GM TECH ? (CRASH?) IAC issue?

[crash2]

Intake leak? Honestly putting it on some sort of computer and watching a graph is going to be your best bet. Like everyone else said it's just going to be a guess.

You have to remember what an IAC does--its a controled intake leak so to speak. An intake leak will typicly cause a high idle condition and depending on how large the leak is can cause a lean miss(which is quite common on the 4.8 and 5.3's)

 

[skinny_pedal]

You have to remember what an IAC does--its a controled intake leak so to speak. An intake leak will typicly cause a high idle condition and depending on how large the leak is can cause a lean miss(which is quite common on the 4.8 and 5.3's)

True, would I be wrong though given his systems to maybe look that way. Maybe just watch live vehicle data, preferrablly fuel trim. Spray some hydrocarbons(carb clean) around the engine and look for a spike in the fuel trim.

 

[Pook]

No found vacuum leaks.

You can hear the IAC spinning and doing stuff while the motor is gulping for air.

The more I read the more its confirmed the bigger cam pulls to much air at idle and IAC can't keep up to maintain enough airflow. By opening up the hole in the valve I get more baseline airflow and then IAC can control just the higher amounts needed to make it run above that baseline.

Seems to be a fairly common issue.

 

[crash2]

True, would I be wrong though given his systems to maybe look that way. Maybe just watch live vehicle data, preferrablly fuel trim. Spray some hydrocarbons(carb clean) around the engine and look for a spike in the fuel trim.

Are you wrong? No because its a good idea to give that a quick check. You just don't want to center around it.....

 

[skinny_pedal]

Are you wrong? No because its a good idea to give that a quick check. You just don't want to center around it.....

Nice. I just seem to find vacumm/ evap leaks left and right. Sounded similar.


Pook...maybe next time

 

[t1h5ta3]

No found vacuum leaks.

You can hear the IAC spinning and doing stuff while the motor is gulping for air.

The more I read the more its confirmed the bigger cam pulls to much air at idle and IAC can't keep up to maintain enough airflow. By opening up the hole in the valve I get more baseline airflow and then IAC can control just the higher amounts needed to make it run above that baseline.

Seems to be a fairly common issue.

a bigger cam wont pull more air @ idle.... the opposite actually... hence the lower vacuum.
the issue you are seing is low manifold vac due to a large cam. the ecm see's les vac than expected. it opens the iac to try to rais the idle speed assuming theres a load. the rpm jumps, it closes the iac. the rpm drops, vac drops, iac opens. by setting the base idle manually with the iac closed you are essentially creating a controlled vac leak ( same thing the iac does) that is stable. you dont want to set it @ base idle tho, you want it just a bit below, otherwise the iac will try to close all the way ( and posably stick)

this is basicly the equivalent to ether drilling the throttle plates on a holey to improve the idle w/ big cams




or maby im just talking out my ass... after all what would i know...:corn:

 

[Pook]

a bigger cam wont pull more air @ idle.... the opposite actually... hence the lower vacuum.
the issue you are seing is low manifold vac due to a large cam. the ecm see's les vac than expected. it opens the iac to try to rais the idle speed assuming theres a load. the rpm jumps, it closes the iac. the rpm drops, vac drops, iac opens. by setting the base idle manually with the iac closed you are essentially creating a controlled vac leak ( same thing the iac does) that is stable. you dont want to set it @ base idle tho, you want it just a bit below, otherwise the iac will try to close all the way ( and posably stick)

this is basicly the equivalent to ether drilling the throttle plates on a holey to improve the idle w/ big cams




or maby im just talking out my ass... after all what would i know...:corn:

Pretty much what i read and what Crash confirmed for me on the phone. :beer: