Installed the new IAC today, first start seemed to be ok. Will see how it goes in the next few days. Our weather is going to drop down to the 40's the so it should be a good test. If this does not work thinking it could be the throttle postition sensor or coolant temperature sensor. What makes it hard to figure out is that after driving a couple of blocks it runs great.

 

With new IAC cold start is better but not as smooth as it used to be. Start engine and rpm will be 1000 rpm then drop to 900rpm and start to run a little rough, after several seconds there will be a loud sound of air (loud air hiss sound that lasts 3-4 seconds) and rpm will go up to 1100rpm and stay there for 10 seconds and then it starts down to 900 again. The air sound only happens one time. Not sure what the air noise is but suspect intake air past the IAC. This may be normal but I can't say I remember hearing it before the cold start surge started. When warm the engine will idle at 650rpm and is very steady. Temp this morning was 32 degrees F. Will try again tomorrow and try to find the source of the intake air noise. Ideas?? Could MAP, TPS or Coolent Temp sensors cause this symptom?

 

I'm going to replace the TPS in my truck. I noticed yesterday, for the first time, that the RPM's were jumping up and down a few hundred rpm's as I cruised at about 55 mph. I guess if the IAC was all gunked up it is safe to assume that the TPS might be as well.

 

TPS should not be effected by carbon/gunk. It is basically a potentiometer which is rotated by a blade connection between the throttle shaft. We now think our problem may be one of the O2 sensors when cold. A friend is going to hook up his scanner and look at O2 output during cold start to see if it has the wrong voltage. Current thinking is the O2 sensor gives bad signal on initial cold start but as soon as it warms up the output is ok. I had replaced the rear o2 sensor because of an CEL but not the front. Will also check coolant temp sensor when testing O2 sensor. This could take a few days since the problem only happens on the first start in the morning. Will keep you informed of our progress.

 

OK, it'll be interesting to see what you learn. Sounds like I might just be replacing the TPS AND both O2 sensors!

 

After a couple of mornings of testing it appears that the forward O2 sensor is slow to react causing the IAC to hunt for position resulting in the idle surge. Replacing the sensor today and will see how this works. Weather turned cold again so we should get some good tests. Will post results.

 

Installed new forward O2 sensor and the cold start surge is still there! I now have $180 invested in parts and it still surges in the morning. The plan now is to drive it until something fails or it fails to start and run so we can hopefully find what is causing the problem. Perhaps someone will find an answer and post it on this site. Only acts up on a cold start, once running and a little warm all is fine.

 

check the fuel pressure off of the fuel rail when you first turn it on. maybe your getting a lot of gas for some reason.

 

o2 sensors are bypassed in open loop mode. your IAC is on it's way out or it's sticking. yank it out and clean it real good and see how it is afterwards.

 

I first cleaned the IAC and throttle body, this may have helped but very little. I then installed a new IAC from DC, same result as cleaning the old IAC. I ruled the IAC out after that and started looking elsewhere. I do notice a loud intake air sound within 10-20 seconds of starting. I assume it is IAC intake air but am not really sure. I never noticed the air noise in the past but I was not listening for it. My guess is with the hood up the intake air noise is easier to notice. Plugs, wires, cap, rotor are new DC.