So my 87 pickup is giving me a hard time....kinda stumped on this one.
When cold, the engine idles normal (700ish). Once warm, it idles at around 1400. But here's the confusing part....if I stop the engine and immediately start it back up, it idles normal. As soon as I give it some throttle it goes back to 1400 rpm.
I've replaced and set the TPS, cleaned the throttle body, checked the IACV (was spotless) and mass airflow sensor seems to also be operating normal.

 

Sounds like the idle up control unit under the throttle body is acting up. It works on coolent temperature to open and close a port bypassing the throttle plate. You can pull it out and clean/check it. Also if your coolent runs low or have lots of air in it, it can cause wacky idle.

 

^^^^^ What he said. Mine did the same thing. Idles around 1500 - 2000 rpms. I had a clogged hose running to the idle air control valve. I clean it all up and now back to normal.

 

Interesting...do you recall which hose it was? I removed and checked the idle air control valve, but cant remember if I checked all the hoses or not.

 

its both hoses that connect to the Idle air control valve. One is coolant in, the other is out

 

I just checked both hoses, all clear. Water flows easily through IACV. Also checked that coolant is flowing through underside of throttle body and the little thermostat housing port. Could the IACV be defective in any way? I tested it with hot water and it seems to work ok. Maybe the electronic sensor is bad?

 

Success! Unfortunately I'm not quite sure what the fix was. I suspect there may have been a clog in the spigot at the thermostat housing because it's the only thing I did yesterday that I hadn't previously checked. My rad was replaced a few months ago and a piece of debris may have become stuck in there during the process.
Thanks for the help everyone!