Thread: 2005 Grand Prix hard shifts Lots of questions

Hi everyone, I just purchased my car a couple of weeks ago.

123,500 miles
Replaced rack
replaced drives side wheel bearing,
New Cooper CS3 tires.

The final thing I did and not sure if this caused any of my transmission issues
Flushed my Coolant system 4 times with Peak cleaner and distilled water.
Got it nice and clear and added the 50/50 mix
also replaced the thermostat.,, everything seems to run good gets great heat..

Now my problem
last night, I drive on this highway that is very crowded and I a have frequent stop and starts.
about 3/4 of the way home about 1 hour, only 23 miles though my transmission started shifting very hard.
the Dash Gauge was half way up,, most of the way it was half way between 1/4 and 1/2 slash mark but with all the stop/starting
it got to the 1/2 way point.

After I got home and let it cool down took out for a 10 minute drive and it shifted smooth perfect.
So now I am wondering what to do.

The transmission was flushed by original owner 2 months ago, Valvoline just a flush no pan pull or filter.
I see no signs of water pump no leaks noises nor does it feel loose.
I do not think there is any air in the system, gets good heat in the cabin, and I tried to purge any air from
the bolt on top of thermostat and got no bubbles
The gauge on dash has not gone beyond the 1/2 point

This morning same drive 23 highway miles the car ran and shifted fine. Morning commute has no stop/starting though.
I am concerned I do not want this to become so major issue by ignoring the hard shifting that occurred.

I talked with Danco Transmission on the phone. he said could be any number of things and that I should leave the
car with them for 2 to 3 hours to diagnose the problem. He mentioned something about the tranny going into limp mode causing
higher pressure which would make the car feel like it is shifting hard.

anyone have any thoughts on this.
I would appreciate any opinions and suggestions

thanks everyone

I doubt the coolant flush is related to your shifting issue.

I would check the transmission fluid level and make sure it's not low. It's possible the previous owner was having similar issues and decided to flush the transmission hoping that would help. Probably wouldn't hurt to drop the trans pan and do a fluid & filter change. You could throw in a transgo shift kit at the same time. It has been known to fix shifting problems and extend transmission life.

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coolant has nothing to do with it. I agree with Jeff Like he said a filter change would be a good idea and also a transgo shiftkit while you are in there. make sure the fluid is full. If not maybe add an additive with it like Lucas or something similar. What gears is is shifting hard in? My transmission did that for years before i replaced it. I kept it going by doing what Jeff stated and using 2 bottles of the Lucas trans fix.

thanks everyone, appreciate the input.

Shift kit fixed mine. Search the threads, it was easy to do.