So I am at a bit of a loss here. Have a sprintex 3.8 JK which has in the last few weeks developed a fault where it turns on its MIL with the code above. It ONLY happens at extremely light throttle. Like when just touching the pedal while coasting down hill. Under load and WOT no missfire.

No symptoms ever but on the trinity I can see the miss fire count climbing and can reproduce it at will but as I said only just off idle.

Have been using octane booster which may have nailed the plugs (going to change them anyway) but I have never had a missfire condition that did not get worse under load.

Any ideas?

 

In case it helps someone else I can report that I found this. I had the jeep on a dyno a while back and they changed the plugs for new stock plugs. When they did this they set the gap way smaller than factory. Seems that this means that the small & puny spark is unable to reliably ignite the mixture at very small throttle opening. I tested and proved this theory by swapping out 5 of the 6 and then datalogged misfire counts for each cylinder with the Diablosport trinity. Guess which one still misfired. Swapped out the last plug and the problem is gone.