I have a 1999 Isuzu Trooper with 81,000 miles on it. I seem to have a problem with the idle. It seems low at 1000rpm in Park and drops to less than 500rpm when I put it in Drive or Reverse. It also dies on me when I take my foot off the gas (slowing for a turn or stop) sometimes. Any suggestions?
The Mass Air Flow sensor (MAF), idle air control valve(iac), and throttle plate and bore could be dirty. Get a spray can of MAF cleaner, and a spray can of carb/throttle body cleaner. Remove the large plastic intake tube, and spray the MAF, iac passage, and the throttle plate and bore generously with the cleaners. Let sit a few minuets. Run the engine at fast idle a few minuets (the exhaust will be smoky). Repeat as desired.
Thanks for the suggestion. I went to disconnect the intake tube and noticed that one of the electrical plugs was loose. It goes to a connection point right above the intake tube and when I plugged it in and started the car I ended up with the oposite problem. Now the car idles way too high (3000rpm in Park, 1500rpm in Drive) and is difficult to control at stops (it even went up a fairly steep, icy hill without me stepping on the gas at all). Could the unplugged contraption have been the MAF?; how do I set the idle lower as a temporary fix?; and what could be the underlying problem? Any advise would be much appreciated.
Unplug it again until you get it fixed? 
I think what was unplugged was the idle air control valve (iac). Did it look like this? (Click on this link): http://www.autozone.com/N,15900207/shopping/partTypeResultSet.htm If it did, you need to re-set the iac. I don’t have the instructions of how to do it. On some cars, you turn the ignition key to RUN for 30 seconds; then, start the engine. Get the repair manual, and get the instructions.
Yes it is the IAC (verified by my brand spanking new repair manual). Resetting it by the manual specs did not help. Is the next step to replace the IAC? Thanks for all your help by the way.