Hey everybody. I’ll keep it short.
On my ‘88 4.0, I have a very high idle. I have refreshed my grounds, cleaned my TB and IAC, and completely cleaned my C101 connection. I have no leaks, I just replaced my vacuum lines, and the rear CCV orifice.

It always starts, shifts, and has never stalled.
it’s an automatic.

what next?

 

It's an 88 so there are more knowledgeable people here regarding those but:

Double check your vacuum hose connections
Adjust the throttle cable
TPS

Suggestions to try but I'm sure others chime in with more.

 

Has it always done this?
What RPM is your high idle?
Ever performed Tip 5 at www.cruiser54.com?

 

Unfortunately I have no tach so I couldn't tell you what the revs are at. It can get alarmingly high though, especially entering Park after a drive.
I will take a gander at the TPS and do Cruisers Tip 5, and will update you guys later.

 

Tip 5 first.

 

Ditto. Check sensor grounds and check TPS adjustment.

On the TPS, I can never get mine to adjust according to the spec. I end up with a high idle. I've done the same as you and have done the applicable tips from Cruiser (still have the C101). I always have to fiddle a bit with my TPS to get the idle correct. Even though the setting is "off", it runs great and recently passed CA smog. The TPS I have is from Standard.

 

Just to add a bit of confusion to this topic on my 1988......I have experienced on occasion the high idle at start up, & turn the ignition off, restart & most often this corrects the idle back to normal.....Sometimes I have to start & shutdown a couple of times, but at the end the idle returns back to normal........

 

Characteristic of a faulty TPS.