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AC blows hot when idle, car overheats

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Nov 29, 2011, 5:07 AM

Post #1 of 8 (5210 views)
AC blows hot when idle, car overheats Sign In

When my car ('04 Suzuki Aerio) is idle, the compressor cuts off and the AC blows hot. Shortly after reaching about 2000rpm, it'll turn back on and it'll blow ice cold. This is related to the RPM and not the car moving, as it also does this while parked in neutral. If too much time passes with the compressor off, the engine temperature light goes on (the instrument panel on the '04 is digital), even when the radiator fan is on (the AC fan is also on, ie both fans are spinning). As soon as the compressor kicks on again, temperature light shuts off. All this makes me think all the heat is coming from the compressor.

This is just another thing on the list of AC issues I've had in the past few months. Previously it would typically blow warmer in the afternoon than at morning/night (which was solved on adding some refrigerant); a professional gauge indicates that the pressure is fine so long as the compressor is on.

Also, previously the AC would typically only work (and rather well) whenever it wanted, usually when the car was in motion (revving in neutral had no effect here). I thought it was an electrical problem, and was able to solve this by replacing the compressor relay. This was about a week ago, before my current problem first appeared.

Of note, about a year and a half ago my serpentine belt broke, and when I had it replaced the mechanic insisted I also needed a new belt tensor. After that job the belt would squeak whenever the car was in anything but P/N; the tensor turned out to be fine, and the problem solved itself when a smaller belt was placed on the car. I mention this because of the possibility of the problem being the belt on the condenser (although I've had no problems with the belt for about a year).

The condenser on the car was rebuilt a little less than a year go.

Any help would be greatly appreciated. Thanks in advance.

Tom Greenleaf
Ultimate Carjunky / Moderator

Nov 29, 2011, 5:28 AM

Post #2 of 8 (5185 views)
Re: AC blows hot when idle, car overheats [In reply to] Sign In

Quote">>The condenser on the car was rebuilt a little less than a year go.<<"

Condensors don't get rebuilt nor have belts so guess you mean compressor there. You need to know the pressures while compressor is engaged at what temp and RPM not just someone saying one is OK?

IDK - if a reman compressor was used they have a nasty failure rate, debris in system may extrordinary causing erratic behaviour. Cooling system and engine's radiator heat may be back/radiant heating condenser and HPCO shuts system down.

I'd first have system evacuated to well held vacuum and filled exactly to spec then take observations but as said, engine heat needs be addressed also,

Long retired now


Nov 29, 2011, 5:47 PM

Post #3 of 8 (5165 views)
Re: AC blows hot when idle, car overheats [In reply to] Sign In

I wouldn't have used a reman unit.
They fail a lot as Tom said.

Your only as good as your tools!

New User

Nov 29, 2011, 7:29 PM

Post #4 of 8 (5158 views)
Re: AC blows hot when idle, car overheats [In reply to] Sign In

Sorry, I did mean compressor instead of condenser. As for new vs. reman, I didn't have a lot of say in the decision at the time as I wasn't involved in the repair process nor did I pay for it; I'll keep this in mind for future occasions. However, if the actual compressor was the problem (as opposed to tension in the fan belt or the clutch or something), wouldn't it consistently never work rather than sometimes?

Today I drove the car without ever turning the AC on and it eventually overheated anyway. Is there a difference in refrigerant pressures/engine thermodynamics between AC off and AC on with an inactive compressor? It could well be that there's too much refrigerant, since the day after adding the refrigerant up until switching the compressor relay the AC would always blow ambient temp (however, the car never overheated). I've been putting off a professional refrigerant vacuum/recharge as I didn't want to incur that cost only to have the problem be something else.

I did notice a new symptom that puzzled me: the condenser fan runs with the AC off. From what I've seen it doesn't happen when the car is cold, and I've only ever seen it doing it when the radiator fan is also running. Combined with the results I got from switching the compressor relay, it's looking more and more like an electrical problem (an assessment with which the mechanic I consulted with today agreed).

