What determines idle?

[QTB]

I have had my car at a tuner here in KC for a bit and he and myself have been struggling with coast down. Basically as you approach a stop and go clutch in, the engine likes to stall.

Back ground: the motor is in a 350z and I'm using the nissan wheel speed sensors for the speedo so I do not have the VSS hooked up for the GM pcm. In theory the pcm has no idea what speed the vehicle is at.

My thoughts: The P/N, in gear, and coast down adaptive idle ignition tables should be the same. So what I think is happening as you lift throttle and begin coast down (say your at 1500 rpms and target idle is 800) you go into overspeed idle ignition tables and it retards timing between 1.5-20*. As i approach 800 rpms ignition is still retarded and it never regains timing and stalls. In the log files you can see timing will goto near 0* just before it stalls. This is why I'm curious as to when 'idle' is determined. Would it make sense to reduce all over adaptive idle ignition tables for overspeed to something more along the lines of 1.5- 3* compared to agressive 20*.

Side note: If I blip the throttle as the rpms fall to ~1000rpms the engine will not stall and returns to normal idle.

Thoughts/suggestions? I know the obvious is to hood up the VSS.

[edcmat-l1]

You need to add some idle air to it. That's not enough idle air for that camshaft.

Log your Dynamic airflow g/sec and Mass airflow g/sec and that will give you an idea of what your idle airflow should be.

[QTB]

Finally got some decent weather here and got out to do some logging and that helped a ton, it hasn't stalled yet since that change. I just took the value it was reading in the log at idle and calculated the percent difference and multiplied that out near the rpms cells near the target idle.

Another question: When it comes down from higher rpms to idle it tends to go slightly under the idle target and then bounds back up. Will adding slightly more air reduce that near the target area?

Thanks!

[DSteck]

Adding more will help. You have to be careful where you add it to avoid hanging idle though. Experiment and you'll find a happy medium.

[doc350]

is that generally the way to dial this in? keep adding until the dip/stalling goes away, but the idle doesn't hang on coast down?

that's the method that i have been using and it seems to work fairly well, but if there is a more scientific approach....