I have a 318 with 3.55's. recently I put a high flow cat and flowmaster muffler with a 3 inch pipe on the truck. Now crusing at 55MPH or higher, with O/D on, the RMP's will be at 1500 - 1800 (55 - 70MPH); however, half of the time the RPM's move to 1700 - 2000 (55 - 70MPH) with O/D on and on flat ground.

Any idea's? Thanks.

 

i have an 01 318 w/ 3.55 and 285/75/16 and at 55mph with OD i turn exactly 1500 rpm. if you rpm's are varying by 200-300 it sounds like your torque converter is not always locking up like it should. if you haven't serviced the transmission recently you should do that and hope it fixes it. just change fluid and filter and leave the bands alone. did this start immediately after muffler/cat, or could it have been there for awhile ?

 

Before you start pulling your tranny apart, clean and/or replace your throttle position sensor. This is a fairly common problem where the torque converter recieves a "noisy" signal from the TPS and locks and unlocks eradically.

 

Yes, in the past 300 miles. I had crusie control on and it did it worse than when I controlled the pedal.

 

if you haven't replaced tps, do that. about $35-40 at your local auto parts.