I have a 98 3.9 automatic. I've been searching for answers on why my truck will start fine, then run rough for about a minute, then fine again. Only seems to do it when it cools off a bit. After it goes through the rough spell it will run fine. Just like most of the other posts, I changed the plugs, wires, cap and rotor. Seems like this common problem is linked to the pre-cat O2 sensor. At least thats what I am reading......well, I can only find one O2 sensor on my truck. It's between the cat and muffler. I just bought this truck a week ago and still trying to work out all the bugs. Is it supposed to have (2) O2 sensors? If so.....where the heck is the other one if all I can find is the one between the Cat and muffler. If somebody removed it...what can I do the fix the problem short of replacing that section of pipe?

 

I have the truck, 98, 3.9L and there is another O2 sensor. Look at where the pipes come together from the exhaust manifold. Where they meet a few more inches down the line and the O2 sensor sits there. Problem is when under the car it's hidden cause it's on the top of the pipe in such a way it's almost invisible. Look good in that area and you will find it. I missed it the first time, had my son crawl under the car and he found it....good Luck!

 

I put it on ramps and now my old eyeys found it......what a relief!

 

ditched the idea of the coolant sensor and decided to try the upstream O2 sensor and it worked.....for now! ran I am crossing my fingers and hoping I'm almost done......until the next thing! LOL I wont know for sure until I get a chance to drive daily though....waiting on a drivers door window regulator. That was the other thing it needs attention on.....window was jammed into the bent up stripped regulator when I got it. $20-$40 part and they couldnt fix it! Thanks for the ideas!