RPM occasionally fluctuates with AC Compressor on

[Andratch]

Interesting point. You could try disconnecting then plugging the line from the brake booster. Maybe when the revs surge it's pulling more vacuum that causes the leak to be of meaningful value?
edit: remember there's a check valve in front of the brake booster, so a smoke test would not identify this vacuum leak if it's there.

This still sounds like a hardware issue to me, and not a tune issue..

My MAP sensor is next to the brake booster for what it's worth.

Yeah my map sensor is in the same spot. I’ll try getting the fluctuations started, pulling the hose off the booster side, and plugging the map sensor housing on the back end.

[Andratch]

Interesting point. You could try disconnecting then plugging the line from the brake booster. Maybe when the revs surge it's pulling more vacuum that causes the leak to be of meaningful value?
edit: remember there's a check valve in front of the brake booster, so a smoke test would not identify this vacuum leak if it's there.

This still sounds like a hardware issue to me, and not a tune issue..

My MAP sensor is next to the brake booster for what it's worth.

Actually one more thought on this - if the vacuum is causing problems, would it not do it ALL the time, and not just when it's in closed loop?

[Bry5on]

Actually one more thought on this - if the vacuum is causing problems, would it not do it ALL the time, and not just when it's in closed loop?

A normal vacuum leak would do it all the time, yeah, but it wouldn’t hunt in open loop, it’d just idle a bit high since there’s enough extra fuel during open loop. It’s not impossible that the leak only shows up when there’s enough vacuum generated given the design of the brake booster. Unlikely but not impossible. This theoretical leak could even be temporary at certain engine RPMs and loads (ie: not leaking at low load AC off)

[Andratch]

Quick update - I plugged the vacuum line after the MAP sensor (which is up by the brake booster), and the fluctuation was still unchanged. Unrelated to that test, I did notice that it fluctuated a few times without the AC on over the last couple days, so I'm not thinking this is tied to that system, but rather just extra electrical load.

At any rate, we found a middle ground at 980 rpm (plus some other tweaks) where when the car tries to fluctuate, it’s able to level out on its own after about 5-10 seconds. I’m going to stick with this for now, and see how it does!

[ac427]

Same boat as well and Ive had a hunch its the MAP sensor location too because if i hold the brake down after starting, it will settle down.

Sadly, the MAP sensor is only a backup device. The barometric pressure sensor in the DME is used for air pressure readings.

[Andratch]

Hey folks, it's been a few months - here's a quick update.

I could not find a hardware-based fix for this. That said, HTE was able to basically tune the problem into relative non-existence. I have not experienced a fluctuation lasting more than 2-3 seconds since February.

[0-60motorsports]

Hey folks, it's been a few months - here's a quick update.

I could not find a hardware-based fix for this. That said, HTE was able to basically tune the problem into relative non-existence. I have not experienced a fluctuation lasting more than 2-3 seconds since February.

Thats great and reminds me to contact HTE to fix mine LOL. But i'll probably do that once i replace the normal M3 engine harness with the OE CSL one.