Could this be right? Fast idle to run A/C

78silver · 05-23-2007, 11:47 AM

I have to have my idle speed set at about 1250-1300 RPMS otherwise my '78 SC dies when I come off the gas and the RPMs drop. I've played with the idle adjustment quite a bit and I can't seem to go any lower than this, otherwise, I'm constantly stalling at stops. When I don't run the A/C, I can keep the idle around 950-1000 and all is well...

Others have told me through this forumn that it's typical to have to increase idle speed a bit to compensate for the added pull of the compressor but this seems high to me?

Anyone else have this problem?

Greek55 · 05-23-2007, 12:30 PM

I run mine at around 1100-1150 during the summer and lower to 900 when not using the a/c.

JBO · 05-23-2007, 12:55 PM

I do the same as Greek55. Raise idle to around 1,200 when using the A/C, and this takes care of the problem. And as someone told me a few years ago, this will not harm anything, as when you are driving you are running a min of 2,400 rpm (or at least should be).

ipacketeer · ipacketeer's Garage

Mine idles 900~950 with the A/C on.

ossiblue · 05-23-2007, 04:59 PM

Can't speak for P-cars because none of mine have AC, but most other cars have an automatic increase in idle speed when the AC kicks on. I know on my other cars, at idle, I can hear the AC solenoid click and the idle speed increases as the AC clutch engages the belt. If I sit long enough, it will click again and the idle speed drops. Sounds like Porsches don't have that automatic feature (?) so, logically, your idle would be lower with the AC engaged as you describe. From this thread, it sounds like most AC cars have the idle manually adjusted to compensate. Is this correct or is there an automatic idle speed adjustment that is malfunctioning?

schoward · 05-23-2007, 05:14 PM

Had same problem at one point following a rebuild and my wrench played with timing and idle speed to get my SC to idle at 950-1000 with drop to maybe 850-900 with A/C on. Car was stalling at sudden decel with AC on. Trying various combinations of timing and idle speed corrected it.

In general though over the years, have had little problem having idle 900-1000 with slight drop with A/C (800 or so) and no stalling.

If you don't have decel valve hooked up, that may be a factor - combined with timing and idle speed.

1350 sounds high to me (and like it would be a bit annoying). I would try to correct.