OK, here's the info. Fuel pressure is about 36 or 37 PSI at idle. Both the mechanical underhood gauge and electronic transducer with the Holley ECU are showing this. This tells me it's very unlikely to be instrumentation.

As RPMs increase, pressure waffles a little bit here and there, rising to around 40, dropping as low as 33 or 34. At a constant 4000, I was dead on 37 psi. HOWEVER....when I pull the vacuum line to simulate WOT, it jumps right to 46, which is where I have it set.

Difference of course is volume of fuel. At ACTUAL WOT the injectors are pulling way more actual fuel than idling without the vacuum line.

First thing of course is a fuel filter. Got that. Will put on tomorrow. What is next? How do I trouble shoot this? Is there a way to test pressure farther back? -to see what the pump is actually pumping? BTW it's a Racetronix 255 with about 10,000 miles.

 

To me, it actually sounds like the FPR is working correctly. With the FPR being vacuum referenced, the fuel pressure will vary according to the intake manifold vacuum.

High RPM and no load mean high intake manifold vacuum. Which pulls the fuel pressure down.

RBob.

 

Yeah, I found this on a few different forums. Big difference when driving vs. in the garage. Ran a datalog and fuel and AFR seems to be spot on so it doesn't look like I'm actually losing pressure. ....this is what happens when you READ a ton, but are actually just a shadetree mechanic without tons of experience. I know enough to be dangerous!

Thanks for the reply.