Rough Idle

[glundblad]

I have a '87 Comache Chief 4.0, 5sp 4wd. I was the original owner and sold it to my dad in '95. He put 10k miles on it in 10 years. It now has 83k miles on it. I have put a lot of money getting the body professionally restored back to stock condition. But I cannot seem to fix the rough idle problem. It has a new fuel pump, fuel filter, plugs, wires, rotor and cap idle speed stepper motor. It runs fine down the road. I had the injectors cleaned professionally. Today I am going to replace the coil. If the coil doesnt solve the problem, does anyone have any ideas?

[glundblad]

I have a '87 Comache Chief 4.0, 5sp 4wd. I was the original owner and sold it to my dad in '95. He put 10k miles on it in 10 years. It now has 83k miles on it. I have put a lot of money getting the body professionally restored back to stock condition. But I cannot seem to fix the rough idle problem. It has a new fuel pump, fuel filter, plugs, wires, rotor and cap idle speed stepper motor. It runs fine down the road. I had the injectors cleaned professionally. Today I am going to replace the coil. If the coil doesnt solve the problem, does anyone have any ideas?

[BLHTAZ]

Two things come to mind right off for me...

-I had a similar problem with mine when I first bought it and it turned out to be a broken valve spring. You may want to pull the valve cover and just have a look.

 

-The ECU's VERY rarely ever go bad in these trucks, but when they do it's typically in the injector circuit and it effects the #4 injector. Try unplugging the injectors one by one while it's running and see if you find one that does NOT make or make little difference in the idle. I know you said you had the injectors cleaned professionally, but were they flow tested as well when cleaned to be sure that they are actually operating properly?

[BLHTAZ]

Two things come to mind right off for me...

-I had a similar problem with mine when I first bought it and it turned out to be a broken valve spring. You may want to pull the valve cover and just have a look.

 

-The ECU's VERY rarely ever go bad in these trucks, but when they do it's typically in the injector circuit and it effects the #4 injector. Try unplugging the injectors one by one while it's running and see if you find one that does NOT make or make little difference in the idle. I know you said you had the injectors cleaned professionally, but were they flow tested as well when cleaned to be sure that they are actually operating properly?

[glundblad]

Thanks, I will give that a try. I pulled plug wires and were getting sparks on all of them. Some didnt impact the engine idle when disconnected. You may be on to something. I thought about getting a new ECU.

[glundblad]

Thanks, I will give that a try. I pulled plug wires and were getting sparks on all of them. Some didnt impact the engine idle when disconnected. You may be on to something. I thought about getting a new ECU.

[BLHTAZ]

If you pulled a plug wire that had no impact, but it has spark, then there may be other internal issues with that cylinder as well. Do the injector test and see if there is no response on the same cylinder. If so...do a compression or have one done if you can't do it.

[BLHTAZ]

If you pulled a plug wire that had no impact, but it has spark, then there may be other internal issues with that cylinder as well. Do the injector test and see if there is no response on the same cylinder. If so...do a compression or have one done if you can't do it.