I am trying to track down an idle problem with my new motor. I have a BBK 58mm TB and have done the "drill mod" and the "tube mod" to it.

The Problem:
The car idles fine in closed loop and the IAC will move around usually in the 80's (the idle screw backed all the way out). The O2's are not being used and there are no problems.

As soon as the car enters open loop, the IAC slowly backs down to zero. I have messed with the idle screw and had it adjusted really open and really closed, but no change to the slow drop. Once the IAC has dropped to zero, the O2 levels will climb until they hit 194 and set off a lean code.

Any ideas? Why does the IAC work in closed loop but not open? A tuning issue? Need more fuel at idle?

Setup: Accel 36# inj, aeromotive FPR, Racetronix pump, BBK TB (mod), New 02's, healthy cam/heads Pretty much any part you can think of.

 

TTT - any ideas?

 

Not sure if you meant what you said.... the engine starts in "open loop" = no feedback from the O2 sensors. After the coolant warms up, and the O2 sensors heat up enough to work, and the timer times out, it enters "closed loop" = feedback from the O2 sensors.

When you start your car cold, depending on the coolant temp, the PCM will set a higher target idle speed. For the M6, the idle drops to 1,000rpm when the temp reaches 24degF, drops to 900rpm at 46degF, and to the normal 800rpm at 111degF. Since the car starts cold, it may be simply a matter of the IAC opening up to 80 to get the required elevated idle speed. Then it will start to close down as the target idle speed drops down.

What exactly did you mean by the O2 levels climbing to 194? What climbs? An O2 reading of 194 "milliVolts"? One side? Both sides?

What is the target idle speed with you "healthy cam/heads" tuning? Is the engine holding the required idle speed when the PCM moves to closed loop? If its idling at the correct speed, and the IAC is at "0", its getting air from somewhere else.

I realize that I didn't offer a whole lot in the way of help, but a couple of the points mentioned above confused me.

 

Sorry about that, I had open and closed backwards. I'll try to clarify:

As soon as the car begins to rely on the O2s, the IAC drops to zero. While in closed loop, the IAC does it's normal thing to maintain idle (all over). Before I did the drill mod on the BBK, it was at 100 IAC+ all the time. After it was better as far as driving and idleing, but my IACs won't hold steady. They are fine and respond like normal until it sits at idle for awhile.

The target idle is 900, and it now holds the idle after the drill mod (it used to almost die and then hover at 1800). The problem occurs when it sits at idle and the IAC closes off. The car thinks its lean and then goes into open loop. The hole that I drilled is only 5/32 (too small to feed the motor alone). I can't figure out why, as my MAP level seems to be fine (no vacume leak). I did the water test and sprayed all around the intake but the idle didn't change.

I have a scan of it at idle, and a light rev below:
http://ahotvent.com/idle%20prob1.csv

The 194 is the value for the interegator both left and right. At 194, the computer sets a code. It sets both codes, 44 and 66 (left and right lean).

What should the vacume be? My MAP is .57-.61? Could it be a bad sensor? What else would cause it to close down?

This all started with my trying to get the car to idle after the BBK. It caused the normal split BLMs, high idle, and now I'm trying to keep it from throwing a code and taking an RPM dive after a rev (see line 335 in Excel where it hits 600rpm after the rev).

 

TTT - I'll give the tuner a call. Any other input?

 

How big is the cam... I/E and LSA? That will determine the MAP. 57-61kPa (about 12"H2O vacuum) would go with something larger than the HOT cam.

The fact that the O2's are pinned at a constant value would lead me to believe they aren't responding at all. Do they ever indicate any other values than 18mv on the left and 13mV on the right (recognizing the whole record is only 64 lines, and that's all I have to look at)? Faulty extensions? wires burned to the headers? bad grounds?

I thought that the "lean" codes are set when O2 sensor voltage stays below 200mV for 50 seconds. Maybe that's because my manual is for a 93??

 

Crap...I attached a short file. It was supposed to be 400+ lines long. Sorry

The O2's are functional, they are brand new as of 30 min. before the scan. The O2's are functional in that they do respond (movement up and down the scale) and when I give it gas they return to low/normal levels. The lean code is set after the O2s have remained at a set level for awhile.

The cam is in the range of a GM847 and is on a 114 LSA for the nitrous.

Thanks for all your time and for looking over the logs. I'll post a new one this evening that is full length.

 

New logs are on my web page.

"Idle Problem 1" references the post above (now a complete file)
"Idle Problem 2" is the car idleing in Open Loop
"Idle Problem 3" is the car in closed loop with the IAC falling and the 02s rising. It had just gone into closed loop and thats how fast the problem occured.

http://ahotvent.com/ (download them or it will be all messed up)