Why does your engine idle high in Park and Neutral but seems to be O.K. in Drive?

This is almost always caused by the Idle Control Valve (IAC), which is located near the firewall, on the passenger side, next to the throttle body. They sell for about $50. On generation 1 cars (1993 to 1996) these are very difficult to remove and install, but it can be done-or you could try to 'SEAFOAM" the intake by running a bottle in the gas tank and taking another bottle and run it through the intake by disconnecting the PVC hose while the engine is running, push the hose (it will suck the Seafoam into the intake) and then let it sit for about 10 minutes. Two bottles will run you about $14. When you start it up, expect alot of smoke-it might even bring the volunteer fire company, but it is the carbon build up being cleaned out of the engine. The engine should be fully warmed up before you do this. After you do this, disconnect the positive battery cable for about 10 minutes-it allows the computer (ECU) to 'relearn' inputs from the various sensors like the IAC. I actually did this on two Generation One marks I have owned, and in both cases, it cured the problem. The high idle in drive and park is tough on the engine and murder (in drive) on the already fragile transmission, not to mention the brutal effect it has on U-joints when you shift into drive at high RPM's, so you want to get it fixed as soon as possible or park it until you can solve it.