91 Explorer Engine Shake when switching to overdrive

[condo0000]

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

[Opera House]

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.

[srercrcr]

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