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Tim's Auto Repair
Tim's Auto Repair, mechanic
Category: Chevy
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Experience:  Have owned a repair shop for 25 yrs.
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96 chevy: service engine soon light came..trouble codes..o2 sensors

Resolved Question:

The service engine soon light came on in my 96 chevy silverado. The trouble codes said 3 of the four o2 sensors had low voltage. I erased the codes hoping it was a one time thing and that it was operating in safe mode but the truck still isn't running right. The rpms only go up to about 1800, after that the engine bogs down. Other than that it idles fine and drives normally up to the point it bogs down. It starts a little harder than normal also. I inspected the o2 sensor connections and they seemed fine. I replaced 2 of them several months ago as well as the catalitic converter. Whats the problem!!
Submitted: 7 years ago.
Category: Chevy
Expert:  Tim's Auto Repair replied 7 years ago.

hi

you need to check the fuel presure...this sounds like a weak fuel pump...these must have no less then 58psi at all times

i would think you need a new fuel pump

thanks

Testing and Inspection

 

Diagnostic Chart
Diagnostic Chart
Diagnostic Chart
Diagnostic Chart
Diagnostic Chart
Diagnostic Chart
Description

Circuit Description

When you turn ON the ignition switch, 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 the fuel pump OFF within 2 seconds after you turn the ON the ignition or if the engine stops.

An electric fuel pump pumps the fuel through an in-line filter to the Central SFI unit. The pump is attached to the fuel level meter assembly inside of the fuel tank. The pump is designed in order 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-E Fuel Pressure Gauge.
  • J 34730-1A Fuel Pressure Gauge with J 34730-250 Fuel Pressure Adapter Kit.
  • d 42873-1 3/8 Fuel Feed Pipe Shutoff Valve and the J 42873-2 5/16 Fuel Return Pipe Shutoff Valve.

Test Description

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

  1. 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 leaving the engine OFF, the fuel pressure should be 415-455 kPa (60-66 psi). This pressure is controlled by a spring pressure within the regulator assembly.
  1. The fuel pressure that continues to fall is caused by one of the following items:
    • The in-tank modular fuel sender is not holding pressure.
    • The fuel pressure regulator valve is leaking.
    • A Central SFI injector is leaking.
    • Fuel line is leaking.
  1. 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 vary somewhat depending on the barometric pressure; however, the pressure at idle should be less indicating the pressure regulator control.
  1. If the fuel pressure is less than 415 kPa (60 psi), it falls into the following 3 areas:
    • The pressure is regulated but less than 415 kPa (60 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, overall poor performance, and may set a DTC. 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 inoperable. However, it 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). Excessive pressure may damage the fuel pressure regulator.

  1. 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 455 kPa (66 psi), as the valve in the return line is partially
Customer: replied 7 years ago.

I replaced the fuel pump about a month ago. That was the fourth new pump I have had to put in the truck in about 6 years. What could cause them to go out so often?

Expert:  Tim's Auto Repair replied 7 years ago.

hi

it is fairly common to go through a couple...it is hard to find a good one sometimes..nothing causeing it,,,just bad pumps,try a different brand

thanks

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