When coasting at say 60mph my rpm's go up and down between say 1600 and 1800 rpm..It goes up and down at a steady even pace but idles perfectly..Any ideas on my humping xj?BTW it's a 1991 larado 130kmiles with auto aw4..

 

1 st. the term is HUNTING

2 nd. either your torque convertor is bad or your output speed sensor is shorted to ground.

 

Where is this output speed sensor located?Tail shaft perhaps?

 

Are you sure? Going up and down sounds like humping to me...

 

Or the T/C is locking and unlocking. Next time it happens use your left foot to lift the brake pedal and see if the surging goes away.

 

Will try that FrankZ..How a bout a vacume leak?It idles just fine tho..Humm?

 

When my 97 had this problem is was a misadjusted brakelight switch. The torque converter lock circuit goes through the switch. Step on the brakes and the T/C unlocks.