'Sawtooth' idle timing and wants to die when AC is on

[Gutless]

I have a LQ4 with a blower and a t56 swapped into a G body. Having issue with the car dying when the AC is on and the car is coasting to a stop on idle. Mainly when it is in neutral. The car rarely (if ever) dies when I am coasting in neutral or clutch in when the AC is off but if the AC is on and the car is not on gear or my foot is not on the pedal to give it a little RPM boost it dies. It will idle when the car is sitting parked and the AC is on but it struggles when its rolling and the compressor is on.

Some highlights are
-Silver blade LS2 DBW TB
-02 Denali DBW harness
-2 BAR speed density
-02 Tahoe flex enabled P01 PCM
-BTR Stg 2 blower cam

I am also doing some dirty stuff like logging aftercooler water temp off the trans temp input, logging fuel pressure and clutch line pressure with fuel and AC pressure inputs. Essentially I am doing EVERYTHING a P01 computer can do.


I have a sanden style AC compressor and it's controlled by the G body HVAC controls and pressure switches. I assumed my problem was that I wasn't bumping the idle speed up when the compressor turns because it idles fine if I give it some throttle with my foot when coasting. I added 200rpm to the idle speed when AC is on and I hooked up the AC request pins so the PCM would think it is controlling the compressor even though it isn't. I put 12 volts to the red 17 pin and grounded the red 55 pin.

The problem is I still can't get the RPM to boost when the compressor is on. I can add the AC request PID and see that the PCM is seeing the AC request.

The other problem is that the timing sawtooths sometimes when it's idling and the AC comp is on. The error between RPM commanded and actual RPM is pretty big at that time, about 100 RPM, so I assume the PCM is entering some mode where it is trying to close the error but the drag of the AC pump is too much.

So the question is
1. How do I get the timing to bump when the AC is on. Is there some table beyond the target idle speed table that is preventing this?
2. What table causes the spark to fluctuate from the commanded 30 degrees at idle to a instantaneous 8 degrees and then back to 30?
3. What is causing the mismatch of 100rpm between my desired idle RPM and actual idle RPM?

Tune and log is attached. The log attached is like 22 mins long, the goofy AC bog and timing sawtooth is in the last 2 mins. I attached a snapshot of the log when it sawtooths.

8-4-21 new timing for e.hpl86GP 8-4-21 8-12deg in 87 14-18 target in E (reduced 2 from last pull).hptHPTTiming.PNG

[Gutless]

Attaching screenshots is a PITA here so for more visual instructions on what I did look here
https://gbodyforum.com/threads/86-gp...57866/page-123

OK so I feel like an archaeologist digging up 10 year old threads and piecing together things but I think I got it where it needs.

To help future people that have the same issues.

It appears that if your short or long term idle trim airflow (STIT LTIT) is maxed out or working very hard, even if the car idles OK, it seems to prevent the AC from being enabled.

I had to to turn on a few channels and determine what tables to edit. After I hooked up the AC request wires (12V to red 17 and ground to red 55) the car idled worse with the AC on than it did before.

I had to zero out the Engine>Torque Management>Engine>Max Retard table, that eliminated the 'sawtooth' timing at idle and it didn't die anymore but it still surged at idle with the AC on

I had to essentially double my Engine>Idle>Idle>Idle Airflow table and that didn't make it idle any different BUT it brought the LTIT and STIT much closer to zero and allowed the Idle to bump when the AC was on

There was still a decent amount of LTIT/STIT error when the AC was on so I increased the Engine>Torque Management>Engine>Inertia Torque table by 50% and that bought both tables into semi-check

[smokeshow]

I've never encountered this myself, but it looks like the torque reserve that the PCM builds to counter the compressor load is far too aggressive for your modifications. I'd bet this is more evident because of both high AC compressor load due to IAT/lack of pressure torque compensation and the cam/T56 swap. You might look over a manual P01 car's toon file and bring over the AC torque/inertia cals so they better match your combo. A C5 vette would be a good option. And if you're using the AC pressure signal input for other things, I'd make sure the diagnostics for those circuits are set appropriately as well.