I have a 1992 Pontiac with 3.8l engine coupled with auto transmission 4T60E or H (I think). The vehicle has about 165,000 km and original transmission and engine. When driving (in D not overdrive) the vehicle at about 40-50 mph, I can feel the transmission switch to a final gear or something is switching within the tranny, there is a jerk sensation feel or shudder. The engine RPM at this speed is about 1600 RPM If I press on the gas slightly, it will shudder will continue but more consistence until I press hard enough that it will switch to a diff gear then the problem will go away until it drop back into the same speed or engine rpm. The problem is more obvious when the engine and tranny is at its operating temperature (about 30 min drive max). When the weather is cold (0F or colder) outside, I can not feel this. Perhaps I will when it get hot enough since my driving distance is usually less than 30 min. If I drive fast enough (70 to 75 mph) it will work fine regardless if the engine is hot or cold.
I have new spark plug wires and plug. Injectors has been cleaned by GM. New oils in tranny. Vehicle drive fine other than the problem above. I also have recently installed new tranny oil cooler. All these were done in attempt to trouble shooting this problem but does not help. I am thinking about replacing vacuum modulator but the more I read on about this, I think it is not is.
Any suggestion or comment is greatly appreciated
Paul