O2's just lose sensitivity over time they are part of the tune up of an efi engine.

I would expect a mileage drop for a tank or two after resetting the computer as it relearns the engine. The 4.0 doesn't get good mileage and your mood effects as much as 30 percent of your mileage.

 

When I changed my spark plugs at around 70K miles, I used the Autolite Double Platinums. I, as you, did found the original plugs with a huge gap. The truck was running fine but decided to check one plug to see how it was doing. That told me to change the plugs right away.

I accidentally gapped the new plugs tighter than spec. I think it was around 0.059. I was concerned after I figured out it was not to spec but I was too lazy to go back and re-gap them. The truck ran great with no issues and I don't recall any change in gas mileage. The mileage is near 120K now and the truck is still running great. The plugs are probably worn enough to be in spec now but I will be pulling one of the plugs soon to see what it looks like.

I changed the spark plug wires at around 100K miles. When I took it out for a test drive it suddenly started misfiring under hard acceleration. When I got it back home to check out things I found one of the spark plug wires was not seated all the way on the plug. Apparently I did not get it all the way seated and it vibrated loose. That solved the problem and all is well. You should consider changing the spark plug wires. They can cause misfire issues if they are not in good shape.

I think the 4.0 SOHC engine is a great engine and plan to use it for at least 300K miles. It has plenty of power for my needs, which includes towing my race car all over the country.

 

ok guys good news. i regapped the plugs to .056 and made sure all plug wires and stuff are set. everything is back to normal, no jolting were it was before, idle perfect, engine sounds just as beefy as it was before, i did a number of hills, steep and not, plug flat road and i didnt feel any jolting. my only conclusion is to stay within the .052-.058 range thats in the FACTORY MANUAL, NOT THE RADIATOR EMISSION STICKER! found that out the hard way. .062 gap is WAY too big.

ill let u guys know after a couple tanks of gas what the mileage situation is. i will also look into the o2 sensors as i will prolly have to do those soon anyway. but throughout this whole thing, not one CEL light. hope this helps someone.