Losing fuel pressure

Tiny
BRIANRONEY
  • MEMBER
  • 1996 CHEVROLET 3500
  • 7.4L
  • V8
  • 2WD
  • AUTOMATIC
  • 246,000 MILES
Truck has an extended crank because of fuel pressure having to build up.I can pinch rubber line in fuel return line and vehicle will hold fuel pressure. I have replaced fuel pressure regulator twice with no change in condition. Is there anything in the fuel rail that the regulator is suppose to be closing off that may not be working?
Saturday, January 11th, 2020 AT 5:24 PM

4 Replies

Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 109,747 POSTS
Hi,

The fuel pressure regulator is designed to reduce pressure, so if you don't have enough pressure, either the pump is bad or you have a plugged fuel filter. Have you checked pressure? It should be between 60 and 65 psi. I have a feeling it is lower.

Although there could be something stuck in the regulator preventing pressure to build, since you replaced it twice, chances are that isn't the problem.

https://www.2carpros.com/articles/how-to-check-fuel-system-pressure-and-regulator

_____________________________________

If you look below, I attached 5 pics. The pics are in order and consist of a diagnostic flow chart related to fuel pressure problems. Take a look through them and see if they help.

_____________________________________

In my manual, this comes after the flow chart.

CIRCUIT DESCRIPTION
When you turn the ignition switch ON, the Vehicle Control Module (VCM) turns on the in-tank fuel pump. The pump remains on as long as the engine is cranking or running and the VCM is receiving reference pulses. If there are no reference pulses, the VCM shuts off the fuel pump within 2 seconds after you turn the ignition ON or the engine stops.

The pump is attached to the fuel level meter assembly inside the fuel tank. The pump is designed to provide fuel at a pressure above the regulated pressure needed by the injectors. The pressure regulator keeps the fuel available to the injector at a regulated pressure. Unused fuel is returned to the fuel tank by a separate line.

DIAGNOSTIC AIDS
Tools Required, J 34730-1A Fuel Pressure Gage in J 34730-E or J 34730-1A Fuel Pressure Gage with J 34730-250 Fuel Pressure Adapter Kit.

TEST DESCRIPTION
The numbers below refer to the step numbers on the diagnostic table.

2. Wrap a shop towel around the fuel pressure connection to absorb any small amount of fuel leakage that may occur when installing the fuel pressure gauge. Turn ON the ignition, the fuel pressure should be 385-430 kPa (56-62 psi). This pressure is controlled by a spring pressure within the regulator assembly.
4. The fuel pressure that continues to fall is caused by one of the following items:

The in-tank fuel pump check valve not holding.
A partially disconnected fuel pulse dampener (pulsator).
The fuel pressure regulator valve leaking.
An SFI injector is leaking.

5. When the engine is idling, the manifold pressure is low (high vacuum) and is applied to the fuel pressure regulator diaphragm. This will offset the spring and result in a lower fuel pressure. This idle pressure will very somewhat depending on the barometric pressure; however, the pressure idling should be less indicating the pressure regulator control.
16. The fuel pressure less than 385 kPa (56 psi), falls into the following 3 areas:
A regulated pressure but less than 385 kPa (56 psi). Amount of fuel to injector OK, but pressure is too low. System will run lean and may set a DTC. Also, hard starting cold and overall poor performance or may not start at all. Refer to Engine Cranks but Does Not Run
A restricted flow causing pressure drop - Normally, a vehicle with a fuel pressure of less than 300 kPa (44 psi) at idle is undriveable. However, if the pressure drop occurs only while driving, the engine will normally surge then stop running as the pressure begins to drop rapidly. This is most likely caused by a restricted fuel line or plugged filter.
A leaking or contaminated pressure regulator valve or seat interface may not allow the regulated pressure to be achieved.

Notice: Do not allow the fuel pressure to exceed 517 kPa (75 psi) because damage to the fuel pressure regulator may result.

22. Restricting the fuel return line allows the fuel pressure to build above regulated pressure. With the battery voltage applied to the fuel pump test terminal, the pressure should rise above 430 kPa (62 psi), as the valve in the return line is partially closed.

_____________________________________________

Let me know what you find or if you have other questions.

Take care,
Joe

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Saturday, January 11th, 2020 AT 6:57 PM
Tiny
BRIANRONEY
  • MEMBER
  • 3 POSTS
Fuel pressure is good at ignition on, but drops like at rock when pump stops running. Step 3 pressure does not hold steady but drops like a rock with key off, so I move to step 4. Pressure holds steady when I pinch flexible hose on return side which is in line with regulator so I move to step 6 to check for partially disconnected fuel pulse dampener, I am thinking that is the FPR, but I am not sure if it is something in the fuel rail.I have removed fuel pump to replace it but there is no check valve in return line in old pump or new pump, so I did not replace pump because I was not sure about that.I will get better pressure reading for you tomorrow, but I believe it is something in fuel rail allowing pressure to leak off, but I am not sure how that pump works to hold pressure when off.
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Sunday, January 12th, 2020 AT 11:12 AM
Tiny
BRIANRONEY
  • MEMBER
  • 3 POSTS
What is a partially disconnected fuel pulse dampness on this vehicle?
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Sunday, January 12th, 2020 AT 11:37 AM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 109,747 POSTS
A fuel pulse dampener is designed to maintain a steady pressure to the injectors. When the injectors open and close, it causes a fluctuation in pressure. If that happens without restriction, you won't get an even flow of pressure to each injector. The pulse dampener is the flexible connector between the fuel pump and the metal fuel line in the tank. It can tear or become damaged. However, the result is usually low or no fuel pressure. If it is torn or damaged, it could cause the drop off you described. Also, it can be the cause of low fuel pressure as you described.

I attached a pic below. When you replaced the pump, was that replaced?
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Sunday, January 12th, 2020 AT 7:32 PM

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