I have an 87 150 with a 302 and a 4sd as the winter truck. Shortly after i bought it the motor started to surge between shifts to about 3000rpms. I cleaned the iac motor and then tried replacing it no change. i started running it with the iac motor unhooked cuz i needed the truck. When i start it i just have to hold the pedal open to give it air to start and then keep my foot on it to hold 1500 for about 20sec and then it idles at around 500. Any way last winter i had to change out lower intake gaskets so the other parts i changed were air charge sensor coolant sensor and the egr position sensor. if i leave the iac hooked up and start it cold it idles at 1500 then comes down to 900. If i let it warm up a little it will surge to 2500 then fall to 1000 like you shut the key off for a second and then it will start climbing again, if you tap the pedal the idle will climb and stay there and take about a minute to come back down. So now im finally sick of this and want it to run right. could an egr or brake booster cause this? Im a diesel guy this spark ignition stuff gots me stumped

 

If disconnecting the IAC makes the high idle go away, I would think that there still could possibly be an issue with it.

It sounds like the TPS may be your problem.

I would also check for vacuum leaks.

 

I know the 87s had an adjustable tps and i replaced it at one time, not knowing what i was doing i just put the new one on in the same position as the old one came off. Is there a correct way to ajust it?

 

So borrowed a service manual and i adjusted and checked the tps. everything with that checks out. the idle does drop down when i put the vac line to the map sensor and the idle fell. and i could rev it and the idle came back down right away. if i just left the line hooked up and pulled the connector to the map sensor it still kept that high idle. Could i have gotten the wrong iac valve at the parts store? Is there even different ones? What else decides how far the iac valve is open?

 

I don't know about adjusting the TPS, but I do know that you have to install it by rotating it clockwise into position. I don't think that you could have gotten the wrong IAC valve.

 

Is there any other sensors that control the iac valve? Would a bad map sensor cause that? Would clearing the computer help?