2001 Suburban and 2008 Mazda 6. Air condition works fine, blows nice and cold. At idle at a light the fan doesn't change but the air becomes noticeably warmer.

The Mazda 6 has done it only 3 random times during the entire summer. The Suburban is becoming more consistent, and is starting to extend into slow speed (low rpm) driving.

Since the AC works most of the time, methinks this is an electrical issue that shows up through the AC? Weak batter or alternator? Or is it the AC compressor or other part?

They are probably just sensitive to the fact that there is no airflow across the condenser hen you are stopped.

Most likely, there is no problem, it's just annoying.

In reply to glueguy:

It's been HOT in NC this summer. The AC is going to be less efficient if air isn't flowing over the condenser at low speed, so that is my first guess.

Check the condenser fan (it may be separate from your radiator fan) to make sure it is kicking on. Pressure wash the heck out of your condenser. If it is clogged that would hurt airflow, too.

Maybe rig up a small fan to help push air through the condensor. That is definitely the cause, I can get my car frigid at highway speeds...it is nice.

Brett_Murphy wrote: In reply to glueguy: It's been HOT in NC this summer. The AC is going to be less efficient if air isn't flowing over the condenser at low speed, so that is my first guess.

I know that AC will be less efficient in hotter weather, but this is like flipping a switch when it happens. Pull to a stop and air out of the vents becomes warm/hot, start driving again and the ice returns.

Do those cars have a cabin air filtration system? I agree with all that's been said, particularly cleaning out the condenser. When the Outback cabin air filter gets dirty, I notice the AC is not as cool. May be part of what's going on here.

another thought Ive had is that when you are at idle, prolly spinning at less than 1k rpm, the condenser doesnt have that much power to condense the refrigerant, so less cooling ability. Perhaps either the condenser is failing or your refrigerant needs changed/charged (ive heard of sludge buildup?). My Infiniti does the same, just not light-switch as your issue, but a noticeable difference in temp. Im not yet concerned enough to be motivated to do anything about it yet though

Probably what everyone else said. Also it is possible that at an idle the compressor clutch releases just leaving the fan on only. But I would go with what everyone else said.

The condenser air flow issue only applies to engine driven fans. The other factor is that at idle your compressor isn't producing the same BTU's as it does at speed. But if I understand you correctly, your problem goes beyond idle efficiency. My guess is that your charge is low enough that your suction side pressure switch is cutting off due to low pressure. Check to see if the compressor clutch is engaged when this happens. If it isn't, but engages as RPMs come up, then check the pressures with a gauge to make sure you aren't running close to or below 0 psi on the low side.

Low freon level will cause that, as well as a weak compressor. Check to see if the compressor is engaged at idle and the condenser fan is running, if so, but some gauges on it to see if it's low on refrigerant. If not, you've probably got a weak compressor.

Thanks all. The cabin air filters are recently replaced. I'll stare at the radiator this weekend and give the front a good cleaning. I'll also look for compressor engagement but not sure I'll be able to catch it since it works most of the time. I have a sense that it is a dying part or something similar (like slightly low freon or electrical system not able to cycle the compressor at idle) because it is intermittent. It is becoming frequent enough that it got a second mention from wife, which means it's time to start working on it.

16vCorey wrote: Low freon level will cause that, as well as a weak compressor. Check to see if the compressor is engaged at idle and the condenser fan is running, if so, but some gauges on it to see if it's low on refrigerant.

Hmm, where have I heard that before...

It's been a hard summer on AC here in the south. I quit driving the miata during July and August because it just isnt an efficient system in 90+ weather. One advantage of the GTO is that good old GM systems are incredibily efficient. I rarely need the max settings unless the car has been in the sun all afternoon.

Go for a drive, come home, idle it in the driveway and let it get that warm thing going.

Open the hood, look at the compressor, is the clutch engaged? I wouldn't be surprised if it is not.

1 blame is low refrigerant. It leaks out. When it gets low enough, the compressor kicks off. It does this most often at idle.

2 blame is a faulty rpm switch. More common on Japanese vehicles. It's a dohicky to disconnect the compressor if the idle speed gets too low.

Thanks again - fun with AC this weekend!