Why is my car's A/C causing my engine to overheat?

I’m driving a 2004 Kia Optima. The outside daytime temperature has been in the low- to mid-100’s F this week, which I think is the hottest I’ve experienced. I’ve noticed my car’s engine temperature gauge will go into the red zone if I have my air conditioner on for more than a few minutes. If I turn off the A/C, the temperature gauge hovers at about mid-level.

Needless to say, I’ve learned to bring plenty of water for my 70-minute commute.

But I see plenty of people driving with their windows up, so I assume that their air conditioners are working fine despite the heat. What would cause my air conditioner to misbehave?

Your air conditioner, when it is turned on, draws a load from the engine. The engine is working to turn the pulley to drive the compressor and the load is not inconsiderable. A/C causes a significant loss in fuel mileage in most vehicles.

Anyhow, it sounds like your cooling system is not working efficiently. If you’re consistently having to add water you have a leak. You could try the pressure cap on the radiator first, or check the hoses to and from the radiator. Barring that, you might have a leak in your radiator, you might have a bad water pump, it could be any number of things, but it’s almost certainly your cooling system.

Do they still have radiator fans in cars? That sounds to me like the symptoms of a mid-eighties GM car I used to drive when the radiator fan thermostat went bad. On highways it was fine, but as soon as you slowed or stopped, it would overheat. Overheating is hard on your engine. If you are actually in the danger zone, you should run the internal heater to cool the engine. I had an early 90’s GM that only occasionally overheated and it was eventually diagnosed with a hose leak. If you aren’t handy, you should get your car looked at by a professional.

Also contributing to it is your A/C’s condensor, the radiator-like (actually, it is essentially a radiator) thing that sits in front of your car’s radiator. When you run your A/C the condensor gets hot too and must be cooled off via air flow in addition to the cooling system’s radiator.

This is especially a problem if you’re sitting in traffic (i.e. less air flowing past) and its extremely hot.

However, your car should be designed to handle even this worse-case scenario, so you may want to get it checked.

In support of what you say: in systems where the condenser is located in front of the radiator (which is quite common), this is the main impact on engine heating by far, not the compressor power.

As others have mentioned, it’s not your A/C that’s misbehaving, it’s the engine cooling system that’s falling short. It can’t handle the extra heat that the A/C condenser dumps onto the radiator.

A low coolant level, most likely due to a leak, is one possible cause. With the engine cold, check the coolant level in the radiator and top up if needed. If it was low, find and fix the leak.

Poor coolant flow could also cause this. Usually this would be from a partially clogged radiator, faulty water pump, or malfunctioning thermostat. These are pretty unlikely on a two year old car.

A failed radiator fan would typically only cause overheating when the vehicle is stopped or moving slowly, as mentioned previously.

And it’s possible that the cooling system is not malfunctioning, but just isn’t designed to handle the load of both 100+ temperature and A/C. As a “bargain” marque, Kia may not have engineered as robustly as most other makes.

Items of interest:

Yes, the A/C helps keep you cool, but it doesn’t contain coolant. It contains refrigerant. When you’re controlling how much hotter than ambient temperature something gets, you use coolant (car engines, nuclear reactors). When you’re making something colder than ambient temperature, you use refrigerant (A/C, refrigerators).

The coolant level in the overflow jar is not a reliable indicator of a full cooling system. You need to check it under the pressure cap (on the radiator in most cars, on the engine or on the surge tank in others).

The condensor is a radiator-like device that sits in front of your engine radiator and cools the refrigerant for the A/C system. Air moves through it, then to your radiator, so the air to cool the engine down has already been heated quite a bit from the A/C condensor.

The cars engine cooloing and A/C systems are designed to work together, even under hot conditions, but any problems in the engine system will show up big time when it is really hot outside, the A/C is on and the car is not moving or moving slowly.

One other thing that could be causing it - it was fairly common years ago, but often a dealer would order cars with few expensive accessories (such as air conditioning) in order to have low-priced cars advertised and sitting on the lot. They would then try to sell the cars with dealer-installed accessories, including A/C. Could be, somewhere down the line, the car was factory built without A/C, which was added on later. In many cases, dealer-installed A/C units are very similar to factory units. The big difference is, the engine radiator was often not part of the A/C add-on package, so the car still had the smaller radiator of a non-A/C car. This means that the radiator, though it may be in good operating condition, just isn’t up to the demands of shedding the heat of the engine with the additional drag of the A/C compressor and being cooled by already heated air. That could explain why the car works normally without the A/C and then heats badly when the A/C is on. The radiator may be too small for the job.

