I have a '95 Blazer 4x4 with 114,000 miles. My problem is when I'm driving at 50mph and above in Overdrive (4rd gear). The engine sounds like it is hesitating or reving up and down. I'm staying an a constant speed when this happens. Also it doesn't happen when I have it in Drive (3rd gear). Any ideas on what could be causing this?

One more question. I'm installing a tranny cooler and need to know which line is the oil return line off the radiator?

Thanks for you help.

 

Return line is the top line.

On the other problem, it sounds as if your torque converter is locking and unlocking... There have been many posts on this issue, most of which point to a faulty valve in the valve body inside the transmission. There are updated valve bodies available, but they are not cheap.