GMC Jimmy service engine soon light

[GlennVnaz]

1998 GMC Jimmy 4x4, 4.3L, 113000 miles

After driving the vehicle on highway for aprox. 2 hours the engine begins to misfire and stutter and the temperature gauge drops to zero and then bounces around. After a while the service engine light goes on and misfire worsens.
Problem goes away after vehicle rests for a while.
Problem does not occur in stop and go traffic or when traveling on winding road with intermittent acceleration.
Problem only occurs when acceleration is constant for about 2 hours.

One mechanic said it was the fuel pump. New fuel pump & filter was installed, $750 later and the problem still exists.

I pulled a plug and it is very black and burnt. It appears to be running too rich.

[Meccboy]

Not sure how the temp gauge is related to this, other than a possible PCM problem.
The fact that it may be running rich could be an injector problem, or the fuel pressure regulator, but wont take 2 hours to cause a problem.
If the coolant were low, this would cause the gauge to show high then go crazy. This may also explain the way its runs- Because if an internal coolant leak from the intake manifold or a headgasket, it could cause the plugs to foul out.
The key is to have the codes scanned.
It may show more than just misfires. It may show codes for other systems, or a bunch of codes- which may indicate a PCM problem, or a wiring harnes problem.

[gwcchatt]

You may have a simple problem, have you checked your "ground" wires, the one from the battery to the motor? and the one from the body to the motor? Sounds like they may not be making a good ground? Just take them off and grind a "shiny" spot then bolt back up? if it is this, that should "solve" the mystery?

This also could cause the "black" plugs, with the coil not firing properly?
Just a suggestion? The service light could be because of misfire that occurs when the coil stops firing?
gwcchatt

[butchkaz]

Get the codes scanned. We need to know if there is more than just engine misfire codes. If there are other trouble codes, that may help diagnose the different problems you are having. The ground wire is a good idea to check. Also may be a bad ignition switch.