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.
96 Chevy 5.7 Fuel pressure issue
- Thread starter Jbarry14
- Start date
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You can pinch off the return line and see what happens to the pressure. If it goes high and holds after you shut off the key, your fuel pump is probably still good. If it acts the same as before, it's probably your pressure regulator. Another possibility is that the hose connecting the fuel pump to the sending unit may have split, blowing fuel back into the tank.
With both banks reading lean, it's probably not your spider leaking. YMMV
Yeah. I am going to replace the pump and the fuel filter because the pressure is way below spec. I am hoping the fuel pressure is dropping because of a bad check valve in the pump or possible problem with the rubber lines as your were saying. 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.
Don't go to the trouble of dropping the tank or lifting the bed, until you've tested the voltage as close to the pump as practical. Usually this is at the most-rearward connector before the harness goes up 'n' over the tank.
The pump MUST be running when you test the voltage.
Low voltage makes for low pressure/low volume.
In a perfect world, you'd verify voltage near the pump, and also verify current draw using a low-amperage probe and an oscilloscope. That way you can test the current draw of each individual bar on the motor armature; and calculate motor RPM.
Don't forget to install a new fuel pump wire harness inside the tank. The connectors corrode; if they're bad enough that alone can cause low fuel pressure--but you can't test for it without opening the tank.
Most fuel pump harnesses are "universal", meaning the wires are extra-long to fit as many applications as possible. Original harness vs. NAPA harness:
Of course, if your fuel pump is the integrated "module" style, the assembly comes with a new harness. Otherwise, you generally have to buy 'em separately.
The pump MUST be running when you test the voltage.
Low voltage makes for low pressure/low volume.
In a perfect world, you'd verify voltage near the pump, and also verify current draw using a low-amperage probe and an oscilloscope. That way you can test the current draw of each individual bar on the motor armature; and calculate motor RPM.
Don't forget to install a new fuel pump wire harness inside the tank. The connectors corrode; if they're bad enough that alone can cause low fuel pressure--but you can't test for it without opening the tank.
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Most fuel pump harnesses are "universal", meaning the wires are extra-long to fit as many applications as possible. Original harness vs. NAPA harness:
You must be registered for see images attach
Of course, if your fuel pump is the integrated "module" style, the assembly comes with a new harness. Otherwise, you generally have to buy 'em separately.
Thanks for the reply. I will test the voltage first. The only other thing is I still need to figure out where my fuel pressure is leaking. If the valve in the pump is bad or the FPR is bad. It drops to 0 immediately after the pump shuts off.
Fuel pump check valve and the hose connecting the pump to the "hanger" in the tank would be verified OK. It's an indirect way of indicating that the regulator has failed.
I don't think so. If the return side is blocked, it wouldn't matter if the regulator leaked. It would be an indication that the pump or the connecting hose has failed.
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.
Replied in your OTHER thread on this same issue.
Please don't start more than one thread on the same problem.
Please don't start more than one thread on the same problem.
Its not the same problem. This thread was about fuel pressure issue, which has been resolved. My other thread was about a bank 2 lean code... thank you though.
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