Okay I have a question about my Cobalt 2.4L Automatic? Why is it that when I am cruizing at 30-50mph that I see my RPMs bouncing 500-1000RPMs? It just started doing this lately. So far the only thing that I have done to the car is put on a Injen CAI, Vibrant Header/Downpipe, Magnaflow Exhaust, and a MRZ Underdrive Pulley. Vince from Trifecta has tuned the car and the RPMs is still bouncing. Me and him are trying to figure this thing out... I have cleaned the throttle body, put Seafoam in gas tank, put Seafoam through intake system to clean motor, and the stupid car RPMs are still bouncing. Anybody got any ideas?

 

If your rpm is going up and down then the car should be speeding up and slowing down is it ?(Surging constantly)
If not then I would say its something to do with your torque converter .
There is a connector on it to lock the converter you could try unhooking it and see if the problem goes away.

 

I wouldn't say it is speeding up or down. It's like reving the motor while riding down the road. It only does it at certain speeds though. I can be running 60-70mph and it wont do it. It usually does it worse at 30-50mph. While cruizing it will rev quickly but come down quickly constantly, so that I am not actually speeding up or down. It gets me ticked off because I have a Magnaflow exhaust and people are thinking I am wanting to race because of the RPM fluctuating, making the exhaust sound change.

 

no expert by anymeans but i bet it has something to do with that underdrive pulley. . . .