I have read dozens of threads trying to glean some info but at this point noting seems to fit exactly so here is the situation. 1998 Jeep XJ I6 running fine. I decided to change out the air filter, spark plugs, wires, distributor cap, and rotor with the intent of getting a little better than 17 MPG and I had no idea when it was last done. The plugs I removed were Autolite Copper Core 985's and were gapped to about 0.020 oddly enough. I replaced them with the exact same plug gapped to 0.035 per factory specs. The plug wires, distributor cap, and rotor looked good but I replaced them just because I already had new parts. So after doing all that I fired it back up.

Now. Rough idle with a mild backfire occasionally (every 10 - 15 seconds). Check engine light would come on likely indicating air/fuel mixture not right. I verified all the plug wires, connectors, and everything in the area I was working and all look ok. Still same issue. So I changed back to all the old parts and still the same issue. I even confirmed spark on every cylinder and the engine definitely runs worse when each plug is pulled. The real problem is, since it is not throwing any codes, I am at a loss what to do next. I have driven it a few miles and it is a dog, basically in limp mode. What I can't figure out is why this happened after the tune-up? And why it continues when everything has been put back to its original state. Which leads me to believe it is a sensor but which one? I can't justify justify replacing every sensor because it might be the problem. All I have is a basic OBD2 scanner which really doesn't tell me much other than trouble codes. And the check engine light does not come on anymore, even when driving it on the road. I did have to pull the battery for a few hours, could the computer have reset and allow this issue to become apparent where it was masking it previously?

As I said, I want to diagnose the issue, not just start replacing sensors hoping the problem is fixed. I am just not an engine guy and the cost of gas driving my TJ is killing me... TIA.

 

Resolved. Plug wire order was wrong,.. 1 and 2 reversed.