I've got a 99 XLT 4x4 with the 4.0 (OHV, not OHC) and the 5 spd auto trans.
About 2000 miles I had an O2 sensor replaced as a result of a check engine light (I don't know if this matters, although I thought I'd mention it).
Just the other morning while cruising at a steady speed with just enough accelerator to maintain speeds the truck started "bucking". It reminds me of an old Buick I had that would buck at the top end of the 1/4 mile when the carb bolts backed off a bit (vacuum leak).
I don't notice it if I give it a little extra throttle or if I'm in a coast - only under a light throttle. The problem is at it's worst when the car isn't fully warmed up. It still exists warm, but it doesn't seem as severe.
The car also "seems" a little down on power, but that could just me thinking that it is. I did run it on a dragstrip about 1000 miles ago and it ran as quick as it had ever gone.
It has 77,000 miles on it.
One thing that may or may not be related is I started noticing a sort of hum or buzz type of a sound that is definitely coming from somewhere in the intake. The sound echoed through the air filter box, then I took it off and could hear that it was coming through the intake tube. This sound happens after the car is warm and when the car is under no throttle (stop sign/light, etc...). With a little bit of gas the sound goes away, but would come back in a situation where I was coasting to a light. Someone joked that it sounded like I put a supercharger on it from the outside.
I don't know if they're related or not, but has anyone running a 4.0 OHV motor had anything like this?
I cleaned the throttle body and the idle air bypass solenoid, but that had no effect - they were actually quite clean for the miles on the car w/o having done this.
Thanks,
Steve (remove the gs455 for email)