Hello all, my 99 C2 has an odd AC issue and seems to be getting more prevalent.
The AC works great at idle with recirc on or off. When at speed it no longer stays cold even with the recirc selected. I can cycle the switch a couple times and it’ll get cold for a couple mins but doesn’t last.
The pressure is good, I can hear the relay engage and the compressor engage.
What else in the system could cause this issue?

Thanks in advance!

 

Are you sure about that..? What is the suction line pressure at 750, 1500, 2500rpm...?

Sounds like its low on freon to me.

At idle compressor is spinning slow. With some Freon in system slow spinning at idle compressor is not able to pull down the low side pressure below pressure switch low limit cutout. So system runs and produces some cold air. Once the engine rpm goes up the compressor can suck down the low side pressure below cutout switch limits and compressor clutch disengages till pressure rises again above cutout limits that will only happen when engine/compressor speed slows enough to maintain pressure above cutout point.

Not sure of the exact cutout and differential psi for our 996. For many cars using r134a cutout is set around 18-21psi and cut in or differential is 10psi above that.

 

From experience get it to an AC professional. They will pressure test, evacuate the system, refill with the proper amount of freon and compressor oil. It takes a professional AC guy with the right equipment to get this down correctly.

 

I actually had it to a shop to be evaluated. Sounds like they checked the pressures but did not evacuate the system. They didn’t do it because the pressures were good and the AC was cold when testing. They did not identify the issue while at speed.
Maybe as the other member mentioned, it’d be worthwhile to get it evacuated and re serviced properly.
Allcool, I also appreciate the education on the AC system.

 

Assuming AC charge is good.
All blend doors operating properly, no failed or missing foam issues?

 

Weird problem. When is the last time you cleaned the radiators?

 

One other thought, when AC is engaged do the low speed radiator fans run? On both sides? Something to check. I live in Florida and after a good run I park in my garage and the fans are on high. Sounds like a hovercraft!

 

I live in the Bay Area where it's expensive and even then it only costs $150 for an AC shop to evac, pressure test, and recharges the AC on a European car. Really, why bother with shot gun diagnostic. Pay for the service and they will tell you what's wrong with the car if anything. Or they will just say it's a little low and all good now.

I had to troubleshoot an Audi B6 S4 AC system where it was intermittent like this. I got it down to the "AC compressor needs replacement." But the job on the B6 Audi is so involved and expensive that I wanted a second AC pro opinion so I paid them $150 to essentially tell me it's the valve on the AC compressor. Money well spent.