Is this GM Fuel Pressure Spec Unachievable?
 

I ran the fuel pressure tests in the helpful "96+ (SCFI/MFI) Fuel System Diagnosis" found here https://blazerforum.com/forum/tech-a...agnosis-23038/ on my '96 Blazer. Step 3 specifies "After the fuel pump stops running the fuel pressure should stabilize and remain constant. Does the fuel pressure drop more than 5 psi in 10 minutes?" With the original 1996 fuel pump, mine dropped from 57-58 psi to 44 psi in 10 minutes. I confirmed the drop was through the check valve of the fuel sender and replaced the original 1996 sender with a new Delphi. I also installed a new Delphi fuel pressure regulator. Now the fuel pressure drops from 57 psi to 42 psi in 10 minutes. After ll this work, it drops more than it did before.

Is this 5 psi drop spec just a nice GM target that should not be taken seriously or does a drop of more than 5 psi indicate a real problem that should be fixed?

did you just swap out the sending unit or the pump also at the same time

I replaced the the pump too. I bought a whole new pump/sending assembly on ebay that was identical to the original. It even had the same "SFR" sticker on it. By "new" I mean it was never used. It was probably manufactured about the same time as the original that it was replacing, 1996, so in that sense it was 18 years old.

with the filter removed and the test gauge in its place the pressure should remain pretty much constant with minimal drop if it dosent my guess the check valve in your ebay replacement pump is bad

I agree ^^^

Deadhead pressure spec is 73psi to 108psi while the pump is running. It must remain above 55psi for at least 10 minutes after the pump shuts off.

Regulated pressure, engine off, tested at the service port by the distributor, must be 60psi to 66psi while the pump is running, and must remain above 55psi for at least 10 minutes after the pump shuts off.

Testing regulated pressure & leakdown checks the entire fuel delivery system. If it fails one or both parts of the test, deadhead pressure and leakdown reading must be taken to determine if the leakdown is in the tank, or in the plenum.

Your intuition is confirmed. The deadhead pressure of my new pump measured at the fuel filter drops from 80 psi to 68 psi in 10 minutes. There are no external leaks. I will contact Delphi for warranty. Thanks odat and Captain Hook.

80psi deadhead running and drop to 68psi after 10 minutes is within spec, the pump is fine. The reading of 57psi and a drop to 42psi in post #1 means the regulated pressure is too low and the leakdown is excessive. Both problems are in the plenum.

In the posts above, I only reported the pressure drop in 10 minutes, not the instantaneous drop after pump shutoff. Here are the complete results of tests at the Schrader valve:

Test 1, battery just charged:
Pump on: 62 psi
Drops instantly when pump shuts off: 56.5 psi
After 10 minutes: 48 psi
Dropped 8.5 psi in 10 minutes after pump shutoff

Test 2 with a different gauge next day, battery not charged overnight:
Pump on: 59 psi
Drops instantly when pump shuts off: 53 psi
After 10 minutes: 43 psi
Dropped 10 psi in 10 minutes after pump shutoff

The deadhead pressure is within spec, so I am concluding that with a fully charged battery the pump-on pressure is adequate (60-66) and my only problem is the 10 minute leakdown. The plenum is open and the fuel pressure regulator and poppets are not leaking a drop into the intake, so I conclude the pressure must be going through the pressure regulator down the return line, which is where I will put my effort.

A certain amount of leakdown is to be expected when the pump initially shuts off, but your drop from 62psi to 48psi is slightly excessive. "Technically" it is out of spec, and depending on what's leaking, it may or may not affect fuel mileage.

There are numerous places in the plenum that can leak. These can all be checked visually. Keep in mind though, you're not looking for a serious leak, it might just be damp with fuel:
The fuel pressure and return line connections to the metering block.
Each injector has an O ring inside the fuel metering block.
Each nylon line where it connects to the injector and to the poppet.
The nylon line itself.
The poppet nozzles.
External leak from the pressure regulator.

The pressure regulator can also leak internally. To check for that, the system must be pressurized with the pressure tester connected. When the pump shuts off, immediately seal off the return line to the tank, (requires some creative plumbing using a ball valve). If the leakdown remains within spec, the regulator is leaking internally. If leakdown continues to drop, the regulator is not the problem.

Generally, if there is a problem with the spider assembly, it's cheaper, (and better) to upgrade to the new style spider. (Delco #FJ10565) The injectors are relocated to the intake ports and the poppets are eliminated. Plug & play, takes just over an hour to do the upgrade.

The original problem was that I was getting leakdown through the fuel pressure regulator from 60 psi to 43-48 psi in 10 minutes. To correct this I bought a new Delphi fuel pressure regulator and installed it, but failed to remove the tiny o-ring from the old fuel pressure regulator. So I had two tiny o-rings where there should have been one. Now the CSFI leaks into the return line without creating any pressure whatsoever at the Schrader test port. It does this regardless of what fuel pressure regulator I install and I suspect the fuel is going directly into the return line without even going through the fuel pressure regulator. As soon at I turn the key the fuel comes out the return line.

I thought the two o-rings may have distorted or cracked the plastic injector body, but I took it apart and can find no leaks. There is one o-ring inside the injector body that seals the return line from the pressurized feed volume, but that connection looks fine.

Any similar experience out there? Hints? Ideas?