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Hot Air 86 Monte Carlo SS

I have a 1986 Monte Carlo SS with 305 HO engine. Recently when driving along suddenly I have hot air blowing out of the vents, then in a few minutes it goes back to the cool air. This happens whether it is set to vent or air conditioner. How can I fix it??

Sounds like you have a vaccum leak. Start checking from the engine to the dash. You can also do a simple test. If the AC goes off while driving just lift your foot off the accelerator suddenly. If the AC comes back then it’s definitely a vaccum leak.

Does the air continue to blow out of the dash vents? And by hot, do you mean ambient temperature or heater on temperature? Maybe the problem is the AC system.

The air continues to blow out the dash vents. it is ambient temp.

It would be worthwhile to get the pressures checked. The car has a CCOT, ‘cycling clutch orifice tube’ control system and the low pressure switch on the accumulator can get sticky while some models have a high pressure cut out switch that can turn off the compressor when the high side exceeds 300 which can occur when driving slowly in stop and go traffic, especially if the fan clutch is getting weak. Is there any pattern to the problem?

Yeah what others said or also you could have a bad HVAC module-available at junk yards.

There is no pattern. It happens at high or low speeds. The fan clutch has not been replaced in quite some time. I will have the pressures checked. Thanks

It does sound like an electrical control instead of a low refrigerant problem to me, especially if the air is nice and cold and stays that way for some time before it cuts out. This is an older system using R12, correct? If it is low, it’s going to cost an arm and a leg to top it up, but if it doesn’t have any significant leaks, it would be better to top it off instead of converting it to R134a in my opinion. Has it been recently charged? Actually having too much refrigerant in the system can cause problems like this too.

Above 45 mph the fan clutch has no effect on AC performance.