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!