Starting A/C kills engine

78silver · 08-03-2006, 06:52 PM

Hi, after years of dreaming, I'm finally a Porsche owner with a '78 911SC. 70K original miles and in excellent condition cosmetically and mechanically. So far I'm very happy with the care put into it by the previous owner. When I purchased the car just a few weeks ago, the A/C system had just been completely "redone". However, anytime I try to crank on the system, the engine dies. It seems like the idle boost is not sufficient to sustain the system. If I let the RPMs down easy, it seems to "hold on" and the system remains running but if I tap the gas, RPMs drop hard and engine dies... again, only when the A/C is on. I've been asking around and tinkering but can't seem to figure out what types of adjustments (if any) I can make to adjust the idle boost. Any suggestions would be greatly appreciated! For what it's worth, I had a friend/mechanic (unfortunately not a Porsche expert) check the refrigerant and he said it looks fine. Frankly, I've never had the A/C running long enough yet to see if it will really get cold. Mildly cool is about as close as I've gotten.

ruf-porsche · ruf-porsche's Garage

Could be that there is too much freon in the system. The original York compressor is notorious for taxing the engine at idle compare with a more modern compressor such as a sanden or a seiko seiki.

Motorhead-45 · 08-04-2006, 02:29 AM

There isn't an idle boost circuit for the air conditioner. What is the idle speed of your car with the a/c off? Perhaps a simple increase in your idle speed adjustment is all that is necessary. I set mine to 1100rpm during the summer to compensate for the increased pull of the compressor and to promote more airflow across the condensor when the car is at a stop.

oneblueyedog · oneblueyedog's Garage

You also may be running a bit rich in mixture.

78silver · 08-04-2006, 06:14 AM

Thanks for the input guys! I let a little bit of freon out and also found that I had a small air leak at the base of the hoses behind the airbox housing... snugging these up seems to have helped a little. I also increased idle speed to right at 1000 and that helps a bit too. All in all, the compressor is still killing the engine when I let off the accelerator quickly. I'll try to do a little more tweaking and see how I come out. Again, thanks to everyone for their suggestions!

'78SC Silver Coupe

JBO · 08-04-2006, 06:20 AM

I do what Motorhead does and it solved the same problem with my SC.