Hi all. My mechanic and I are getting stumped on this one.
I have a 2001 gmc jimmy, 4.3L 4x4 SLE with 72,000 miles.
Whats happening is,, while driving between 42 and 60 mph and keeping your foot steady on gas or putting it in cruise control,, the rpm'* keep fluctuating up and down a couple hundred rpms,, it feels like the car doesnt know what gear it wants to be in. It stops when you go to accelerate or take your foot off the gas,, but once you get steady again, its starts again. Once I am in overdrive or over 60+ mph, it does not do it. From starting at a dead stop, it shifts and accelerates with no problem,, but once your over 40-42 mph and steady your speed, it begins again.
I changed the tires from 235-70-r15 to 235-75-r15, so we figured it was throwing off the computer, because i did not notice this problem before the tire change,, so he went in and changed the tire size in the computer. No luck, still doing it.
Has anyone had this issue or have any thoughts,, any help would be greatly appreaciated.