Are the IAC Step values in a data log coming from the ECM or from the IAC? In other words, if the IAC was stuck open would the IAC step value in the datalog show it at a higher then normal value?

I have a 6E MAF system. I continue to run into an intermitten high idle condition after I initally start the motor from cold and drive it around to Op. temp. If I then stop the motor and immediately restart it, it will idle normally forever. Until I stop it and let it cool down over night. I've done data log on the high idle condition and the IAC steps are showing 95 where a normal idle shows 44. If the 95 is from the ECM. then the ECM is holding the idle high. If the 95 if coming from the IAC, then it's sticking open.

 

Sometimes a worn throttle body will cause a high idle. Check and make sure the TB is closing all the way with the engine running. If the shaft is worn they won't close all the way with engine vacuum pulling on it.

If it seems OK, check and make sure the min idle is adjusted properly. I think the procedure for that is in the Tech Section of the board.

 

To clarify your question, though: NO, the IAC does not send a signal to the ECU telling it where it's at. The ECU only commands it to move, and the desired position is reported, not actual.

It bases it's "knowledge" of the IAC position on a reference position, where upon shutdown, the IAC is slammed shut completely, and then opened a set amount of steps. At key on, it assumes the IAC is at this reference position, and bases the position from that. So, if you ever put a new IAC in, you'll have to allow it one reset before it's correct.

 

So if the data log shows the IAC position at 44 steps during normal Idle and shows the IAC positon at 95 steps during the fast idle problem, then it is safe to say that something in the $6E code is setting the Idle fast, correct?

If the throttle body was worn or the IAC was at an incorrect start point prior to start up, the data log at fast idle problem should be 44 steps. The ECM would not be aware of the IAC or Throttle body problem, correct?

 

There's still a chance that the throttle is sticking open. See what the TPS reads during the problem.

 

TPS is .055 volts at slow idle... .057 volts at fast idle. Is that enough to cause 600 rpm increase in Idle?

 

I doubt it, unless it pushes the ECM out of idle mode.

 

When that happens, just open the hood and force the throttle closed. See if the idle returns to normal. Then you'll know, and we wouldn't have to guess anymore.