91 Explorer Engine Shake when switching to overdrive

Hi,

I just replaced the wires, spark plugs and EGR valve on the car. Originally, the idel was very rough, and the car seemed to lack horsepower. The engine would "shake" when I stepped on the accelerator. Now, the problem seems to have gone EXCEPT for when I get on the highway and it shifts into overdrive. As soon as the overdrive kicks in the engine "lugs" and shakes/vibrates. I also seems to loose horsepower at the same time.

Any ideas as to what may be causing this??

Thanks for your help,

Craig

Overdrive causes the output shaft to turn 33% faster than the motor is. This puts a very heavy load on the engine. Any engine problems will become very obvious in overdrive. It is likely that you still have unresolved engine problems.

Turn off the overdrive and drive it to high speed.
If no problems, it could be the lock-up torque converter.