Fuel or electrical

GARY SCHELLHAMER

MEMBER
1993 GMC C3500
7.4L
V8
2WD
AUTOMATIC
150,000 MILES

The problem started a few days ago. When it would start, it would not idle. Going down the highway was fine. I changed the idle air control, and the throttle position sensor. It did not help, only got worse. It sounded like it was flooding. So, we changed the ignition control module, plugs, wires, cap, rotor, coil, fuel filter, replaced both injectors, all new gaskets in TBI, water temperature sensor, oxygen sensor, and went as far as a new ECM, also new MAP sensor. Still no improvement. Will not idle, or if you try to accelerate it sounds like it is starving on fuel. There are only two or three vacuum lines. I removed the vacuum line from the MAP sensor, and it started to run on its own and did better. Then forty five seconds or so later it shut off. I have sprayed carburetor cleaner along the intake to check for leaks. It really did not change. We tried starting fluid along the linkage side of intake, it sounded better. I have no water leaks etc, on the intake. Could the intake gasket still be bad? I am at a loss on where to go from here. I have tried everything I know. Help please!

1 Reply

JACOBANDNICKOLAS

MECHANIC
108,171 POSTS

Hi and thanks for using 2CarPros.com.

Wow, you have done a lot. If spraying starting fluid along the intake didn't make much of a change, I doubt the problem is related to the intake gasket.

Interestingly, when you disconnected the MAP sensor vacuum, it ran better for a short time. Since disconnecting it would have allowed more air into the system, I question if there is an issue with fuel pressure (too high) causing the problem.

You did not mention checking it, so start there. Here are general directions for checking both pump pressure and the pressure regulator:

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

Here are the directions specific to your vehicle along with the manufacturer's specifications:

FUEL SYSTEM PRESSURE TEST
PROCEDURE

A fuel system pressure test is part of several of the Diagnostic Charts and Symptom Checks. To perform this test, follow this procedure:

Note: To perform residual pressure check refer to: Fuel Pump, Testing and Inspection.

1. Turn engine OFF to relieve system pressure.
- Disconnect negative battery terminal to avoid possible fuel discharge if an accidental attempt is made to start the engine.
- Loosen fuel filler cap to relieve tank vapor pressure.
- The constant bleed feature relieves fuel pump system pressure when the engine is turned "OFF".
2. Uncouple fuel supply flexible hose in engine compartment. Install fuel pressure gauge between steel line and flexible hose.
3. Tighten gauge in line to insure no leaks occur during testing.
4. Connect negative battery terminal and START engine and observe fuel pressure reading. It should be 62-90 kPa (9-13 psi). If not, refer to DIAGNOSTIC CHARTS/FUEL SYSTEM PRESSURE TEST (CHART A-6).
5. Turn engine OFF to relieve system pressure.
- Disconnect negative battery terminal to avoid possible fuel discharge if an accidental attempt is made to start the engine.
- Loosen fuel filler cap to relieve tank vapor pressure.
6. Remove fuel pressure gauge.
7. Install new O-ring on fuel feed line. Reinstall fuel line.
8. Connect negative battery terminal and START engine and check for fuel leaks.
9. Install air cleaner assembly.

Let me know what you find or if this is something you have already done.

Take care,
Joe