08 E38 2 Bar - Random idle issue.

[jakebdb56]

Just curious if anyone else in here has run into any odd issues with idle on the 2 bar OS's, trucks specifically. There's one turbo 5.3 that's been eating my lunch and can't trace down the issue. It only happens with the A/C on. A/C compressor will begin to engage, then kicks out since RPM drops too much. The cam is rather large for a 5.3 so I'm not sure if that's the issue or not (23x/23x range). Air fuel is dead nuts, even tried to richen it up a little but no luck. Timing is fine and doesn't go astray when the compressor kicks on, even tried locking in 28* and no dice. TPS hangs around 16% but doesn't "hunt" when the compressor begins to bog the motor down. Manifold pressure definitely drops out to 90'ish kpa. No vacuum leaks at all. Every other one I've done like this has no issue. Even throwing a ton of airflow at it doesn't help. More or less, has anyone the same situation before? Any remedies to the problem?

Things I've tried:
Static timing, even tried up to 32* and no luck
Richen and/or leaned the afr in these cells.
Crazy high idle RPM, will idle perfect and drive with higher RPM and even the normal 800 RPM idle, but as soon as compressor engages it goes to shit.
Crazy high idle airflow, same as RPM situation, no change.
Zero'd out modifies for trans shift and a/c airflow models. No dice.

I did have a Gen III before do the same thing on a motor application, had too much freon in it. I had no way of testing this at the track last week, but I'm almost leaning towards a mechanical issue.

Another note: In park, hold throttle to increase revs to 1800. Compressor will engage, but brings RPM down to 1300 while A/C system is doing it's thing then goes back to 1800 after compressor clutch disengages.

[MikeOD]

Have you tried a lot less idle timing? I've started to get away from trying to idle with a lot of spark...instead I just run the stock timing in the idle table, which gives the PCM a LOT of ability to save the falling RPM with spark advance...the closer you are to optimum timing, the less chance it has to save it when something tries to bog it down.

[MikeOD]

...you'll HAVE to re-learn your idle airflow tables if you change it that much, as it'll need a good deal more airflow at idle with that much less spark.

[DSteck]

Run less idle timing. You've completely eliminated reserve torque by using that much to idle.

[jakebdb56]

Run less idle timing. You've completely eliminated reserve torque by using that much to idle.

Makes sense, just thought it was odd that when holding revs at a static rate the motor still dipped significantly. I should see the guy Friday, so I'll give 12-16* a shot. I think the situation is exaggerated due to the donkey dick cam and having stock-like manifolds for the turbo setup. It makes absolutely no torque. I'll let y'all know what it does. Thanks again for the input.