RPM occasionally fluctuates with AC Compressor on

[0-60motorsports]

I ordered Eldor ones because they were consistently very highly regarded across a few forums. I know Bosch is an industry standard, nothing wrong with them, but folks were actively singing the praises of Eldor versus saying "Bosch are fine too" so I'll give Eldor a shot.

Ok sure try them out. My experience will eldor hasn't been great so I just switched all the coils in my M3 and X5M to Bosch.

[Andratch]

-- UPDATE --

Installed the new NGK spark plugs and Eldor coils.

The fluctuation is still there, but I'm able to "throttle blip" out of it better. Before, I could blip the throttle and it would immediately start fluctuating again. Now, I blip the throttle and it'll stay steady, at least for a bit. So, partial improvement. But this does lead me to believe I'll see more improvement from the O2 sensors later today. Bry5on agrees that it's having a hard time settling into the right spot on the map, perhaps due to old O2 sensors giving crappy readings.

[dukeofchen]

Tested AC pressures -

COMPRESSOR RUNNING:

LOW: 32 psi
HI: 200 psi

COMPRESSOR OFF:

LOW: 50 psi
HI: 150 psi

As far as I know, this is within normal ranges.

Purely anecdotal, but over the summer I decided to throw caution to the wind and fuss around with refrigerant instead of taking it to an expert.

In the process I technically overfilled the system if one were to go by the standard ambient temp / PSI charts, and interestingly found that not only were the idle dips gone when the compressor kicked on (which I expected starting from an underfilled state) but that the engine generally felt smoother/happier across the board. My car is stock tune / intake btw.

Don't remember the exact readings but this was at our peak summer ambient temps of 90-100F, and while the high side readings were high they weren't near blowout territory. Low side was just slightly north of 35PSI which is where the system seems to regulate itself to.

I'm far from understanding the intricacies of AC, but I wonder if at 20 years / 190k miles my compressor is just worn to the point of creating higher drag on the system, which is further exacerbated by low charge.

Not an endorsement of overfilling the system – just a thought.

[Andratch]

I replaced the Pre-Cat O2 sensors this afternoon, no change. Still fluctuating. I'm working with Bry5on to see if there are tweaks to the mullet tune that can figure this out, but frankly other than that I'm getting low on ideas.

My only other thought, kind of around dukeofchen 's line of thinking, is that perhaps since this was a Canadian car, the AC was rarely used - and as such, it needs to be flushed and recharged. I suppose when I had the AC belt off, I should have enaged the compressor to see how it spun. It didn't feel "chunky" when I spun it by hand with the car off and compressor disengaged, but maybe that's not a good enough test.

[0-60motorsports]

Some real CSLs do this too and many of them dont so that leads me to believe there is another mechanical component that is causing this issue. I never for once believed it was the tune as all the CSLs running the same factory tune and SOME of them do this and MOST dont.....hhhmmmm

[Andratch]

Some real CSLs do this too and many of them dont so that leads me to believe there is another mechanical component that is causing this issue. I never for once believed it was the tune as all the CSLs running the same factory tune and SOME of them do this and MOST dont.....hhhmmmm

I would tend to agree. What I'm curious of is -- are the ones that experience this using MAP sensors and the ones that aren't experiencing this on Alpha N? It seems like there are more folks that just buy the intake and a Alpha N tune than do it "properly" with the upgraded ECU and MAP sensor.

At this point, I've replaced NEW spark plugs, coils, ICV, O2 sensors, done a vacuum test, checked AC pressure....I can't really think what else it could be. The only thing in my mind that's left is replacing the AC compressor, but then again, this fluctuation doesn't happen 100% of the time, so it makes me feel like there's something 'triggering' it. But I don't really suspect the AC pressure switch because the compressor clutch is engaged both when it's on and stable, and on and fluctuating.

Bryson's next suggestion was to clean the injectors, suspecting a fueling issue. He's generously reviewing my latest log with all the lambda integer values as well.

[0-60motorsports]

I would tend to agree. What I'm curious of is -- are the ones that experience this using MAP sensors and the ones that aren't experiencing this on Alpha N? It seems like there are more folks that just buy the intake and a Alpha N tune than do it "properly" with the upgraded ECU and MAP sensor.

