RPMs don't drop to idle in neutral. AOS? Vacuum leak? Throttle cable?

Hi all, I have a somewhat strange issue that I've been experiencing with my car. Here are the basics: 1999 C2 with a six speed manual. Newer OEM clutch, flywheel, AOS (within the last 15-20k miles but at least 5 years old). New OEM MAF, plugs, coils. Aftermarket headers and mufflers. New air filter with helmholtz delete on the airbox. When I'm cruising in gear about 3-4k RPM and let off the gas the car wants to continue cruising at speed. Depending on the gear/speed the car will sometimes hold the current speed or even accelerate with no throttle input! When I clutch in or shift to neutral under these circumstances the RPMs will drop to somewhere between 2000-2200 RPM and remain there. If I slow down in gear to below 2000 RPMs or so and then clutch in or shift to neutral the idle will drop to a more normal speed. What could this be? It seems like a good indication of a vacuum leak but then again I don't know why it's RPM dependent. Is the throttle cable or plate sticking?

Smoke test for vacuum leaks. 20 year old plastic breaks/cracks.

I had a problem once with high idle following removal of the TB for cleaning. The black plastic throttle cable piece on the TB wasn't snapped back into place on the TB frame. So I did this...undid the throttle cable from the TB and gave it a pretty good tug. I could feel that the end of the throttle cable snapped into place behind the black plastic piece, and the black plastic piece snapped into the TB frame. All was well after that. Now I know to check and double check this.

I'd have a look at your idle air control motor. could be clogged up

Weird scenario. Assuming you have the cable throttle on your 99, I would first inspect the cable at the TB end first. See that the plastic bracket is fully snapped into TB housing (as mentioned earlier) and manually rotate the throttle to feel for snags.

Hi all, Sorry for the late update but I think that I've pinpointed the issue. I removed the idle air control valve today and cleaned it out (for the second time in a year). There seem to be some deposits in there that the carb cleaner cannot remove so I figured that it might just be irreversibly gunked up. I applied power to the valve with a nine volt battery and found that there are three positions: fully closed, slightly cracked, and fully open. After cleaning I reinstalled the valve and went on a short drive. I didn't have too many issues as the car warmed up but once it reached operating temperature the RPMs just kinda got "stuck" around 2,000 on their way down. The main symptom was: Driving at 45 mph in fourth gear - clutch in, lift off the throttle, RPMs drop rather quickly down to around 2,000 and then either get stuck here (or rise slightly to 2100-2200) and then finally proceed down to a natural idle. The "stickiness" here can last 1-3 seconds or so. After this I pulled over and unplugged the idle air controller. The car was a little harder to drive as the revs dropped much more quickly between shifts and it would try to die if I didn't catch the revs on their way down with a little throttle input as it approached a ~600 RPM idle. The exhaust popped a LOT louder than normal on decel events too. Obviously I can't drive the car like this but I had no issues with any kind of "stickiness" around 2,000 RPMs. It seems that the idle valve not only helps you maintain a steady idle in a variety of conditions (and with A/C on) but also serves as a bit of a damper so the revs don't fall so quickly between shifts and the engine doesn't die as it approaches a natural idle. It's probably also a good emissions device based on how freaking loud my exhaust would pop off with the valve unplugged. My best guess is that my valve is a bit sticky around in the "slightly cracked" state so that it doesn't want to fully close - at least not easily. Anyways, I'll buy a new valve and try to remember to update this thread once it's installed.

Well, I think you have correctly identified the problem. I have cleaned my ICV several times and cycled it with a nine volt battery. At first, with all the carbon in there it would do like yours...stick and not rotate smoothly. I spent a lot of time cleaning it out with carb cleaner, alcohol and Q-tips. I was surprised to see that the inside rotating valve is actually shiny smooth as opposed to the ugly black. Once I got it cleaned out, apply nine volt battery to watch it operate, and it rotates lightning fast from closed to open. So let us know how the new one works out. I was too cheap to buy a new one, but probably will eventually.