Electrical, transmission, what's the problem?

I recently had an engine replacement on my 97 Cavalier, 2.4L. Since the replacement I’ve been having some trouble with the car. The car accelerates and shifts fine. When I let off the gas however, it feels like I hit a small bump in the road and I hear a small knock, almost like the transmission disengages before it should and lets the engine run free for a second. Afterwards everything is fine until the car reaches 45mph. As the car decelerates the RPM’s jump from 900RPM’s to 1500RPM’s and back again, over and over until it reaches 35mph where it just idles around 900RPM’s until I stop, when the idle drops to around 600RPM’s. This only happens if I have my foot completely off the gas peddle. If I give it some gas the RPM’s jump to about 1700 or so and stay there. The engine runs pretty smooth for the miles that were on it (it was from a wreck, but is a solid runner) so I’m confident that the engine is not the issue. The car also throws a code for an IAC over voltage. I replaced the IAC and it did not help. I also replaced the transmission fluid and filter since the engine replacement. The problem happened before and after I replaced the fluid.

I would like to add that this problem doesn’t happen all the time. It seems to go in cycles of about 3 or 4 days at a time. For 3 days the car runs fine with no problems, then for 3 days the RPM’s fluctuate between 900 and 1500 when I slow from 45mph to 35mph. Also, it doesn’t seem to be leaking any fluid, but this transmission doesn’t have an easy way to check the level so I cannot say for sure. There are no leaks under the car in my garage since I had the engine replaced a few months ago.

I have run out of ideas. Any help would be appreciated!

My first guess is a problem with the torque converter lockup.
This part was fiddled with for the engine replacement.

Could you provide the DTC code of the IAC overvoltage. Are there any other transmission related DTCs.

I would check the operation and voltages of the Throttle Position Sensor. That is one of the critical sensors for the electronically controlled transmission along with the Vehicle Speed Sensor, air flow meter, and the MAP sensor. There is a Transmission Fluid Pressure switch in the transmission that is mentioned in TSB that may or may not be related to your symptoms.

Keep us in the loop on your progress.

Thank you for the replies! Sorry for the long delay in answering…
I haven’t been able to get anywhere over the holidays to get the codes read, but the check engine light doesn’t stay on very long anymore when this problem happens. I do have some more information about this problem though.

I’ve noticed that the problem with the transmission only happens when the cooling fan comes on. If the car is running normally and I get to a stop light for a few minutes so the cooling fan kicks on, the car will instantly start acting up. This problem now goes away the next day after the car had a chance to cool down overnight. This happened yesterday on my way home so I stopped and removed the fuse (relay?) from under the hood that controls the cooling fan for the duration of my trip home. This did not help the problem. I don’t understand how the cooling fan turning on would affect my transmission.

Also, when the car was acting up a few months ago, the cooling fan would always turn on when the car was running, which in turn would coincide with my transmission issues. Maybe because it’s so cold outside, but now the fan only turns on when the engine is over optimal temperature.

Any ideas?

Try cleaning the engine and chassis ground connections under the hood. The higher current needed by the fan and a bad ground connection may be causing a voltage drop to occur.

Another thing you might check for is a intake manifold vacuum leak.