Engine running hot but not overheating

I have a 2001 mustang v6 that runs slightly warm at highway speeds only, especially at higher speeds of 70 mph or more. I can idle the car for a long time or drive at city speeds and the temperature gauge stays right in the middle. However when I start going faster at 70-80 mph or more, after a while the temperature gauge will start creeping up (but not overheat.) When I notice it getting warmer, I slow down slightly to 60 mph it will gradually go back to normal temperature in the middle, but hypothetically if I were to keep going at a faster speed and not slow down it might keep getting hotter and overheat. I haven’t tried testing this as I don’t want to find out, because I slow down everytime I notice it getting hotter and haven’t tried not slowing down to see what happens.

The heat and AC in my car work fine and I can idle/city drive with AC or heat on with no problems. Only when i’m going 70 mph or more does the temperature start increasing and running warm. What could the issue be? I’ve tried basic stuff such as making sure the oil is good, checking the coolant level to ensure proper amount is in the radiator and replacing the radiator cap. I also did a radiator drain and fill and added radiator flush And drove around for 30 min before doing the drain and fill. All those checked out.

What could the problem be? If the radiator was clogged wouldn’t it be more likely to overheat at lower speeds since the air flow at lower speeds isn’t as strong? If it was a water pump issue wouldn’t the car would overheat all the time and not just at higher speeds? Also the temperature wouldn’t fluctuate and go back down, it would just overheat completely. I’m at a loss…It could be the thermostat but this seems unlikely because then the car would probably run warm at lower speeds too and likely overheat completely as well. If the gauge itself was broken it would be sporadically reading all the time because the car is definitely running hotter at faster speeds but the gauge doesn’t move when city driving at slower speeds.

What could the problem be?


If it’s not overheating, then that’s called operating within its normal range.


The temperature gauge goes 3/4ths to the red instead of halfway where it should be though

The temp gauge should be straight up and down if it is normal. It isn’t overheating, great, but this is a hint that all is not quite right. Not a thermostat, probably not a radiator cap. Maybe a water pump that has eroded away a bit internally… stick a pin in that for now.

How many miles on this car? How is your mile per gallon? How many rpm do you see at 70 mph?

Has the radiator ever been replaced? Is the car losing coolant at all?

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I don’t have anything to add to what @Mustangman has already said but good on you for 1. paying attention to your car’s gauges and 2. not wanting to ruin your engine. You’d be surprised at how rare those are.

It’s an old car with 200k miles. The strange part is I replaced the radiator cap and this seemed to fix the problem for a few weeks at highway speeds but then it started doing the whole running warm thing again yesterday…the car isn’t losing coolant to my knowledge but it could be leaking very slowly or just leaking while i’m driving and then burning off/evaporating, so i never actually see the evidence of a leak. I’m checking the coolant though and the level looks fine so even if it was losing coolant it is a negligible amount that shouldn’t be contributing to the problem. I thought it could be a water pump at first too, but if it was, wouldn’t the car overheat all the time at all speeds and idle as well and not just at higher speeds?

The rpm at 70 mph is around 2.5k give or take. The mpg isn’t the greatest but it’s acceptable. As for the radiator as far as I know it hasn’t been replaced

Check the color of the transmission fluid.



I suspect that you may have a small head gasket leak or the radiator has lost some of its efficiency due to corrosion inside it.

Would a full radiator flush help remove corrosion? (Only did drain and fill before)

Also if it was a head gasket problem wouldn’t it be overheating all the time and not just at higher speeds

It’s not the radiator.

If it were, the engine would run hot when idling or driving at slow.

Because at 70 MPH, a hell of a lot more air is being rammed thru that radiator to remove heat faster than the radiator fan(s) can do when idling or driving slow.



Right that’s why I’m baffled as to what the problem is

Did you check the transmission fluid?


The transmission fluid is somewhere in between okay and good from the chart you posted. I did a drain and fill when i first got the car about a month ago

What do you consider acceptable?

Do you have actual numbers? The reason I ask…

Either something in the cooling system is not working as designed, but only at higher speeds. This is the opposite of normal failures.

Or, there is something causing an additional drag on the car making the engine work harder at 70 mph than it should. In other wards, it isn’t a cooling problem at all, but a problem with another part of the car entirely. A problem causing a drag, heating the engine and hurting your fuel economy. This is what @tester was suggesting when he asked about the trans fluid.

After driving the car on the highway for a few miles, pull over nice and easy into a rest stop and park. Get out and touch each of your wheels. Are any very warm or hot to the touch. They should all be pretty cool even considering you just slowed from highway speeds. If they are, warm to hot, you have a brake problem, not a cooling problem. Check that and report back.

Sounds like the fins of the radiator are plugged up with dead bugs.

Put a bright light behind the radiator, can you see it from the front?

No I wasn’t.

If the transmission fluid were dark or black, it would mean the transmission fluid is burnt, which would indicate that the transmission is running hot, which would cause the engine to run hot.


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Even if it was a brake problem that wouldn’t cause the engine to overheat right

@Tester, I like your point about the transmission fluid.

I understand the thought about the higher air flow across the radiator at higher speeds and agree with that, but I also wonder about how much more temperature change there is for the engine when it has to move the car down the road at 70 MPH or higher, rather than going around 35 MPH. I also think that a SMALL head gasket leak could cause this kind of thing to happen due to the higher RPMS required from the engine. Just my thoughts anyways and I could very well be wrong about both of the ideas. I would have the engine checked to see if exhaust gases are getting into the coolant.