993RS

Why does the 993 run less smoothly at low revs when the a/c is on?

CalvinC4S

I believe the 993 idle speed is bumped up when the A/C is switched on normally due to the additional load.

If you are seeing this, it may be harmonic and or balance at that RPM.

If not, it may be an aftermarket chip.

gmorat

Same for me. When the A/C compressor clutch engages the engine idling gets lumpier.

doublecabmel

Also check the freon level

x50type

Mine idles as smoothly as ever.

993RS

Thanks, but it is OK, and my 993 has always done it since I bought it 10 years ago...

pp000830

I suspect the DME looks at the engine rpm drop from the compressor load and adjusts the Idle throttle to compensate. The added load causing higher input torque pulses from each cylinder firing needed to maintain the specified idle rpm works counter to the flywheel inertia that encourages the engine to rotate smoothly. It probably also causes the DME programmed throttle increment adjustment made to maintain idle to slightly under and over shoot the specified idle rpm value causing what may be perceived as rough idle as it tries to regulate the RPM.
A guess from a non-automotive guy. Any others' thoughts?

993RS

Apparently very few have this problem...

Kika

Mine actually seems to run a little bit smoother with the AC on.

PRSWILL

^^ same here^^ with the AC off, I have a little rumble in the idle- with it running the RPM's go up to about 750 and it smooths out really nicely.

gmorat

Mine idles at the same speed its just a little lumpier when the A/C engages.

timothymoffat

Mine idles smoother with AC engaged as well.

CalvinC4S

I'm curious what re-adapting the idle memory in the DME/ECU with the AC on would do.

TMc993

My engine idles at 750 RPM with AC off and increases to 850-900 with AC on. The idle is equally smooth at both speeds. I always assumed that turning on the AC switch activated a slight advance in ignition timing at idle so as to compensate for the additional load of the compressor.

993RS

The funny thing is that driving at low revs with the AC on, the car is rougher, not as smooth as when it is off...