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Overheating only when AC is on

I’m not sure what is wrong with my 2003 pathfinder (approx 78K miles). Last Sept I took it on a road-trip from Atlanta to New Orleans and the temperature gauge approached the “Hot” level. I turned on the heater, took a break, and finished the trip with windows open. Took it to the mechanic but they didn’t find anything. Then the weather turned cooler and it didn’t happen again. Now it’s hot again, and the problem is back. I’m able to replicate it by driving fast (70 mph) with the AC on high. It only happens when I run the AC, specifically on high. The weird thing is once I turn it off, the gauge drops immediately back to normal within 1 - 2 minutes.

There are several things that it could be but I think your particular vehicle uses a fan clutch so that would be my first suspect.

Other possibilities could be a clogged radiator (internally or bug clogged condenser), A/C pressure problem (excessive on the high side), etc.

thanks so much for your reply! i really appreciate it!

also, neither of the “warning” or “check engine” lights come on. don’t know if it makes a difference in diagnosing the problem.

That should not make a difference. Hopefully it’s something like the fan clutch as that’s an easy fix.

The AC’s putting extra heat into the radiator, so something is clogged. Easy check is to make sure the condenser and radiator have nothing blocking them. Next is to check that the thermostat is opening fully. Given it’s happening at 70 mph (where the fan makes littel difference) my next guess would be a partially plugged radiator.

Some follow-up…

I’ve noticed it also over-heats when I’m stuck in traffic, not going anywhere. Just idling. And this is without the AC even being on… I avoid traffic at all costs, so I never noticed this problem. And since it’s been consistently hot outside, I’m now able to replicate it going random speeds. Now it turns out I don’t have to go fast for the gauge to approach the “hot” level.

Anyway, thanks for the help!

Here is something you might try that won’t cost you anything. Take a garden hose with a nozzle and squirt water through the radiator from under the inside out (from under the hood through the grille). An air hose would work as well. It just might be that the radiator air passages are clogged with bugs.

It turned out to be the radiator. Depsite all this advice, it ended up being a fiasco… One thing I’ll add: if the radiator is cool on a hot day, it suggests that it isn’t working. Why, oh, why, it was only after they changed the fan clutch and the thermostat they noticed that problem, I will never know. (well… $$$)

This was fixed a while ago (not sure why I just got a notification of sorts), but thought I’d follow up.

That’s continues to point to the fan clutch or a clogged (either the airflow or the water flow) radiator.