1989 GMC C1500 Fuel or Ignition problem

MARADAMX3

MEMBER
1989 GMC C1500

Electrical problem
1989 GMC C1500 V8 Two Wheel Drive Manual

Once the engine warms up, at certain rpms and throttle positions the engine will spit, sputter, pop, and run rough. The engine seems to "clear" out once the rpms get to a range above 3-4000. This happens in every gear. However, if the throttle is easily applied and the engine is allowed to accelerate slowly, the spitting and sputtering does not occur. This issue does not happen every time the vehicle is driven, however it has been happening more lately. I have changed the spark plugs, wires, fuel filter, and added fuel conditioner. The engine idles nice and smooth and once set at the desired spped it runs nice and smooth - the condition only occurs during acceleration.
Thanks!

5 Replies

RASMATAZ

MECHANIC
75,992 POSTS

Hi maradamx3, Welcome to 2carpros and TY for the donation

Check and test the following

Oxygen sensor
Dirty fuel injectors (cleaning the injectors often fixes this).
Bad MAP (manifold absolute pressure) sensor
Bad TPS (throttle position) sensor
Bad or dirty MAF (mass airflow) sensor
Low fuel pressure (leaky fuel pressure regulator or weak fuel pump)
Vacuum leaks (intake manifold, vacuum hoses, throttle body, EGR valve)
Bad gasoline (fuel contaminated with water or too much alcohol)

Sometimes, what feels like a hesitation is actually ignition misfire rather than lean misfire. The causes of ignition misfire may include:

Dirty or worn spark plugs
Bad plug wires
Weak ignition coil
Wet plug wires
Cap and Rotor

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Tuesday, January 19th, 2010 AT 3:26 PM

MARADAMX3

MEMBER
6 POSTS

I was hoping to get a more specific response to my issue, not the same response that has been given for several other questions. Here is some more information.

No codes are detected. This problem is intermittent. Yesterday, the problem occurred in the morning and afternoon, but not the evening. It seems to be random. The truck will pop/backfire through the TBI. Being that the issue is intermittent, most of the causes stated above are not applicable.
Can this issue be reduced to just a couple of really possible causes?
Thank you!

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Wednesday, January 20th, 2010 AT 11:31 AM

RASMATAZ

MECHANIC
75,992 POSTS

What I gave earlier is my standard approach of your problem and needs to be check and tested any one of them will cause your problem.

I'm hinting your wanting a direct fire answer-

Check the ignition timing, test the throttle position and manifold absolute pressure sensors-and the fuel pressure and get back with some testing results so that I can continue to assist

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Wednesday, January 20th, 2010 AT 3:32 PM

MARADAMX3

MEMBER
6 POSTS

OK. Checked the timing - 1 degree BTDC. Checked the fuel pressure - 12 psi. Again, no codes when scanner is hooked up.

Would the fuel pump work intermittently? Seems like it would either pump, or not. Same with a coil. And the cat - it would either be plugged or not, right? Wouldn't most of these parts be black and white - either they are working or aren't? Just asking.

Something I noticed is that the rough running seems to be more prevalent when the humidity is high, or when the air is heavy. In the morning, runs rough. Afternoon with sun, no clouds and dry - runs great.

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Tuesday, January 26th, 2010 AT 4:15 PM

RASMATAZ

MECHANIC
75,992 POSTS

An intermittent problem are the worst one to deal with it has to be duplicated to chase it down, the dealership are afraid of this type of problem especially there's no DTC's

It seems that you are concentrating on the fuel pump and coil there others involved in this problem as I've mentioned earlier you need to check and test all of them and comeback with the testing results for further evaluation