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Discussion Starter · #1 ·
Have an odd A/C problem, that I was hoping somebody had seen before.

Outside temp < 95F A/C works fine.
Outside temp > 95F A/C blows cool air, maybe ~75F in bursts, with hot, humid air in between. It won't ever get cold, as long as outside temp is in the 90s.

Carmax did a leak test, pressure is fine, compressor is running. Of course, since they tested on a cooler day, there was "no trouble found" and want to charge me a diagnostic fee and another to test it again.

2000 M Roadster

Thanks.
 

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eswimm said:
Have an odd A/C problem, that I was hoping somebody had seen before.

Outside temp < 95F A/C works fine.
Outside temp > 95F A/C blows cool air, maybe ~75F in bursts, with hot, humid air in between. It won't ever get cold, as long as outside temp is in the 90s.

Carmax did a leak test, pressure is fine, compressor is running. Of course, since they tested on a cooler day, there was "no trouble found" and want to charge me a diagnostic fee and another to test it again.

2000 M Roadster

Thanks.
Most likely its cutting out under high head pressure, Head pressure is quickly affected by outside temps and in your case, when it gets humid in the car is whne the comp. cuts out from headpressure building and the blower is still blowing across the evap which is coated with water from moisture condending on it..
Since it was leak tested I am assuming it is not low in r134a, if it was low the same thing could happen but the low pressure switch would cause it to cycle.
You either have
1. bad fan clutch
2. bad electric fan or not always comming on when it should
3. to much refrigerant
4. to much oil ( a few ounces over spec is to much)
5. waterpump going out and the higher radiator temps are spiking the condesor coil and causing a high head pressure
6. a weak compressor that has been overcharged to compensate for the poor cooling( basically its overcharged but that is the only way to get a weak comp. to cool at all)

There are other things but they would not be affected by outside air temps
 

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My car is the same exact way. Just had to get used to it. There are a lot of people complaining about our A/c system.
 

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Or German engineering where they have never seen high temps. :)

A leak test doesn't tell you the state of the charge (high or low). That is the first thing I would have checked.
 

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Discussion Starter · #5 ·
Pinecone said:
Or German engineering where they have never seen high temps. :)

A leak test doesn't tell you the state of the charge (high or low). That is the first thing I would have checked.
When I checked the low side, pressure was 56-58PSI. Air temp was around 95-100F at the time, car had been sitting in the sun.

Carmax says the low side was 35PSI and high 142PSI when they tested, it was a cooler day and I have no idea how long the car had been sitting in their garage, so obviously much lower temp.

I've been able to narrow down the problem a little more. When the RPMs drop, the A/C cuts out as soon as I get back on the gas. So, if I'm cruising at constant speed, the A/C blows cooler air. If I slow down for traffic, then accelerate again, I'll get a burst of hot/humid air. The hotter it is outside, the more pronounced the effect is. In stop and go traffic on a hot day (95F), the car is a sauna. Even cruising at highway speeds, the car will never get cold inside, on a hot day.

If temps are in the low 80s, the A/C works perfectly and the car will get uncomfortably cold.

Is 55-60PSI too high and if so, is it worth just letting a little refrigerant out?
 
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