My 88 Gmc S15 w/4.3 V6 has begun to idle too high when put in gear, I can then shift vehicle to park and idle speed will remain high. If you then turn key off and restart vehicle it will idle fine, put truck in gear idles too high, around 2000rpm. Changed the IAC valve but engine still does same. Can anyone help me with this problem?

Make sure you got a good one.
Check the wiring on it and check to see if the computer is controlling IAC valve.
Is the TBI butterfly good and clean?
PC valve good and no vacuum leaks?

Thanks for the reply, changed the TPS and it cured the problem!!

Thanks for the reply, changed the TPS and it cured the problem!!

You are welcome andthanks for posting back how it went.
Glad you got it fixed.