96 Chevy 5.7 Fuel Pressure Issue

[jbarry14]

96 Chevy K3500 with the 5.7l It has about 215K miles. I am trying to diagnose a fuel pressure issue. My truck first started to run rough like it had a miss. Check engine light came on. I had codes for both banks being lean. I cleaned the MAF sensor with no change. I checked for vacuum leaks. I hooked up a scan tool and checked the LTFT for both banks. They are both at 25%. I hooked up a fuel pressure gauge to the rail test port. Key on engine off it is around 45. As soon as the pump shuts off it almost instantly drops to 0. I am trying to figure out if I have a bad check valve in my fuel pump or if my FPR is bad. Before this all started, I noticed a loss in power. It could barely get a trailer up the slightest incline. I was trying to see if I could find out the difference in the speed of fuel pressure dropping when the check valve in the pump is bad and when the FPR is bad, to try and isolate the issue.

[richard wysong]

How does the oil smell? My $ is on the FPR being the problem. sometimes they leak externally into the intake which would cause the rough idle but doesn't account for the lean codes

[jbarry14]

I will check the oil. The FPR could still be bad, but the pressure is not to spec either. The other thing is that when the pump shuts off, the pressure immediately drops to 0. If the FPR is bad, wouldn't it take longer for the pressure to go down? I am thinking the pump is bad and the check valve in the pump is bad and the fuel is going right back to the tank.  

[richard wysong]

Very possible. You could remove the fuel filter and check the pressure there to determine if it is the pump or the FPR. You may have to make an adapter for your gauge to do this though as most adapters would just go inline with the filter

Edited December 7, 2022 by richard wysong

[asilverblazer]

I would normally say the fuel pump is bad, base don your pressure dropping when you shut it off, but sounds like it could be the regulator letting off too much pressure the pump can't build up enough, causing drivability problems. I can't recall regulator test procedures from memory though.

 

[SILVER SLED]

I had a 98 1500 with a 350.  I went through a lot of fuel pumps back then for one reason or another.  Bad check valve, weak pump… 

[mjm-1957]

Pinch off the return line and watch if pressure still drops off.If it does than the check valve in the pump is bad.

[jbarry14]

I just wanted to give an update. I had the fuel pump replaced on my truck. That helped with some of the problems. But now I am getting a lean code for bank 2 only. I had a shop put a camera down the intake to look at the FPR and Injectors. The previous owner had the FPR and Injectors replaced with the new style and they are all working perfectly. All of the O2 sensors have been replaced very recently as well. My LTFT on bank 2 is at 25%. What else can I look for or test to figure out why the only code I get is for bank 2 being lean? It seems to run a little more rough when the air temperature warms up. It has been very cold here in Minnesota recently.

[asilverblazer]

No miss fires?

 

I'd do a compression check and throw a vacuum gauge on it all the basic diagnostic steps...

 

Do those checks cold and warm. Sounds like a bunch of parts have been thrown at it without a thorough diagnosis.

 

Start looking for air/vacuum leaks.

[jbarry14]

11 minutes ago, asilverblazer said:

No miss fires?

 

I'd do a compression check and throw a vacuum gauge on it all the basic diagnostic steps...

 

Do those checks cold and warm. Sounds like a bunch of parts have been thrown at it without a thorough diagnosis.

 

Start looking for air/vacuum leaks.

 There are no miss fires.

 

Compression is good

 

I will be smoke testing the engine later tonight

 

The parts that I have replaced, had to be replaced. The fuel pump was only putting out max 45 key on engine off, way under spec. The check valve in the pump was also bad and making the pressure drop to 0 instantly after the pump would shut off. Since I replaced the pump, I wanted to do the fuel filter at the same time. Now I have spec fuel pressure and it holds after the pump shuts off. I had codes for 2 bad O2 sensors, The others didn't throw a code, but they could still be giving off bad readings and they were cheap enough that I wanted to just do them all. Now the only code I have is for bank 2 being lean. 

[LDM]

Has the intake manifold gaskets ever been done on this truck?  This will cause all sorts of issues when it decides to start leaking and they all do eventually.  With 215K I would assume it has at some point but maybe not.

 

It looks like you've had the other stuff done, spider assembly and regulator, as well as a new fuel pump that is holding properly pressure now.

[jbarry14]

I am starting to think it is the intake gaskets. I am not sure if they have ever been done, but the smoke test I am going to do tonight should give me a definitive answer. I was just hoping there might be a sensor or something besides a vacuum leak that I could check in the meantime that would only effect bank 2. 

[asilverblazer]

20 minutes ago, LDM said:

Has the intake manifold gaskets ever been done on this truck?  This will cause all sorts of issues when it decides to start leaking and they all do eventually. 

I was thinking along the same lines.

[asilverblazer]

1 hour ago, jbarry14 said:

Compression is good

 

I will be smoke testing the engine later tonight

 I think you're on the right track.