank with a welded in sump, feeding a Walbro 255LPH, to an Aeromotive FPR, to an aftermarket fuel rail, through a 100 micron filter, and then back to the tank again.

Tests performed:

-Blocked off sensing manifold pressure to the FPR; no difference: not vacuum problem.

-13.8V to the relay board and 13.2V after the fuel pressure relay (voltage drop due to resistance?) Fuse gets very hot, and new fuse made no difference ( as well as getting hot too ).

-Get anywhere between 12.4V to the potential 13.2V I see at the relay board at the pump positive. Oddly enough, very little resistance along wire from relay to pump.

-Connected pump straight to battery positive momentarily; no difference.

-New ground straight from pump to battery ( sometimes shows some resistance, sometimes none...oddly shows voltage at times ). No difference.

-Turned pump on without engine. Pressure peaks to 50psi as normal ( for my car ), but slowly loses pressure ( I monitored to about 35psi and it was still dropping ). The higher resolution gauge on the fuel rail bumps around ever so gently, even while the engine is off. Points toward mechanical problem.

The pump is a few months old with less than 100 miles on it. The tank has run very low before while testing the car. The fuel tank has been coated in a sealer that very well may peel off despite its claims to be perfect for fuel tanks. The previous owner suspected he might have some blockage before he sold me the car, so he dropped the entire fuel system and blew it out. Eveything was clean.

Again, the car runs great while not in boost ( even runs great while in boost, just loses pressure ) and the shaky fuel pressure at idle doesn't seem to affect it that much. It's just that it drops from about 40psi to 30psi or lower while under boost and should increase.

Should I be looking to drop the tank and system again? Fuel pump the culprit? FPR? Should I look somewhere else? Sorry for the huge post!

Thanks in advance!

Edited by usafdarkhorse
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Pulled the filter and found a few bits of crud in there. It's a 100 micron filter so it mattered quite a bit.

Fuel pressure's rock solid and when clamping off the return line, the fuel pressure pegs the gauge and I have to turn it off before it breaks.

Sounds good to me haha.

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