A friend is having a fuel system problem. As he builds boost fuel pressure builds a bit and then just falls off barely in positive manifold pressure. He lets off the gas and fuel pressure returns to normal.

He has 2 pumps, both wired on their own setup straight from battery each with their own relay but both using the same trigger (an alway on ignition source). We have replaced both pumps and verified that everything in the tank is secure so there are no leaks back into the tank. We have verified both pumps are working at a rest. He has one 6-an line from each pump to a fuel block then a 8an filter (less than 50 miles on it), then another fuel block splitting back to 2 6-an lines each going to a rail. Then from each rail a 6 an line runs to the FPR and a 6-an line returns to the tank. He is running 32psi at idle.

He has also put a bulb on the FPR and it will build pressure fine in the driveway all the way up to 70psi. He changed the relay trigger sources to directly to the battery but the same thing happens so it doesn't appear to be a relay trigger issue and I don't see how the pumps would be turning off based on the boost pressure so we are assuming both pumps are working even when the drop happens.

I had a FPR fail and it just stopped seeing manifold pressure all together. These symptoms indicate to me that there is either:

a) not enough fuel flow to build pressure as the engine starts consuming more fuel.

or

b) the fpr has an issue and is not holding the pressure.

The bulb test points to fuel flow but I cant imagine how 2 pumps that are working at full voltage are not providing enough fuel for a 1psi boost.

Could the FPR fail in this way? Any other ideas?

 

Im running a system alot like your friend with no problems, base pressure at 42/45. The regulator could be bad and not rasing pressure, but you should be able to see it raise when you rev the motor.

 

Most likely is something so simple that you are overlooking it.
The fuel system is a simple system.

Check all fuel reloated lines and make sure they are CORRECTLY connected.
Verify that they all are not stopped up.

 

Where is the FPR vacuum/pressure source? is it a verified good source?

 

He moved the line to the Uim where the boost gauge was connected (Known good).