Rapid engine speed change

[Peder]

I've searched, and could not find anything. 99 Grand Prix GT.

What happens is my engine speed changes very rapidly up and down about 500 RPM, usually around 1500-2000 RPM/45-55 MPH. My guess is that it's a vacuum leak somewhere. It happens randomly, and keeps doing it unless I give it much gas, or let off the pedal and get back on it. I can tell when it's about to happen, as the gas pedal gets really stiff. This occurs 2 or 3 times during a 10 mile trip, and sometimes it won't happen at all.

No unusual smells or anything, It's not overheating either. I've pulled the cruise control fuse, it still does it though. May also be linked to a tranny problem. Occasionally, maybe once or twice a week or so, the car will shake a little when it shifts into 2nd. All stock engine. Thanks in advance for any help!

[BNaylor]

The rpm range where your problem is occurring is typically when the torque convertor (TCC) will lock/unlock when in Drive. This effect is called "chuggle". It can be caused by tranny issues such as a TCC-PWM solenoid or by engine isssues such as spark plugs, wires or possibly throttle position sensor (TPS).

Start with the simple stuff like spark plugs and wires. I'd recommend getting some STP throttle body spray cleaner and clean the throttle body.

GM has a Technical Service Bulletin that states the TPS could be the cause of "chuggle or fishbite".

You could get it scanned with a trans capable scanner for any possible tranny issues and see if there are any stored DTC error codes. Many do not give an SES light.

[Peder]

Thanks for the help. I thought about the torque converter slipping, and I was actually thinking about cleaning the TPS when it gets a little warmer outside. I'll give it a shot tomorrow or Monday, and report back.

[maxwedge]

Can't clean tps, sealed unit.

[BNaylor]

Originally Posted by maxwedge

Can't clean tps, sealed unit.

Yeah, he'll find that out Max. But I would clean the throttle body with STP throttle body spray cleaner....not while engine is running of course.

The only other thing that can be done to verify TPS is to check the 5v reference signal from the PCM to TPS and then output of TPS back to PCM. Idle will show around .4 volts and WOT around 4.0 volts. The TPS signal should be consistent with the throttle opening position and linear as rpms are increased. No dips or erratic readings.

In the GM TSB even a Tech II may not detect a flaky TPS and if suspected replacement is the only way to tell for sure.

[Peder]

Oh ok. Thanks for that. I'll see what I can do about the throttle body.

-David