Too much fuel pressure

I Have a 1996 Chevrolet 2500 series with 5.7 multi-port injection.
My problem is I have 85-90 psi fuel pressure. I have tried 2
fuel pressure regulators, two fuel pumps and blew through
return line. At initial Key on I have 60psi, once engine is
started pressure jumps to 85-90 psi. I have a constant 20 in/hg
vacuum at manifold. Truck runs fine for 10 minutes then fuel
pump starts whinning, fuel pressure drops to 40-50 psi and I
get low fuel pressure drivability problems. Let truck set for 15-20
minutes, start up and cycle begins all over again.
I appreciate any help anyone can give me here. I stumped. Thanks

Have you visually checked the return line to see if it isn't crimped or twisted somewhere. It really sounds like a restriction on the return

Yes, I have actually disconnected the return line from the fuel tank and
stuck it in a can and watched it return fuel. The half gallon can filled
in maybe 15-20 seconds. The fuel pressure was 85-90 during this test.
Also the fuel spider I put on was from GM, not aftermarket. I keep
wondering if the fuel pump is wrong and putting out more fuel than
the regulator can handle. I called NAPA (where I got the pump) tech
service and they told me this pump is designed to put out more than
100psi max pressure. I thought about pinching the return line off and
seeing what max pressure is, but I am afraid that i will damage the
regulator or that $408.00 fuel module. Thanks again for the info.

As Jeremy said it sounds like a return line problem.

A good fuel pressure regulator should control the fuel pressure.
60 lbs no load or full engine vacuum.
Engine off or no vacuum it should go to 65-66 lbs of fuel pressure.

With engine off jumper fuel pump with a 12 volt fused jumper and check fuel pressure.
Also let it run about 10 to 15 minutes.

If it goes over 66 lbs.
Your fuel pressure regulator is not bypassing pressure or you have a restricted fuel return line.

You could be getting a bad batch of fuel pressure regulators.
Or the fuel pump could be restricted inside fuel pump module.

To test the return line you would almost have to hook another or a test return line into the intake where the return lines goes and drop the other end in in a proper container or feed it back into the fuel tank filler pipe.

Or hook a fuel pressure gauge in at the fuel filter and remove and jumper the pressure and return lines with a rubber hose at the intake.


As tech support says the pump should put out around a 100 lbs full pressure.
If testing fuel pressure just do not run pump longer then it takes to check full pressure.
When they start whining they are at full pressure and will burn up the pump if ran very long that way.


Did you replace the complete fuel injector assembly or just fuel injectors?
Was it doing it before you replaced fuel pump or spider assembly or fuel pressure regulator?

This model 5.7 has the fuel module or spider under the intake manifold.
The fuel lines come into this module at the top of the module. I have taken the return line off at that point and blew air threw it all the way to the tank.
I have also taken the fuel sending unit out of the tank and blew through it.
I have taken the return line off at the tank and started the engine, It seems to return plenty of fuel. Also the fuel module or spider is new, it came from
GM dealer. The fuel module came with everything, housing, injectors, regulator, and lines. The trucks original concern was badly contaminated fuel.

I know what fuel system is on your truck- I have the identical part on my 96 305, called MPFI instead of CSFI and the injectors are at the ends of the "spider" lines instead of poppet valves.

The fuel pressure issue is baffling if there isn't a restriction. At idle I would expect it to be around 55psi and at WOT- about the same as engine off. Do you know what the NAPA part number of the pump is? On my truck I was able to just replace the pump itself, was less than $100 from autozone over 3 years ago and since you truck uses the very same injectors as a 305 the pump shouldn't need to be any different. Possible NAPA part #s NFP74167. NFPP80001S, FE0114, BSH69225. Same results for 96 K2500 5.7 as k1500 5.0

There is only 3 ways it can build to much pressure.
Restricted return line or fuel pressure regulator.
Or a restriction in the fuel injector block before it gets to fuel regulator.

I would suggest testing with engine off with fuel pump jumpered.
Let it run for 10-15 minutes and watch pressure.
If pressure comes up replace fuel regulator and then if pressure still runs high bypass fuel return line with another line.
If still high jumper inlet and test return lines at intake and run test Lin into tank filler pipe and check fuel pressure at fuel filter.
If pressure is low with inlet and return line jumpered together and test return line run into filler neck.
You need to get another fuel injector block.

What kind of contamination was in the fuel tank to start with.
Was the tank replaced or cleaned real good and all lines cleaned and a good fuel filter installed?

In your first post you said it was losing pressure after 10 minutes.
Is it still doing this to.
If so there is a fuel pump module problem or restricted line or fuel filter or tank is low on fuel.
Make sure you have over a 1/4 to 1/2 tank for testing.
Or with pump jumpered with engine off give us your fuel pressure readings when first jumpered and then after 10-15-20 minutes.
MT

Is this a dual or single tank fuel system?

Also on a 96 K-2500 2 wheel I show a balance fuel pump on left inside fram rail.

I cleaned fuel tank. fuel lines and replaced fuel filter initially. After I noticed
the high fuel pressure issue I removed fuel module under plenum and replaced
regulator, the new regulator came with a new screen. I didn't see much
trash inside fuel module. Tested after reassembly but still had same
problem. Replaced fuel regulator again. Still had same problem. This is when
I decided to replace the complete fuel module, which comes with regulator,
injectors and all. Reassembled with the same results. It will crank and run
fine for maybe 10 minutes, but pump will start whinning, pressure drops to 40-50 psi and then start running bad. I can make pump run continuously and never exceed 60 psi, but once engine is started pressure exceeds 85 psi. The only difference between the two is 13.65 volts at pump and 20 in/hg vacuum in manifold. I'm to the point now where I'm checking things that I've checked
three times before. I think tommorow I'm going run separate fuel lines as you stated and run these tests with fuel module and pump in open where I can see whats going on. Thanks for the input. This is a single tank-ex propane burner. I have traced fuel lines a few times from tank to plunum and have't seen any other pump.

Sounds like a tough one or may be two problems.
But the high pressure at start up is not good.
If jumping to 85 or more it could over load fuel pump and cause problems on fuel pump end.
Like overloading it and then causing the fuel pump whine and then low pressure.

Engine running or engine vacuum on regulator should only raise fuel pressure about 5 lbs.
And or put a little more voltage to fuel pump.
Pump should run on 12 or 13 volts with no problem.
But you might check for good ground and or for voltage drop at fuel pump.
Some had a plug in connection problem with bad or lose wires and some had a rewire change over.

Keep us posted on how it goes.
And give tech support on fuel pump another phone call.
Some of them people can be a lot of help.
Good Luck MT

I found problem today. Went back and blew through return lines again. This time I blew with my mouth and I felt a little resistance. I removed metal return line from union above fuel filter to where it is attached at fuel sender unit. Took line and ran a piece of wire threw it, I got all kind of rust and trash out of line. By the time I finally had the wire moving easily all through the line I had thimble full of what looked like rust. The line was not stopped up, it was just restricted from rust enough to reduce flow from regulator. I guess you learn something new everyday. This thing had a good (visual) flow from regulator when engine was running,just not enough volume. I appreciate the help and input on this problem.

You are welcome and thanks for posting back with the fixed.
Glad you found it.
You might kinda watch the inside of the steel lines.
I have had a couple of steel lines rust inside and start flaking off rust.
Sometime something like that can be a pain.
One time I had a big bug get in a fuel line and it took about 6 mo. to find it.
That rascal would float around in there a stop up a fitting and then float lose and you could not find the critter.
Good luck.
MT