1993 4.3 L Misfire and Dieseling

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Pete1949

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I have had the dealership install new plugs, rotor, cap, wires, ignition module, coil, map sensor for tune up. However i have a misfire or backfire periodically and some times a dieseling issue when running as apposed to when you turn off the ignition. The dealership has checked the timing and injectors. They have also run the scanner with no faults.
 

Urambo Tauro

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Welcome to the forums!
...some times a dieseling issue when running as apposed to when you turn off the ignition.
Not sure what you mean by that? Dieseling is when the engine keeps firing without spark, allowing the engine to keep running after you've killed the ignition. You're saying that the engine still runs while the key is in the RUN position? That's... supposed to be a good thing.

What are the conditions under which it misfires/backfires? Idling? Driving? Accelerating? Coasting?
 

Pete1949

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Welcome to the forums!

Not sure what you mean by that? Dieseling is when the engine keeps firing without spark, allowing the engine to keep running after you've killed the ignition. You're saying that the engine still runs while the key is in the RUN position? That's... supposed to be a good thing.

What are the conditions under which it misfires/backfires? Idling? Driving? Accelerating? Coasting?

Thanks for your response. The engine when taking off from a stop or some times if you let off the gas and start to accelerate it wants to diesel. Not every time,it is occasional, same with the misfiring. dealership had it for three days and run 120 miles with out an issue. after i picked it up and drove 8 miles it back fired about 4 times then quit. It will back fire while driving,nothing at idle, coasting or accelerating. my experience for dieseling was when the ignition was turned off and the engine still wanted to run.
 

Urambo Tauro

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Hm, that doesn't make sense. Are you shutting off the ignition while driving? Why? And how long does it diesel for?

Also, that dealership test-driving it for 120 miles sounds fishy to me.
 

Pete1949

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Hm, that doesn't make sense. Are you shutting off the ignition while driving? Why? And how long does it diesel for?

Also, that dealership test-driving it for 120 miles sounds fishy to me.

Thanks for the reply. No the dieseling i have noticed two times was brief and one time for maybe 5 to 10 sec with the ignition on. it occurred once from a take off and another idling down and one time just briefly while accelerating.
 

Urambo Tauro

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Dieseling is not a thing that happens with the ignition on. Even when a vehicle becomes susceptible to dieseling, it only happens after the ignition has been shut off. I think you're using the word to describe something else.

As for the backfire, well that's a somewhat tricky term. It's sometimes called afterfire if it takes place in the exhaust. When you hear that loud pop/bang, does it sound like it's coming from under the cab, or inside the engine compartment?

And does it make any difference if the engine is cold vs when it's at operating temperature? (This would probably also be a good time to ask- It's not overheating at all, is it?)
 
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Pete1949

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Dieseling is not a thing that happens with the ignition on. Even when a vehicle becomes susceptible to dieseling, it only happens after the ignition has been shut off. I think you're using the word to describe something else.

As for the backfire, well that's a somewhat trickier term. It's sometimes called afterfire if it takes place in the exhaust. When you hear that loud pop/bang, does it sound like it's coming from under the cab, or inside the engine compartment?

And does it make any difference if the engine is cold vs when it's at operating temperature? (This would probably also be a good time to ask- It's not overheating at all, is it?)


Thanks for the reply. I'm using the term dieseling as a ( sound ) that i remember from older engines when the timing was off and you shut the ignition off and the engine would try to still run. Only this makes that sound with the ignition on while driving. The after fire you are describing also happens sometimes from taking off from a stop or just driving down the road at 45 mph but will stop. Generally at operating temperature. Have never experienced it while cold.
It is not overheating. I just had the water pump, radiator, and hoses replaced
 

Pete1949

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Thanks for the reply. I'm using the term dieseling as a ( sound ) that i remember from older engines when the timing was off and you shut the ignition off and the engine would try to still run. Only this makes that sound with the ignition on while driving. The after fire you are describing also happens sometimes from taking off from a stop or just driving down the road at 45 mph but will stop. Generally at operating temperature. Have never experienced it while cold.
It is not overheating. I just had the water pump, radiator, and hoses replaced
It sounds to me like it is under the hood.
 

Urambo Tauro

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Okay. Assuming that all those new parts are good and installed properly, all gapped/adjusted/timed as appropriate (I should expect nothing less from a dealership), I think the next thing that I would check would be the EGR. Do you have any stored codes?
 

Pete1949

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Okay. Assuming that all those new parts are good and installed properly, all gapped/adjusted/timed as appropriate (I should expect nothing less from a dealership), I think the next thing that I would check would be the EGR. Do you have any stored codes?

Thanks for the info. It was put on a scanner and showed no fault codes.
 
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