Not only quite common, it is the only way I have ever seen it done in all the years I have worked on cars.
Gary T did his usual superb job, two little things I might add.
If you live in the southwest (or anywhere dusty) radiators and condensors can get plugged with little bits of gravel and crap from the road.
You can check for this when the engine is off, and cool by placing a bright light on one side of the radiator (say behind) and looking at light level from the other side (say in front of the grill. You should be able to see some light. If you can’t you probably have a pound or so of gravel disrupting your air flow and causing the O/heat.
My other suggestion is to call or stop by the KIA dealer and ask the service guys about this. If the response is no they are not seeing this from all the KIA owners it is time to get the car checked out. don’t put this off, you can destroy an overheating engine very easily.

IIRC my 1979 Civic mounted the condenser on one side of the front of the car, and the radiator on the other, but I could be wrong. I’m also wondering if my Subaru Justy did that as well…

I’m curious - on a modern car, the radiator draws coolant from and returns coolant to the overflow tank as the system fluctuates in temperature. If the coolant level in the overflow tank is within range, and there isn’t a problem with the return line, then the radiator should show full as well. Why do you advise that the level in the overflow/surge tank is not a reliable indicator of system fill-state? Thanks.

It’s a good indicator if everything is in good shape. And with many system leaks, the level in the reservoir will go down as coolant is drawn from it to replenish what leaked. BUT - with some leaks, instead of coolant being drawn in from the reservoir, air is drawn in through the leak. The coolant level gets lower and lower in the system itself while the level in the reservoir maintains. Thus it is not a reliable indicator. The only way to be sure that the level is correct in the radiator (and engine) is to remove the cap (cold engine only!) and look.

By the way, my post above applies to an overflow jar, which is external to the pressurized part of the cooling system. A surge tank resembles a reservoir in that it’s usually translucent plastic, but by definition is under pressure. Generally the pressure cap is on the surge tank itself. In a way, it’s an extension of the radiator and its fluid level is a reliable indicator. The problem with surge tanks is that they often get some build-up inside which obscures the coolant level, and you just can’t see (accurately) how much is in there.

To help keep everyone confused :smiley: , most parts people call surge tanks “overflow jars,” even though that’s technically incorrect.

Una 'K I didn’t think about some of those well oddball Asian cars. Its been awhile since I looked under the hood of an old Civic, and I doubt I have ever looked under the hood of a Justy, but if I recall correctly the condensor runs the full width of the front and the radiator just runs 1/2 or 2/3 of the width. In addition many times the condensor has its own fan on the part not covered by the radiator (which also has its own fan.
Like this

Front of car
C - O - N - D - E - N - S - O - R
RAD - I - ATOR            F A N

Rick - I cheerfully admit I could be wrong - I haven’t owned either car for a long time, and the only car I’ve worked on in the last 8 years is my Mustang. That’s why I said “(which is quite common)” originally, rather than anything more definitive.

Hey same here. The point I was trying to make is that the condensor is always exposed to cool air, never to the warm air behind the radiator.
Anyway, no harm no foul.

Thanks for the expert response.

I waited until my radiator cap was cool enough to touch, and popped it off. It appears that it’s full of coolant - the coolant was about level with the gasket. The spill tank is just slightly lower than the “HOT” line.

I couldn’t find a second radiator at the front of the car, though the radiator that is there is quite wide, and has two fans. Also, I think I’ve found the compressor, and it has tubes leading to the radiator, as does the water pump. Is it possible that the A/C radiator is incorporated into the main radiator?

I’ll agree with that. The efficiency of the A/C would be abominable if it was behind the radiator.

No way is the A/C condensor incorporated in the coolant radiator unless you car is different from every other car make on the road. Putting the condensor inside the radiator would cost extra and make your A/C performance SUCK.
Pretty much a lose lose. I can’t see any car maker being that dumb.
Better go look again.

While all of the comments have been spot on, I’d like to add one that just happened on my car showing the same symptoms.

I’d replaced the thermostat, fan, and was about to have the front clip removed to have the radiator replaced. (yes, you have to remove the front clip to get to the radiator on a Taurus/Sable.) I decided to take it to my corner garage and have them look at it before seeing the dealer.

Apparently, the way that the condensor was mounted on my '98 Sable, it easily collects leaves, dirt, and accumulated crud in it. This caused the condensor to be inefficient, and the engine temp to rise.

It took the corner garage about 30 minutes to lift my car and wash out around the condensor. For about $30, instead of the $500+ that the dealership wanted.

It’s at least something to have looked at.


I had a 1980 Volvo (this was in like 1999.) Anyway, this thing was so old that some of the coolant hoses were cracked and leaking, so not only was I going through a lot of coolant but the engine was always in danger of overheating and this was one hot summer. As xbuckeye said, you can cool the engine down some by running the heater. I did it a lot until I could get the new hoses, so I can personally attest that it works. Sure, it looks kinda funny to be driving with the windows open and the heater running full blast, but it keeps the engine cool, since you’re now shunting waste heat into the passenger compartment.