At this point, I've replaced NEW spark plugs, coils, ICV, O2 sensors, done a vacuum test, checked AC pressure....I can't really think what else it could be. The only thing in my mind that's left is replacing the AC compressor, but then again, this fluctuation doesn't happen 100% of the time, so it makes me feel like there's something 'triggering' it. But I don't really suspect the AC pressure switch because the compressor clutch is engaged both when it's on and stable, and on and fluctuating.

Bryson's next suggestion was to clean the injectors, suspecting a fueling issue. He's generously reviewing my latest log with all the lambda integer values as well.

I've done all this and it's still F'ing there lol. Maybe a component in the dme failing….we will never know until we know when someone discovers it 10 mins later or 10 years later.

My car was an alpha n and then I went to full or CSL setup with flap and it was fine and then it started doing this one day out of the blue many years later. So failing mechanical or electrical component for sure, we all just don't know where and what.

[Slideways]

I would tend to agree. What I'm curious of is -- are the ones that experience this using MAP sensors and the ones that aren't experiencing this on Alpha N? It seems like there are more folks that just buy the intake and a Alpha N tune than do it "properly" with the upgraded ECU and MAP sensor.

At this point, I've replaced NEW spark plugs, coils, ICV, O2 sensors, done a vacuum test, checked AC pressure....I can't really think what else it could be. The only thing in my mind that's left is replacing the AC compressor, but then again, this fluctuation doesn't happen 100% of the time, so it makes me feel like there's something 'triggering' it. But I don't really suspect the AC pressure switch because the compressor clutch is engaged both when it's on and stable, and on and fluctuating.

Bryson's next suggestion was to clean the injectors, suspecting a fueling issue. He's generously reviewing my latest log with all the lambda integer values as well.

Does this issue impact drivability?

[0-60motorsports]

Does this issue impact drivability?

Not for me since it's only when I'm at a stop, if I flick off the ac compressor by pressing the button (ac blower fan is still running) immediately the fluctuations are gone and the M3 is sitting there on a stable idle. So so strange and that's because no one and I mean no one has been able to resolve this issue even on real CSLs LOL.

[Andratch]

I've done all this and it's still F'ing there lol. Maybe a component in the dme failing….we will never know until we know when someone discovers it 10 mins later or 10 years later.

My car was an alpha n and then I went to full or CSL setup with flap and it was fine and then it started doing this one day out of the blue many years later. So failing mechanical or electrical component for sure, we all just don't know where and what.

Perhaps I should ask Kassel to swap ECUs with me as an exercise, and see if that does anything? I'll see what they say.

Does this issue impact drivability?

Yeah, in two ways. First, since I have SMG, if it's fluctuating and I touch the throttle, it "drops the clutch" pretty quick because the rpm is up at 1200 and it thinks I'm trying to do a bit more of a launch. So that can be juttering.

Second, it's horribly embarrassing. Sitting there with the idle revving up and down constantly...not a good look with the wife, clients, in public areas...

It's not a "screw this I'm selling the car" impact on drivability, but it's just not ideal for someone as OCD as me.




I uploaded my latest datalog from TestO if anyone wants to take a look.

[heinzboehmer]

First, since I have SMG

Just thought of something. All the local cars over here with CSL airboxes don't do this, but they're all manual.

Does your car do this with the SMG in both neutral and D?

[0-60motorsports]

Just thought of something. All the local cars over here with CSL airboxes don't do this, but they're all manual.

Does your car do this with the SMG in both neutral and D?

Yes to your question.

As you say ALL but its not ALL it probably MOST. They'll either do it sooner or later or not at all. It is the issue causing this, mech or elec failure of a components. maybe even degradation of wiring.

[Andratch]

Just thought of something. All the local cars over here with CSL airboxes don't do this, but they're all manual.

Does your car do this with the SMG in both neutral and D?

I’m SMG too, does it in both positions.

[Slideways]

I’m SMG too, does it in both positions.

Have you tried recharging the A/C system? Outside of that, you've replaced everything but the compressor.

[Andratch]

Have you tried recharging the A/C system? Outside of that, you've replaced everything but the compressor.

I haven’t recharged it, but I tested the low/high pressures and they were “in spec”