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2001 GMC problems.


skywalker45
09-08-2007, 10:33 PM
I've had this very strange problem with my 2001 GMC Jimmy 4WD. It has the 4.3L Vortec with the csefi fuel injection system. It has been starting rough for awhile and the other night it finally died. I thought maybe a tune up was in order so I replaced plugs, cap and rotor, fuel filter, air filter and PCV valve. After all of this it still wouldn't start. It is getting good spark and fuel is making it to the fuel meter module at the rear of the carb. I bought a fuel system pressure tester. You hook up this device to the Schrader valve on the high pressure line. I hooked it up and turned on the key. For a couple of seconds the fuel pump hummed as it pressurized the system. According to the manual for the pressure tester the system should hold pressure even after the fuel pump switches off. It went up to about 30psi and then when the fuel pump kicked off the pressure dropped to zero. An auto parts guy told me it's probably the fuel pressure regulator. I bought one ($50) and then discovered this little device is located UNDER the fuel meter assembly (plenum). How in the world do you remove this plenum? I can get the injector electrical plug off and the fuel lines but how is the plenum supposed to come off? Do I have to remove the entire manifold? Also what are the chances that this could still just be a faulty fuel pump and if so why didn't the line hold pressure? Any advice would be appreciated. Just as a side note: when cranking the engine fuel sprays from the intake line at a fairly high rate.

MT-2500
09-09-2007, 10:09 AM
Parts place people are just want to be mechanics.:grinyes:
Let what they tell you go in one ear and out the other.
Proper testing is the way to go.
Saves a lot of throwing parts at it.

One test is to block off or pinch off the return line or the only true test is to check pressure direct at the fuel line going into the fuel filter.
And do not forget to check that fuel filter.

Here is some guide lines on fuel pressure testing.


Hard to start cold.
Check cold start fuel pressure.
Check engine running fuel pressure.
Check engine running on the road fuel pressure.
Then shut it off and watch for fast leak down.
Pinch off at rubber part or block off return line and check full pressure.
Later V6 and V8 gm engines should have 60-66 fuel pressure.
Cold start should be 64-65 lbs of pressure.
If you do not have full fuel pressure on a cold start the injectors will not squirt fuel.
Do not leave home without it.
Engine running should be 60-66 lbs pressure depending on engine load.
Full pressure with return pinched off should be 75-85 or more.
And if it has a fast leak down after shut off you have a leak.
Post back fuel pressure readings.
MT

skywalker45
09-09-2007, 10:44 AM
Thanks MT. Only problem is that the car won't run at all so doing a test with engine running is not possible. I will say it runs fine while spraying ether into the intake. In my opinion (I'm not sure about yours) the fuel line should hold pressure after turning the key to on and allowing the pump to pressurize the system. The drop to zero psi after this initial pressurization tells me that either:

1. There could be a leak somewhere (I did change the fuel filter) but I don't see any leaking fuel.

2. The pressure regulator attached to the fuel meter body is bad.

3. The fuel pump is able to initially pressurize the system but then somehow fails either through backflow through the pump if it's faulty.

I'm going to try to take some more measurements today. Right now the battery is low from trying to start it so once that's charged I'll try this again.

In your opinion do you think a bad pressure regulator at the injector module would cause the pressure to drop to zero? It seems to me that if this device is causing a low pressure situation then the poppet valves will not open. Since I already purchased the part there's no way I'm not going to replace it. I'm almost $300 into this repair and I'm wanting to pull out my hair. Thanks for all your consideration.

MT-2500
09-09-2007, 12:08 PM
Thanks MT. Only problem is that the car won't run at all so doing a test with engine running is not possible. I will say it runs fine while spraying ether into the intake. In my opinion (I'm not sure about yours) the fuel line should hold pressure after turning the key to on and allowing the pump to pressurize the system. The drop to zero psi after this initial pressurization tells me that either:

1. There could be a leak somewhere (I did change the fuel filter) but I don't see any leaking fuel.

2. The pressure regulator attached to the fuel meter body is bad.

3. The fuel pump is able to initially pressurize the system but then somehow fails either through backflow through the pump if it's faulty.

I'm going to try to take some more measurements today. Right now the battery is low from trying to start it so once that's charged I'll try this again.

In your opinion do you think a bad pressure regulator at the injector module would cause the pressure to drop to zero? It seems to me that if this device is causing a low pressure situation then the poppet valves will not open. Since I already purchased the part there's no way I'm not going to replace it. I'm almost $300 into this repair and I'm wanting to pull out my hair. Thanks for all your consideration.

If engine will not run you can hook up a 12 volt fused jumper to the fuel pump power gray wire for pressure testing.

Pressure can leak of several places.
But first make sure the fuel pump is capable of putting out full pressure of 75-90 lbs of fuel pressure.

Blocking off return may help test it but the true test is the direct pressure from fuel pump while it is running.
Unplug the in line to fuel filter and test direct pressure there.
But do not run fuel pump at full pressure for longer than a fast test.
Mt

skywalker45
09-09-2007, 09:31 PM
OK there is no doubt that my fuel pump is bad. It is only putting out around 40psi, and the drop in pressure after intial charging is due to a bad check valve in the fuel pump allowing pressure to fall. Should be in the neighborhood of 60-66psi and hold there until the engine starts. I removed the upper fuel meter plenum anyway and changed the regulator, cleaned the spider body, etc. Now there is a lot of build up in the intake -- carbon, grease, etc. Before I put this back together I want to clean the intake. What do you recommend to clean it? I've blocked all the intake ports with rags to keep things from falling into the intake. Any suggestion on a good carbon cleaner and degreaser for this job? Thanks.

MT-2500
09-10-2007, 10:14 AM
Carb cleaner cuts carbon.
But whatever you clean or work on make sure you clean it good.
Sometime it is better not to break it lose.
But if you do get it all off.
Good Luck
MT

skywalker45
09-10-2007, 09:44 PM
Are there any tutorials here on fuel tank removal for this vehicle?

skywalker45
09-11-2007, 07:58 AM
Nevermind :D I just found a decent tutorial in the tech help section. Thanks for all your assistance.

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