Dear Car Talk:

I have a 2009 GMC Envoy. It looks and runs fabulous and has less than 40,000 miles on it. On a recent long driving trip, the air conditioning blew lots of cold air at first. But it would become less cold as the miles wore on. If I stopped to buy gas for 15 minutes, when I started driving again, it would blow lots of cold air.

If I drove for a long enough period, it would not just blow less cold air, but would sometimes start blowing mist out the vents. I could get the mist to stop by turning the temperature all the way to hot, but that wasn't so comfortable.

The dealer could find nothing wrong. What do you think? - Charles



I think you should stash a half dozen Axe Deodorant sticks in the glove box, Charles.

I suspect you have a failing air conditioning compressor clutch. The compressor clutch is the electro-mechanical device that turns your AC compressor on and off. The AC compressor will normally cycle on and off while it's in use, depending on the demand for cool air. And the clutch is what starts and stops it.

So if the compressor clutch is faulty and failing when it gets hot (like after a multi-hour drive), it would essentially turn off your AC. That would pretty well explain the lack of cold air, huh, Charles?

Plus, while the AC's not working, warm, moist air enters the cabin and builds up around the AC evaporator under the dash. Then, when the clutch DOES kick back (randomly, or after a rest), that warm, moist air will hit the newly cold evaporator and make fog. Just like what happens around the Golden Gate Bridge most evenings.

If your Envoy had come with the theme music package, Tony Bennett would've come on the radio singing "I Left My Heart in San Francisco" as the fog poured out.

Your dealer found nothing wrong because he didn't drive it for five hours like you did and really heat up the compressor clutch, Charles. And since he's unlikely to be willing to take a five-hour test drive in your 10-year-old AC-less truck, your best bet is to diagnose this yourself.

Bring your mechanic a cup of coffee and a couple of donuts one morning and ask him to show you where the AC compressor clutch is. Then, by turning the AC on and off, he can show you what the clutch looks like when it's engaged, and what it looks like when it's not engaged. Take a little video on your phone for reference.

Then, next time you're crossing Death Valley and you feel warm air coming out of the vents, hop out, open the hood and see if the compressor clutch is engaged or not. If it's not, you've diagnosed your own problem, Charles.

That'll make you feel so good about yourself that your five-hour drive back home without AC will feel like a mere four and a half hours.