A/C freon blow off

[SWFLHOMES]

I have been having some unusual A/C issues lately and decided to go to the dealership to see what was going on.

Symptoms:

1) A/C gets cold (drivers side only) when sitting at a stop light but gets warmer while driving.

2) At stop or idle the coolant temp goes up to the next bar above 180

3) As coolant temp rises A/C freon blows off pressure (steam like) out near drivers side wheel well but only when the freon is full.

4) I have the typical servo motor issues and they workk whenever they're in the mood.

I brought it to the dealer. They checked all pressures of the system and balanced the freon. It did not blow off there. They determined that everything was functioning (through their diagnostics) and that the control unit was bad. We ordered a new control unit and they installed it but it did not resolve the mixing flap servo issues or anything thing else. They put the old control unit back in and said that 5 servos are bad by diagnosing them. Why didn't they determine that the first time? I told them that I don't believe the servos can cause a blow off issue. I asked them to check the fans and sensors. They then came back and determined the coolant reservoir tank had a crack which was not allowing the engine to cool correctly causing the temperature to spike and causing the freon to get hotter and blow off? We are not talking red line here, we are talking the temp only going up one bar over 180.

Any thoughts???

P.S. How difficult is it to replace the res tank?

Thanks!

[mudman2]

I have not seen this posted before SWFL, I will sniff around

[SWFLHOMES]

Update:

I replaced a cracked reservoir tank. The coolant temperature is back to normal (a tick over 180). I am in the middle of replacing the temperature mixing servos (I will update that post for anyone attempting a diy). I had the freon topped off but had another blow out tonight. My question is, is there anything in the unit that the servos control that can cause the freon to build pressure? Is there something else that might cause pressure to build?

Thanks

[deilenberger]

Strlctly WAGs, but a plugged expansion valve might cause excessive pressure buildup... and most AC systems have a blowoff valve of some sort.

Question - when refilling the system, was it evacuated with a vacuum pump and then refilled or just "topped up"..?

The right way to fill a system is by weight, after pumping it down with a vacuum pump to remove any moisture in the system. If moisture is in the system, it will tend to freeze in the expansion valve, and might be causing your symptoms. The really right way to refill a system is to replace the receiver/dryer, then pump it down, and then fill it by weight.

[SWFLHOMES]

It was evacuated by a proffessional A/C tech and refilled to the proper spec (pounds of freon). It runs fine and cold but with no warning it will blow off. The ONLY thing I find not right is that the system gets warmer while driving 40+ MPH and colder when stopped.

[wvicary]

The blow off valve is a mechanical system and not related to the blend door servos for the air box. The pressure is excessive causing the system to purge which could be caused by a restriction/blockage, cooling fans not working correctly or the incorrect compressor output. I believe the Cayenne uses a variable displacement A/C compressor which is controlled via the climate control module. I'll try and find out more info and report back.

[SWFLHOMES]

Update. I replaced the coolant res tank. No change in the AC (as expected). I replaced the driver and passanger temperature servos (at least the system isn't confused). FYI, the old servos are made in spain. The new ones are swiss. I have an AC guy looking to see if I have an intermitent fan problem. Can anyone tell me the freon capacity of the system for an 04 CS with NO rear air? It seems the sticker I should have under the hood is not there and the dealer could not confirm the level but thought it was 2.2lbs.

Thanks!