RPM occasionally fluctuates with AC Compressor on

[Andratch]

The AC in my 05 Coupe works great - blows ice cold. However, frequently when I’m idling with the compressor on, the RPM will fluctuate/rev up and down between 800-1200.

I don’t notice this when I’m in gear - just idling. It doesn’t happen all the time, but it happens quite often. The AC is very cold while this is happening.

as soon as I press the AC button, the fluctuation stops and it sits steady at 800 rpm. When I turn the AC back on, it will start fluctuating again over about 10 seconds.

But oddly, I’ll drive for a bit, come to another stop, and it will idle smoothly!

I have a cheap refill bottle from Auto Zone with a digital gauge that reads about 36 lbs, but who knows if it’s accurate.

Any ideas on what to check would be appreciated! Video sample below.

https://imgur.com/a/t7U6sYm

[cobra]

I think that's normal. It goes from being under load from the A/C and then you disengage it and the load is suddenly gone, but the throttle was slightly open to maintain idle RPM, and it surges until it restabilizes at idle. Mine does a similar thing.

[Andratch]

I think that's normal. It goes from being under load from the A/C and then you disengage it and the load is suddenly gone, but the throttle was slightly open to maintain idle RPM, and it surges until it restabilizes at idle. Mine does a similar thing.

That's backwards. It fluctuates with the compressor on....for minutes straight. But instantly stabilizes when I turn the compressor off. Perhaps an idle control valve?

[Amos]

Mine does this too but only once every few minutes, not every 10 seconds. I have cleaned my ICV before and it didn't help.

[dukeofchen]

Mine does the reverse, where the RPMs dip for a few seconds when the compressor kicks in. While it makes sense that this would happen due to the sudden change in load, the accompanying engine vibration and magnitude of the RPM dip in my case seems a bit excessive, as if the car is needing a second to compensate.

In my case the A/C blows cold-ish, but compared to the E39 we have it feels like it should be blowing colder. Having never serviced the A/C I suspect that some refrigerant has just slowly leaked over the years, so I'm planning to get the system evacuated, then test for vacuum and refill to spec by mass. My understanding is that when the system is low on refrigerant, cavitation can occur in the system due to an excess of easily-compressible air in the system, introducing an interval where the compressor is building up to the proper pressure. Because of the delay, there's an abrupt change in the flow rate of the system once the proper pressure is reached, and a corresponding spike in load on the engine that drags down RPM.

I haven't validated this theory yet with the evac / refill, but it sounds like your car is doing the reverse. I wonder if there's a chance your system is overfilled?

[0-60motorsports]

Mine does the same and im full OE CSL converted. NO one has found a solution yet but for non CSL or non Alpha N cars it should be solvable.

[sapote]

In my case the A/C blows cold-ish, but compared to the E39 we have it feels like it should be blowing colder. Having never serviced the A/C I suspect that some refrigerant has just slowly leaked over the years, so I'm planning to get the system evacuated, then test for vacuum and refill to spec by mass. My understanding is that when the system is low on refrigerant, cavitation can occur in the system due to an excess of easily-compressible air in the system, introducing an interval where the compressor is building up to the proper pressure. Because of the delay, there's an abrupt change in the flow rate of the system once the proper pressure is reached, and a corresponding spike in load on the engine that drags down RPM.

20 years without refrigerant refilling and still this good, then I wouldn't waste money on doing system evacuation as this can be worst than what you have. Just add some refrigerant properly by checking the high side pressure with AC on.

there is no air in the system unless there is a big leak that all pressurized refrigerant had escaped and air entered. The refrigerant in the system is always exist in two phases: liquid and gas at any moment.

[Andratch]

Mine does the same and im full OE CSL converted. NO one has found a solution yet but for non CSL or non Alpha N cars it should be solvable.

Aaaaah interesting. I am also a OE CSL convert. I don't remember if it did this before or not, because I did the conversion immediately after purchasing. I didn't do Alpha-N, though, I did the IAT sensor + Kessel tune.

[0-60motorsports]

Aaaaah interesting. I am also a OE CSL convert. I don't remember if it did this before or not, because I did the conversion immediately after purchasing. I didn't do Alpha-N, though, I did the IAT sensor + Kessel tune.

I have the HTE tune with MSS54HP conversion and all oe MAP sensor and OE CSL rail, OE CSL intake with working flap etc

[Andratch]

I have the HTE tune with MSS54HP conversion and all oe MAP sensor and OE CSL rail, OE CSL intake with working flap etc

I have OE CSL tune, HP Conversion, OE MAP, Turner Intake, No Flap.

[Arith2]

Get an HTE tune.

[LSB4Me]

FWIW, I have an Evolve airbox, HP ecu from factory (04 car), IAT + MAP sensor setup, and HTE non-dyno tune … idle surges as OP describes with AC on at rest.

It eventually goes away (I think) but I chalk it up to increased airflow.

[L0okitzRaj]

What this sounds like to me is that the a/c refrigerant is low or the sensor is bad. The compressor won't run if there isn't enough pressure, so it goes on and off as the sensor feels pressure, then doesn't. Creating the load/unload of the rpm.

Can you hear the compressor turn off/on or a relay when the rpm is fluctuating?


this has happened to me in two e46s, both needed more refrigerant

*edit, pretty much what the dukeofchen wrote.

[Andratch]

What this sounds like to me is that the a/c refrigerant is low or the sensor is bad. The compressor won't run if there isn't enough pressure, so it goes on and off as the sensor feels pressure, then doesn't. Creating the load/unload of the rpm.

Can you hear the compressor turn off/on or a relay when the rpm is fluctuating?


this has happened to me in two e46s, both needed more refrigerant

*edit, pretty much what the dukeofchen wrote.

I do not hear any relays clicking. I’ll try to get a flashlight on the compressor next time it does it to see if the clutch is kicking on and off.

which sensor are you referring to that could be bad?

[0-60motorsports]

This is not necessarily a freon issue! I have had my car recently flushed and refilled but its the same as always. There IS an issue on the intake converted cars and REAL CSLs, no ones been able to find the solution yet LOL.