Occasional High Idle Speed - $8D 383 fully modified TPI

[RobertISaar]

i don't know about all of the time..... but with the 60V6 stuff that has an AIR setup, it's generally only cold start and when in PE for an extended period of time.

[zaut]

That is what it said in the air system service section of my GM Service manual. I agree with you, I don't need it to work, I am just tying to keep it look stock under the hood. Actually I plugged the hose with a cork in the middle of the hose so you cannot tell that it is not working.

[zaut]

This is a thread I stated last year when I was having a high idle problem. I thought I got it worked out, but I thought wrong. When I start the car, it idles correctly. After driving it, it starts to idle very high. If I turn the car off and back on, it will idle correctly until it take it for a drive.. When it is idling correctly the IAC position will be in the high 30's-low 40's. When the engine is idling high the IAC will be in the 80's and 90's. Does anyone know why the EMC will not lower the IAC counts to slow the engine down?

[zaut]

Well, I think I finally got to the real root cause of my high idle. I noticed that when the engine was idling high that the TPS percent was not zero. After close examination of my TPS on the BBK throttle body, it turns out that the follower arm of the TPS was dragging against the throttle body. So the TPS was not returning to zero every time. This was then making the IAC follower function think the throttle was open and therefore opening the IAC motor. Amazing. 10 seconds on the bench grinder and the TPS now has plenty of clearance. One thing I don't understand is why the IAC motor does not move for a set period of time when the engine is cold started? Is that the IAC learn timer setting? And does anyone know what the maximum count the IAC go to? On cold start, the 160 counts is not enough to keep the engine running. And lastly, what does IAC open loop command offset do?

[EagleMark]

with the MPFI and SFI 60V6 applications i'm familiar with....

they check voltage at key-on, and the lowest voltage it sees before cranking is considered the 0% TPS point. with the SFI stuff, this is done with every key-on, but with the MPFI, it only does it if the NVRAM failed(battery disconnected).
while running, if at a high enough RPM and low enough MAP, the TPS 0% point can be changed in a downward direction, but not upwards.

Since you found the throttle hanging up you may want to disconnect battery for awhile and let things reset... Robert quote above says it won't reset to TPS 0% without! Then see how it does.

For a modified engine sometimes IAC is not enough for your cold start issue? Do a data log and when warmed up see what IAC counts are doing? If like 40 or so, well you have plenty of room to open throttle blades so more air can get in at cold start. Do want to end up with some IAC counts for idle control at end.

[RobertISaar]

it may not be necessary with 8D, but id rather err on the side of caution.

[zaut]

The car is running the best it ever has since the rebuild and tune. The TPS % is set to zero on every restart with the 8D. I did a little playing around with restarts with partial open throttle and it set TPS % to zero. I am still perplexed as to why when I cold start, the IAC stays on the 160 park position for a very long time. EagleMark, are you saying that a IAC down as low as say 10 at hot idle is ok? I have the throttle blades set at 100 RPM below my normal idle set point with the IAC port blocked. I can open then up to close to the idle set point and see what happens with IAC counts.

[EagleMark]

I don't know $8D code well enough to say what the IAC should do at cold start? But 160 is full open and seems to need that air a long time so throttle blades may not be open enough.

I know the GM procedure says how to do this at 100 RPM below Idle speed. But if the engine is not fully warmed up like after a drive on a warm summer day? This can be way off! TBI is way more sensitive then MPFI, but I still like to see IAC data at end of log, a minute in drive and a minute in Park. As long as you have some IAC counts, like 10 in park at that point it's OK, may be 20 or so in drive.

I forget what all you did to this motor? But if getting the throttle blades set and IAC counts in order does not help cold start issue then it's time to look at some bin changes. BUT!!! Before you mess with any cold start issues you have to make sure your VE tune is in order first. Then look at Open Loop AFR tables or $8D equivelent...

[zaut]

Thanks again EagleMark. I will start with opening the throttle blades and monitor the IAC counts as you suggest to see if that is why the IAC counts stay at 160 so long.