My 4x4 1989 F150 is stalling when I come to a stop. It runs fine at all other times. I replaced the IAC about eight months ago, so I don't think that is the problem.

When it idles, the RPMs go up and down from almost stalling to about 1000.

The check engine light was on, but I ignored it because it was running fine. Now the light is not coming on and I have this problem.

Any ideas?

 

Have you pulled the codes yet? Since the light had been coming on, the codes will still be stored. Click here to learn how without a scanner. All I use is a jumper wire and a test light (instead of multi-meter). It may be something as simple as an O2 sensor.

Since you put a new IAC on 8 months ago, it would be worth it to pull it to inspect for carbon and clean it out. If it has carbon built up, the valve can stick and become inoperable. If you do need to clean it, take the sensor half off (held on with two screws).

Good luck.