86 305 TPI. Starts and idles fine. When you come to a stop and and let off the brake slightly the motor will idle up with out touching the peddle. If Some times it idles fine at a stop but if you let of brake slightly it will idle up by it self. Any ideas I just replaced the IAC but it didnt help any. ??? Something else while it is idleing high if you put in reverse it will go back to normal idle but when you go to drive it idles high again.

 

Check the TPS. Adjust it if it need be, Check that the brake booster line is connected with no leaks from it.

 

I replaced vac hose to booster. Checked TPS. Idles fine in park when you put it in gear the idle increases to very high idle. Is there a sensor from trans to computer that would make it do this?? I am going to replace TPS .It's stilll under warranty so no cost to replace. Strange just started doing this. Why would idle increase when I put in gear??

 

My buddies 89 GTA had the same problem. I believe we replaced the intake manifold gaskets since they were leaking and idle would change when you spray starting fluid around the intake. Also check the 02, or just replace it for peace of mind since its just 20~ bucks. Also check the timing, we bought a cheap $15 timing gun from Harbor freight that did the job fine.

He had the same problem. I believe thats all we did besides swapping out the ecm. Then again he is running a L05 with a TPI that was running a LB9 chip.

 

Well when you put it in gear you are putting more of a load on the engine so it compensates by increasing the idle. And just because you replaced the IAC doesnt necessarily fix it, make sure you are getting readings from the IAC to the PCM, get pinouts and voltage specs. Check the wires from the PCM to see if it is sending voltage to the IAC.