1980 up and down idle.

[mixybro4]

After you drive the car and put it back in park the idle will go up to about 1400 and then drop down to 600, climb back up to 1400 and repeat the process. If you give it gas while it's doing that sometimes it will stick at about 1800rpms. I'm not sure why it does this

[Maleficio]

Vacuum leak.

[mixybro4]

Best way to find out where?

[Maleficio]

It can be tricky. Some people prefer to simply replace every vacuum line on the engine.

One vacuum line that tends to sneak past people is the line going to the transaxle.

It's a hard line coming off the engine, runs along the passenger side of the body and is coupled to the aft hard line with a rubber vacuum hose. It's about four inches long, and is usually rotten or not even there. Mine was hanging free.

The coupler is just aft of where the line exits the engine bay, and is up high. You'll need a bright flashlight to see it, and some very long needle-nose pliers to fix it.