Bad idle after replacing battery

91710zx10 · December 9, 2014

2000 GMC with 6.0 with 135k miles. Replaced the battery recently and thats when the problems started. It ran perfect before the new battery. Yes I've searched and learned what my problem could be which is the computer reset and has to learn new idle I've ran 2 can of seafoam in gas tank one with full tank and second can in there now with only 1/8 tank of gas. Also seafoamed the throttle body with entire can of seafoam. Also pulled out MAF and cleaned with MAF cleaner. Pulled intake tubeing off and sprayed and cleaned throttle body best I could with tooth brush and wiping clean with rags and yes they were coming out black. (I've since learned how to remove the throttle body and will do that next and super clean it) I am running a K&N filter which I've also cleaned and oiled when I cleaned MAF. New Plugs and fuel filter done at 100k miles. So after all the cleaning it idles good now, BUT there is slight struggle once you give it gas which was never there before. Also had a flashing check engine light come on for about 1 minute then turned off. Now that things are clean should I disconnect the battery again for a few hours then drive around again? This truck is not my daily driver but I can drive it more if thats whats needed to help computer re-learn. What am I missing or doing wrong?

8.1HD/ALLISON · December 10, 2014

P0302 is cylinder 2 misfire. Try swapping plugs, wires, and coils with another cylinder, one at a time, and see if the code changes to the other cylinder.

91710zx10 · December 10, 2014

P0302 is cylinder 2 misfire. Try swapping plugs, wires, and coils with another cylinder, one at a time, and see if the code changes to the other cylinder.

Yeah I just looked up the code thanks. But the code only showed once and flashed a few times then dissappeared. I will keep my eye on the codes. But I just now took of throttle body and cleaned it out real good all the while disconnected the battery. All buttoned up and I started it with the AC still on and it still idled rough BUT much less. So I will drive it around this whole week and see what happens. I'll keep you all posted and also look out for any codes as Im sure this will happen to others here.

8.1HD/ALLISON · December 9, 2014

What code did it store?

Doug_Scott · December 9, 2014

2000 GMC with 6.0 with 135k miles. Replaced the battery recently and thats when the problems started. It ran perfect before the new battery. Yes I've searched and learned what my problem could be which is the computer reset and has to learn new idle I've ran 2 can of seafoam in gas tank one with full tank and second can in there now with only 1/8 tank of gas. Also seafoamed the throttle body with entire can of seafoam. Also pulled out MAF and cleaned with MAF cleaner. Pulled intake tubeing off and sprayed and cleaned throttle body best I could with tooth brush and wiping clean with rags and yes they were coming out black. (I've since learned how to remove the throttle body and will do that next and super clean it) I am running a K&N filter which I've also cleaned and oiled when I cleaned MAF. New Plugs and fuel filter done at 100k miles. So after all the cleaning it idles good now, BUT there is slight struggle once you give it gas which was never there before. Also had a flashing check engine light come on for about 1 minute then turned off. Now that things are clean should I disconnect the battery again for a few hours then drive around again? This truck is not my daily driver but I can drive it more if thats whats needed to help computer re-learn. What am I missing or doing wrong?

It sounds like you are expecting too much too soon. It takes time for the computer to relearn, and running seafoam through it will make the computer learn with that fuel. Once that tank is gone, and you are back to straight fuel again, the computer will again have to learn that. Actually it continuously learns, but every time you make a change, it has to tune itself for the effects of that change. Try to stop making changes for a couple of tanks of fuel and see how it is.

91710zx10 · December 9, 2014

codes saved are P0302, P0302 pd

Filled up the tank with 29 gallons fresh gas and drove 15 miles. Same problem till I turned on the AC and WTF, Idle was terrible but weird it comes and goes and not when the compressor kicks in. Like I can drive for a few blocks then stop light and idles fine. Then next block or so and stop then the truck feels like the cylinders are misfiring and shaking the whole truck. How long am I supposed to let the computer re-learn? You suggest a few tanks of gas. I will try but I need this truck to tow my trailer to go camping this Xmas and want to take it to mechanic if it dont work itself out. Thanks for the suggestion. But Im going to unplug the battery again, clean out throttle body by taking it completely off and try this whole thing again.

Out of curiosity then, if its something else wrong, what could it be? I mean for it to run perfect until a battery change makes no sense.