Engine surges intermitently at cruising speed

[ktownman75]

I have a 1999 Grand Prix GT with the non super charged 3800. The car has 130000 miles on it and runs perfect but at the cruising speeds between 45 and 60, well when you have been at the one speed for some time the engine will surge intermitently then stop and manot do it again for the rest of the drive. I have alreaddone a full tuneup and replaced the coil packs due corrosion on them as well replaced the plug wired with ac delco wires. Also i already replaced the upper intake and the lower intake gaskets with the newer upgraded ones not the piece of junk plastic ones GM has forced on us in all their engines it seems. Additionaly i have cleaned the IAC valve and the Mass air flow sensor and the throttle body with the proper cleaners. The car runs very strong and my wife loves it but the surging issue has me stumped. When it happens if you ease into the throttle a little it will stop but when you get back to the speed and rpm you were at it will start again. Any body have any ideas i have never had an issue i couldnt fix until now.

[SlowNA06]

Check for a vacuum leak.

[ktownman75]

Nope no vacum leaks at all checked every port and the entire manifold system. Its acting like the IAC is trying to adjust do to some sort of sensor malfunction. But the malf light isnt on and there are no stored codes. Gonna do a drivability test with my brothers scanner this weekend and see what the different sensors are doing.

[BillBoost37]

You've removed almost everything except the TCC surging issue.. which is a rebuild.

[01GTLAK]

Does it kinda chug? Does it only do it when the vehicle is warmed up? if so, possible clogged cat

[01GTLAK]

Do a pressure test on the cat
I had a clogged cat and it would chug once the car was warmed up or if i put any rpms on it

[Buggsy]

Does the surging actually change your speed, or just engine RPM? Maybe it's a transmission issue.

[lawdawg]

I have this same issue on my 3.8L Series 2 in my 2000 Grand Prix GT. I have spent some time searching for an answer and have found several referrals to the transmission pressure control solenoid. I haven't swapped it yet, but am strongly leaning toward trying this as a possible fix.

[ktownman75]

The engine doesnt chug at all and the surrge is warm or cold condition. I will look into the tcc and the cat the car spent its life in a major cit before i got it so i imagine there is alot od idling time on it.