Honda Accord overheating when AC is on



My 1997 6-cylinder Accord sedan has started running very hot, almost to the top of the temperature gauge, when the AC is on. The car usually starts out ok, the temp stays in normal range, and then if, for example, I get stuck at a long traffic light the gauge rises well above the mid-point and gets to about three-quarters. Once the car gets moving again the temp comes down a little but not all the way to normal (which is about one-third up the gauge for this car).

When driving home today (it’s 91 degrees here) the temp got very close to the top of the gauge. I turned off the AC and the temp came down fairly quickly, maybe 6-8 minutes. It never got all the way to the normal level but it got below half way.

When I turned the AC back on the temp climbed back up in about ten minutes.

I thought it might be a partially stuck thermostat but I’m not sure, would appreciate advice. It seems the thermostat on this car is a bear to replace so I don’t want to get into it unless I’m pretty sure it is the culprit.

One other piece of data - I recently replaced the alternator which is on the same belt as the air conditioner compressor. Is it possible I snugged the alternator too tight and that is causing the overheating when the AC compressor is turned on?

Oh yes, I checked, both fans behind the radiator are running.

I sure would appreciate some guidance and I thank you in advance for it.


If the thermostat has never been replaced, I’d start there. Also make sure the air flow through the AC condenser and the radiator is not blocked by dirt, insects, etc.


I agree. If both fans are running then make sure the condensor isn’t plugged with debris.


OH… a place that is often overlooked is the space between the condensor and radiator. You might have to remove part of the cover over the radiator to gain access.


My guess is that the radiator may be starting to clog up and is not able to dissapate enough heat. The fans are pulling all of that A/C condenser heat right through the radiator.

After that, possibly an A/C problem with the high side pressure being too high. This would require putting the gauges on it.


New info - and I’m super embarrassed about this. I opened the hood, which I’d not done because I’d just checked everything a couple days ago, and saw what I captured in the two attached photos.

I believe this shows a coolant leak - true? If so, I cannot get it to actually leak now and I cannot see the source but at least I know more than I did.

I also, more embarrassment, found that the coolant level is way down, even though I’d topped it up just a week or so ago.

OK, brain now in gear, at least as far into gear as it goes.

Any wisdom to offer?


Find the leak, and repair it. Let us know what is the source, like a hose or a bad connection. Looks like a terrible mess.


My goodness! It looks like something exploded or is spraying. My best guess is that it’s coolant…I wonder if you’ve got a cracked hose that is spraying around or something. Can you trace the leak with the engine running?


Have you checked your oil? If its creamy, you have blown a head gasket


The top radiator hose is known to crack and spray coolant when under pressure. Squeeze the hose and listen for the sound of air or liquid escaping.

The hose that connects the engine block to the heater core, via a temperature control valve, is also known to crack and spray the entire contents of your radiator quickly. This hose is hard to get to and I don’t know where it is on your model/year. Start squeezing all the hoses and you’ll quickly find your leak.


Have your radaiator checked. Could have a partial blockage.Make sure radiator is filled with proper solution. Have fan belts checked to make sure they are properly installed and AC compressor is turning at maximum rpm.


Have you looked at the pictures??? There is a serious leak. He needs to do more than check the radiator for a blockage.


Thanks for the comment about the pictures! I was thinking people had missed their significance.

Interestingly, after I refilled the radiator and reservoir I cannot get the darned thing to leak - nothing. I’ve only driven two 15 mile trips (one 30 mile round trip with a long pause in between halves) so it’s not much of a test but no leak thus far.

I truly, truly hate intermittents!

Any wisdom offered would be appreciated


And yet more information …

Yesterday I drove 35 miles on the highway, all went fine until we hit stop-and-go traffic. The temp gauge stayed in its normal range, the AC worked fine, etc. After ten minutes of stop-and-go the temp started climbing and I had to turn the AC off so we did not overheat. Interestingly, even in extended stop-and-go traffic the temp stays nice and low as long as the AC is not turned on.

This morning I checked - the coolant in radiator and reserve tank are at their proper levels. Also, I used a tester to confirm the mix is 50% and the boiling point would be 264 degrees (or thereabouts).

I have checked the air flow through the radiator, there are no obstructions.

And, my leak has never reappeared.

How to proceed? What should I check?

Any ideas?


I would be curious to know what the A/C pressures are when this happens.

If it only overheats when the A/C is on and in stop and go traffic, I’m wondering what the high side pressure is at that time.

If there is excessive pressure, that could be putting enough of a load on the cooling system that it can’t dissipate enough heat causing it to overheat. Again this would lead back the condensor restricted/dirty or the radiator having a internal restriction.



Thanks for the quick reply. I need to drive off on vacation with this car in 9 days so solving this is a major issue for me.

I have no way to measure the pressures but I can say the system has not been recharged in 6 years, though it cools me nicely. So, I would imagine the pressures are likely to be lower than in prior years rather than higher.

I did recently replace the alternator - could I have brought the tension on that belt too high and thus cause excessive load?

Also, a friend suggested the cooling system might be partially plugged and therefore not able to handle sufficient flow to accommodate the added load that comes from the heat dissipated by the air conditioner’s cooling coil. Any thoughts on that? Should I just go and have the system flushed? Should I also have it pressure tested?

Thanks again for everyone’s help.


Have you replaced the radiator cap?

Even though your coolant levels are ok, the cooling system may not be at the correct operational pressure. A leak that large tells me that you probably are not at working pressure!

You need to find the source of the leak. It’s either a hose that needs to be replaced or the rad. cap, assuming the leak was coolant (which it looks like).


Also, an obstruction in your radiator is an internal problem, not one your can test by checking the airflow across the fins or with any other visual method. Find a radiator shop and they will properly test your radiator for internal blockages and operating pressure.


The leak’s source is a mystery. I have poked into every place I can manage to see - no source. I washed off the leak’s residue thinking that would make any subsequent leak easy to see - but no subsequent leak has occurred.

And no leakage around the radiator cap either. When I try to remove it there’s a lot of pressure underneath so the cap isn’t leaking.

Did you mean the pressure might be too high because of a bad cap?

And - any thoughts on my questions about the alternator replacement making the belt too tight or about fleshing and pressure testing the system?



And even more info - I had the cooling system flushed - everything came out brown!

The mechanic thinks that means the head gasket is gone and oil is getting into the coolant. But I always thought a blown head gasket lets coolant get into the oil.

Is he LIKELY to be right about this?

Or, is it possible that the brown color of the flushed cooling system was just due to all the old crud from 220,000 miles of use?

Should I flush it a couple more times and see if the color clears up or should I just get rid of this car ASAP?