Instant 10psi drop in fuel pressure

[pkufeldt]

I have a 1999 K1500 Suburban with a 5.7l. It exhibits an instant 10psi drop in fuel pressure when the fuel pump stops while doing a key on engine off test. It goes from 61-62psi (in spec) while the pump is running to 51-52psi (out of spec) when it is off. It holds there.

I have followed the service manual's fuel system diagnosis flow chart and isolated the fuel system at the fuel filter with a J 42873-1 shut off valve. I get an acceptable 2-4 psi drop when the valve is shut isolating the front 3/4 of the fuel loop. So it seems like something in the supply line before the fuel filter is causing the majority of the 10psi drop. Before replacing the the fuel line from the sender to the fuel filter, what could cause such a large instantaneous drop and then hold? Any help before plunking down $150 bucks would be greatly appreciated.

FWIW: I have a new fuel pump and a new MPFI spider injector that came with a new regulator.

Here is video of the drop. [If the video does not play click on the YouTube icon in the player]

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[Caman96]

I have a 1999 K1500 Suburban with a 5.7l. It exhibits an instant 10psi drop in fuel pressure when the fuel pump stops while doing a key on engine off test. It goes from 61-62psi (in spec) while the pump is running to 51-52psi (out of spec) when it is off. It holds there.

I have followed the service manual's fuel system diagnosis flow chart and isolated the fuel system at the fuel filter with a J 42873-1 shut off valve. I get an acceptable 2-4 psi drop when the valve is shut isolating the front 3/4 of the fuel loop. So it seems like something in the supply line before the fuel filter is causing the majority of the 10psi drop. Before replacing the the fuel line from the sender to the fuel filter, what could cause such a large instantaneous drop and then hold? Any help before plunking down $150 bucks would be greatly appreciated.

FWIW: I have a new fuel pump and a new MPFI spider injector that came with a new regulator.

Here is video of the drop. [If the video does not play click on the YouTube icon in the player]

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Curious which brand fuel pump and MPFI Spider you used?

 

[pkufeldt]

Delphi FG0092 fuel pump. GP Sorenson 800-1513N spider.

 

[pkufeldt]

BTW: The J 42873-1 shut off valve has schrader valves on both sides of the valve and I have measured the pressure on both sides. On supply side the pump is capable of 100+psi.

 

[Caman96]

So it seems like something in the supply line before the fuel filter is causing the majority of the 10psi drop.

Leaking check valve on the fuel pump?

 

[pkufeldt]

I replaced the fuel pump because of this issue and this drop remained unchanged from the original pump to the new pump. Even so, wouldn't a leaking check valve cause a continual drop in pressure? After this 10psi drop the pressure holds.

There is very little from the fuel filter and the sending unit - the filter, a handful of O-rings, a pressed connection between the metal pipe and high pressure flexible tubing, and finally the high pressure quick connect to the sender. In the picture below, the pipes at the top and bottom are the ones in question.

BTW: I also put a brand new ACDelco fuel filter in there.

I am stumped.

 

[L31MaxExpress]

I have never seen one not drop 10-15 psi when the pump shuts off. As long as it holds some residual pressure and is within spec while the pump is running, I am not seeing an issue. The pump is going to run on the prime function when you turn the key on before you crank the engine anyway.

 

[Caman96]

I have never seen one not drop 10-15 psi when the pump shuts off. As long as it holds some residual pressure and is within spec while the pump is running, I am not seeing an issue. The pump is going to run on the prime function when you turn the key on before you crank the engine anyway.

I was just thinking that maybe this sounds normal actually.

 

[L31MaxExpress]

This is the exact behavior I have seen on many of these. Van behaved the same way with both the stock style manifold and the marine manifold. I think the fuel pressure regulator and check valve have a slight amount of leakdown before they seal up, just the nature of how they work using spring pressure to seal up the openings.

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[pkufeldt]

I was afraid of that. But the larger context is that the engine is misfiring (See https://www.gmt400.com/threads/misfires-return.62862/) and fuel pressure is an issue I can't seem to resolve.

From the service manual:

If the fuel pressure is less than 379 k P a (55 psi) at idle, it falls into the following 3 areas :

The pressure is regulated but less than 379 kPa (55 psi). The amount of fuel reaching the injector is sufficient, but the pressure is too low. The system will run lean, hard starting cold, no start, over all poor performance, and may set a DTC.
....

When it is idling I drop from 51-52 to 49-50psi - I assume that is due to intake vacuum/regulation. I have done a lot to diagnose the misfires and the fuel pressure seems to be the last thing. Here is the reader digest version. Replaced plugs, coil and wires. I did compression and leak down tests (all looks good). I had a bad disty (worn down cam gear), replaced and cam retard adjusted. Had high fuel trims caused by lazy O2 sensors and intake leaks, both fixed and FT are significantly lower. Fuel pressure is the last thing on my list. At a loss.