I have the EXACT same problem at the moment on a 94 SL1. It only does it after the car reaches a certain temperature (closed loop I assume). I have checked for vacuum leaks on the EGR, EVAP and brake booster circuits and cannot detect any, although a leak can show up at higher temperatures too.
Things that it may be:
- EGR bleed hole is clogged up causing the EGR to close too slowly when throttle is closed. Only way to check this is to disable the EGR and drive it to see what the RPMs do. It could also be that the EGR valve does not fully close due to carbon buildup or the EGR switch (if vacuum type) is sticky as well. If you are going to test it, best thing to do is disable it, and tap it so that its valve seats well.
- IAC valve is not closing fast enough and is sticky. The IAC is used to reduce emissions by preventing a fast throttle close condition in order to burn off residual vapors still in the intake. As well, it helps to prevent the RPMs dropping too fast to aid in smoother shifting.
- Vacuum leaks. This one is something I think would normally show up with a rough idle. My car idles very well, so I think this is low on my probability list.
- deposits in the throttle body. Again, mine is super clean, so not for me.
It will be interesting to see if one of us can solve this though, because I can't find a solution from the dsaturn discussion forums at all.
dave mc