So I should replace the O2 sensor first? Sometimes the truck is hard to start and the spark plugs have a little carbon build up. replaced them last weekend. And I would have thought that the check engine light would come on but i guess not? Thats why I did not think to change out the O2 sensor.

 

I suggest you do a search here on DF for Plenum .

 

I would test the O2 sensor first and the O2 sensor doesn't always make a light come on. Once the exhaust warms up it fools the computer into thinking that the sensor is working. I let my truck sit over night and used a scan tool to test it when it was cold. That is how I found it. You can back probe the sensor and test it that way but again I used a scanner. If you aren't comfortable doing this yourself, take it to a trusted mechanic. I am not saying for sure that it will fix your problem, I am just saying it fixed my symptoms. I am not sure on the plenum, that seems to be everyones answer to just about every symptom. You would definitely be wise to do research first, much cheaper than throwing parts at it. Let me know.

 

It's a lot of peoples suggestion(not everyone) to check the plenum, because it is the MOST common problem that at the very least, needs to be eliminated as such, first. Which needs to be done by removing the TB and looking around inside with a small mirror and a flashlight. Not by removing the air cleaner assembly and opening the butterflies..

 

I guess I am pretty fortunate because mine was completely dry of any oil. I asked a few mechanics and they hadn't really heard much about the 3.9's having this problem. I am just saying that if he is having identical symptoms that I would go with checking the sensor first. The front o2 sensor affects the air/fuel mixture and will cause all of the symptoms that I was having. I figured this out on my own, no offense to this site because it is great. I had 3 mechanics look at the truck and not one of them suggested the sensor as being the cause of the problem. They more or less had me chasing my tail and wasting a pile of time. I would start with the simple causes first and if 99gtp is comfortable taking the throttle body off as well, then more power to him.