wpatters1229

Hi, I have a 2005 Express 3500 that seems to do it's own thing when I start it. I cleaned the throttle body and the idle control solenoid. Replace the throttle position sensor. I disconnected the battery while doing this. I reset the throttle position sensor by holding down the throttle pedal for 10 seconds with the key in the on position and then off for 30 seconds.
Sometimes it starts and runs about 1500 rpm and then slows down to about 850. I believe that is normal. Other times it races about 2000 rpm and I can only get it to slow back down by shifting into Reverse or Drive. Then it stays at the correct idle speed. Seems to me that something is going on with the computer.
Any idea on how to fix this? I get that P0507 fast idle error. This is when the engine is cold. Have not tried to see what happens when it is hot.
Trying to get it to pass smog and get the readys to set.
One other thing. Why is it so hard to find a IAC at a parts store? Rock auto does not seem to show them. Only the wiring harness. They are not built into the throttle body. Is there a difference between an Express 1500, 2500 and 3500? With 136000 miles i figure the IAC might just need to be replaced

mountainmanjoe

This person says otherwise.

https://chevroletforum.com/forum/exp...21/#post444617

wpatters1229

Rock Auto sells the whole throttle body not just the IAC. Ugh!
I think I have to look at the 2500 model as it also uses the 6.0 L V8 Vortex.

Twscarp

Hi, I had the same code, I think about 1-1/2 years ago on my 2008 Express 1500 Cargo, 4.3 with about 230k. My mechanic removed the throttle body cleaned and reinstalled, went through the relearn process and never got the idle right, even after driving about 150 miles, if I remember right. Decided to put a new throttle body on from RockAuto, the idle was correct right away, and gas mileage improved. I would recommend getting a new TB from RockAuto.

wpatters1229

I have taken the throttle body out or at least partway off as I was not able to unhook the cable that goes to the cruise control. There is a little plastic connector that is a mystery on how to pull off. The throttle cable was no problem. Anyhow, I cleaned the body with cleaner and removed the IAC and cleaned it and the cavity it goes into. Replace the position sensor. Still has a weird problem.
I did the reset of 3 minutes idle with one minute off and then 3 minutes again. The first time it worked just fine but then a few minutes later I started it and it was really fast idle that did not drop. Ran the reset again and it did the fast idle again. I can get it to kick down by putting my foot on the brake and shifting into drive and then back to park and it stays at normal idle. Next time it idled just fine. Walked away for several minutes and then reached in and started it again and it raced to a fast idle and again shifting into a gear brought it down and it stays at normal after putting it in park. Is this just the computer or is the throttle body that messed up. It was doing this before I worked on it so basically my cleaning and replacing the sensor did nothing.
Anyone think at this point replacing the whole throttle body will fix this? 136000 miles.

I want to remove the whole throttle body but it has a cruise control cable with a weird plastic piece that connects to the throttle linkage. Does anyone know how to remove without breaking it? Nothing on Youtube. This throttle body also has a water hose connection that passes under the bottom of the body. Small input and output hoses. Is that to cool the throttle body or heat it up?

wpatters1229

OK folks. I figure out how to take off that cable...it was silly. Anyhow, I took the throttle body completely off. Cleaned it out super clean. Put it all back together with a new gasket. Pretty much the same results. Sometimes it races way high like 3000rpm. Again shifting it into gear brings it down somewhat to normal but it is not consistent. I guess the next thing is to drive it for 20 minutes going above 44 mph and back down over and over. My biggest concern is that with this going on I can not get the ready items to clear. Three of them are still not ready. I will have to run that crazy script that GM says you need to run to make them ready. The hardest thing is to find a place in CA where you can do that without getting run off the road.
Anyone got new ideas?

Twscarp

I had the same issues with mine, TB was removed, cleaned by the mechanic, reinstalled but the idle still raced. Did the relearn, and driving many miles no luck.
Try a new TB, I can't guarantee it, but it worked for me.

wpatters1229

I know. I have not done the driving portion yet. I also have to do the GM driving process to get all the items for smog ready. Such a pain. The biggest problem is the expense of the Throttle Body, $251 on rockauto. I hope the driving works. This is the chassis for a Class C RV, 28ft. Runs fine but will not pass smog. I was out trying to get it ready for smog and got pulled over because the tags had expired. In CA they have better things to do but got to do what ya got to do.
Thank God this virus has not gotten in the way.
Stay home and be safe.

wpatters1229

New update...hope everyone is safe and healthy. I finally took the RV out for a ride to try to get the drive cycle done. Almost impossible around the SF Bayarea. Anyhow still have some items showing incomplete. I took off the housing yesterday to look at some of the spark plugs. They are in great shape and the ones theys used are the ACdelco 985's which are the best. Will leave them alone. I did hear an exhaust sound on the right side of the engine (passenger side). Was able to tighten a few of the exhaust manifold bots just enough to stop the leak. That should not have anything to do with smog or fast idle. OK so I start it up and it is idling at around 900 rpm. I hit the accelerator to give it a "goose" and it starts running fast. I waited but it did not slow back down. I then shut it off and started again and it was running fast and again would not go back to idle.
Does this pretty much say I need a new IAC that controls the position of the idle? I can not figure anything else that would do that. I replaced the position sensor but not the IAC. Cleaned the throttle body and just can not see why that whole assembly would need replacing. Also replaced the gasket for the throttle body.
Your thoughts?

Read some other forums and many say the throttle body does not need to be replaced unless the throttle is mechanically sticking. I replaced the position sensor and cleaned the IAC motor but now I will replace it. Hard to find. Found one on Amazon but places like Rockauto does not sell them separately. They only sell the IAC with a whole throttle body. Seems unnecessary to buy the whole thing. Apparently the IAC motors do go bad.

I will now have to find the sequence to retrain the PCM for the IAC Motor. Anyone know that process?