[tirwin]

At cold start the engine should idle at around 1200rpm and then settle down to 850-900rpm after it is warmed up. Is your idle normal after the engine is warmed up? At 500 rpm it is barely running.

As T77911S, it is a good idea to be thorough and methodical. There are a lot of possible contributing factors and sometimes previous owners (or even the wrong mechanic) will change this or that in an attempt to put a bandaid on the underlying problem.

There is a link to the test procedure for the AAR in the CIS for Dummies thread. I generally like to do the easy/accessible things first. Example: Take the oil fill cap off with the car warmed up and engine idling. Do the RPMs change? If no, you know you have a vac leak somewhere. If yes, well... It doesn't mean you still don't have one so check for cracked or loose vac lines, intake runner boots and the fuel injector o-rings.

There is a procedure to test just about every CIS component. You can tackle each one systematically, but make sure you've done all the easy stuff first.

When was the last time the plugs, cap, rotor, etc were changed? If it's been a long time or you don't know, go ahead and do that stuff and you'll have some basic maintenance knocked off. Do you have a timing light? That's also easy to check.