I'm having an electrical mechanic take a look at the car tomorrow. I'll keep you guys updated on any further developments. Thanks for your input.

Tom Greenleaf
Ultimate Carjunky / Moderator

Nov 30, 2011, 1:56 AM

Post #5 of 8 (5145 views)
Re: AC blows hot when idle, car overheats [In reply to] Sign In

OK - Hope I read you right. Take care of overheating first as A/C is just making that harder when normal to keep engine cool. Both engine and A/C are or can be very expensive but no engine leaves you walking!

Not unheard of is fans blowing the wrong way for whatever reason AND unencumbered air flow thru both condenser and radiator - look for debris hidden between them if you suspect it.

May need flushing out cooling system, new radiator, or more but a must fix no matter what if you want the vehicle.

Note on reman compressors: Said - high failure rate and not always much more for new. When or if one throws debris it costs more work, $$ and time to fix. I seriously doubt a compressor itself makes heat enough to overheat engine but they can break belt(s) , seize. Keep in mind they are two things, a compressor when operating and bushings are the bearings - when off it's just another idler pulley, compressor just dead weight and bearing in the clutch is really all that's needed for belt to operate. When a compressor is noisy it will probably fail so best unplug it at once and tie away the plug end out of harm's way or defeat it via relay or fuse.

They routinely come on/engage with defrost or defog request so just not using one by setting isn't enough. Condensers are even higher efficiency and several can't be flushed out effectively so can take that with it. Stinks but if you don't do it or have it done right the new stuff fails again and back at best to square one or needing more than doing it right the first time. Do understand that compressors are lubricated by carrying oil in a properly charged system and run dry if too low and can choke if too much refrigerant so very important to know the charge by weight or high end knowledge of pressures under what conditions - see/read top locked thread here for what it takes just to charge up a working system! Scary!

Long retired now


Nov 30, 2011, 11:03 AM

Post #6 of 8 (5129 views)
Re: AC blows hot when idle, car overheats [In reply to] Sign In

Do your cooling fans work?
When you turn on the A/C do any of the fans come one?

Your only as good as your tools!

New User

Nov 30, 2011, 4:34 PM

Post #7 of 8 (5120 views)
Re: AC blows hot when idle, car overheats [In reply to] Sign In

Latest theory: it's both a heat and an electrical problem.

When the car was in the shop the mechanic noticed two things:

1) My radiator fan only has two cables but is hooked up to a connector with four cables, two of which are currently not connected to anything. I was aware of this, but never made much of it.

2) The radiator was low on coolant. After topping it off, during a day's worth of driving all of the symptoms magically disappeared (no overheating, AC working perfectly).

The radiator fan was replaced a few years ago (another job I wasn't much involved in). Mechanic thinks that a generic fan was purchased to save some money, and the missing wires are meant to modulate the fan running at various speeds; and while the fan DOES run without the missing input it may not run at the lower speeds, causing the engine to heat up more than it should.

In this car, the radiator and AC condenser are next to one another. Perhaps the excess heat from the radiator was dissipating into the condenser, making the AC not run except when the fans kicked into overdrive (ie, RPM > 2000)?

We also noticed that the condenser fan or the fan clutch may also need replacing, as it's not generating much air (certainly much less than the radiator fan) and there's a slight wobble to it as well as not locking up when it's not running (ie, when the AC/car is turned off it can take a full 20-30 seconds for the condenser fan to stop spinning, and spins freely when turned by hand).

At any rate, I've gone ahead and ordered an OEM radiator fan motor, hopefully having all the appropriate inputs will solve all my issues. If not I'll move onto the condenser fan next.

Thanks for all the help!


Nov 30, 2011, 5:17 PM

Post #8 of 8 (5112 views)
Re: AC blows hot when idle, car overheats [In reply to] Sign In

Great, It was a good idea to order the OEM fan.
Keep us posted.

Your only as good as your tools!

AC blows hot when idle, car overheats

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