Transmission

99 Grand Prix.

Everything runs fine. Starts, stops great. Runs good 90% of time with the exception of the following.

Once warmed up. Usually cruising above 40MPH in high gear or 4th gear or overdrive - whatever it is called.

Sometimes, going up a slight incline, the car has a constant, repeated, hick-up - skip, shudder, buck, or whatever it is called. Then it will disappear.

On one hill/incline while this was happening, the transmission shifted into a lower gear and all disappeared. The car ran fine.

This skipping, shuddering, happens every day when car is warmed up. Then may disappear and run perfectly the rest of the day.

Is this a transmission issue or some "module" or "code"?

The fluid has been changed and one additive has been added to fluid.

77,000 little old lady miles. No accidents ever.

This can be caused by an ignition misfire, a bad tps on rare occasions or the TCC slipping. You should scan it for misfire codes, how old are the plugs and wires?

Well, took car to Chevrolet Mr. Goodwrench. They wanted to replace everything from the spark plug, spark plug wires, O2 sensors (2), and more plus probably replacing the transmission for up to $3,000.00. But they are not sure. Hmmm . . .

Later, I took it to my local mechanic and he diagnosed it as a bad transmission. He spent about 15 minutes checking and rechecking all the codes. And no, he is not suggesting replacing all the spark plugs etc. Hmm.

Only a full scan for misfire counts and trans slippage/tcc performance will reveal the source of this problem, careful, what you do here.

Your post has been moved from Pontiac Non-Specific to Grand Prix.

what was his reasoning behind the bad transmission diagnosis? A bad PCS can cause a "chuggle" condition

99 Grand Prix.

Everything runs fine. Starts, stops great. Runs good 90% of time with the exception of the following.

Once warmed up. Usually cruising above 40MPH in high gear or 4th gear or overdrive - whatever it is called.

Sometimes, going up a slight incline, the car has a constant, repeated, hick-up - skip, shudder, buck, or whatever it is called. Then it will disappear.

On one hill/incline while this was happening, the transmission shifted into a lower gear and all disappeared. The car ran fine.

This skipping, shuddering, happens every day when car is warmed up. Then may disappear and run perfectly the rest of the day.

Is this a transmission issue or some "module" or "code"?

The fluid has been changed and one additive has been added to fluid.

77,000 little old lady miles. No accidents ever. I would replace the spark plug wires. Usually, a high speed miss is caused by the electricity that is supposed to go to the plugs, jumping through the spark plug wire insulation to ground. This is a cheap fix. The tranny, if you changed both the fluid and filter, you have taken care of.
I would also change the spark plugs while I was at it.

I had the same problem with mine. I bought new sparkplugs and wires from the dealer. The wires were expensive. I put the sparkplugs in and the car seemed to do better. I am cheap so I took the wires back. The next day it was doing it but not as bad. I went back and bought the wires again and put them in and have not had any problems since. (The counter man told me the first time that it was probably the wires but I am hard headed).

Guys look at the date of the last post, this is old and has had no responses here, to continue please start a new thread, Thanks.