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Weird Overheating!

I have a 2003 Ford Taurus that has recently had cooling service, a new water pump, thermostat, flushed system - even with a lower than normal temp thermostat, (160 degree) the car overheats … but not right away. After driving for about an hour and a half, then the temperature gauge goes up to about 3/4 way on the scale … keeping the car moving drops the temp, as well as draining off heat thru the heater. It will also overheat if the A/C is turned on. I can’t figure this out at all, and I know the gauge is not.faulty. What’s going on here??

Turning on the AC loads the engine more, and creates more heat in the engine compartment, so if the car is overheating anyway, its not surprising the AC makes it worse. The fact that this started after a coolant service suggest three things: (1) there is air remaining in the coolant loop;
(2) the replacement water pump is failing; or (3) you need a new radiator cap.

There are other possibilities, like the ignition timing is off, the valve timing is off, the head gasket is compromised, etc. But for now I’d focus on the above. About the only other thing I can think of is the thermostat. I know it makes common sense that if the thermostat opening temp is lower than spec’d for the car, it would seem like that would help. But it often doesn’t, and makes overheating worse. The thermostat is what controls how much water goes though the engine vs how much goes through the radiator. If it opens too early, you may have too much water going through the radiator, and not enough through the engine. Unlikely, but worth considering. Also make sure the radiator cooling fans are working, and spinning at the proper rate. You might need to eventually take this to a radiator shop and ask them for their ideas. Keep cool, and Best of luck.

Have you checked to see if the radiator cooling fan(s) come on?


The key phrase you used is " keeping the car moving the car moving drops the temp". Does this mean it doesn’t overheat at highway speeds? If so. that points right at the fans. The fact it takes so long to happen points to something electronic or electrical failing after it gets hot long enough.
I am not familiar with the Tarus system but I would look at whatever controls the fan. It could be a relay, temp sensor, fan controller, electrical connection or the fan motor itself.

Thanks for your replies. The fans are working as they should. The radiator cap was replaced. Doubt the water pump is bad … but I’m leaning to going to a radiator shop.

How old is the radiator? I once had an old Ford and nobody could figure out why it was overheating. It turned out the inside of the radiator was clogged, but not clogged enough to block the flow of coolant in and out of it.

Either get the radiator flow-tested or just replace the radiator, depending on the costs involved.

@Whitey I agree that the radiator may still be partially clogged, and incapable of handling the maximum cooling load.

Of course when you check the fans they are working as they should, you said it takes an hour and a half for the car to overheat, that means the fans are working fine for a long time. You need to check to see if they are working when the car is overheating.

If it is the radiator, just replace it. Modern plastic/aluminum ones can’t be repaired or cleaned, at least not for less than putting in a new one.

Unlikely, but don’t totally eliminate the fans as the cause just because they are spinning when they should. To cool the engine properly, they have to spin at the correct speed too. Have you ever had one of those box fans for room cooling, and it starts to spin more slowly than it used to because the bearing is wearing out or gummed up? Same thing can happen to car’s radiator fans. Mechanics have test methods to check for this. They may measure the fan motor current for example. Or use a strobe light to verify fan speed.

Try a pressure test on the coolant system, Low pressure is one possibility for the symptoms.

And, just for the record, running a cooler thermostat in a modern EFI car will cause issues with gas mileage and may cause a never-ending CEL issue. The programming was designed for the OEM temp thermostat, and running the engine cooler upsets the program.

Thanks all, oldtimer 11, I have started to suspect the fans are stopping when the car gets hot, that’s why the temp drops when the car moves (haven’t been on the highway in awhile). Also suspecting that radiator as well … oy

After a quick check, the fans are running as normal with the temp gage at center … not sure about speed, I understand that the Taurus fans run at different speeds, going high when A/C is on or temp goes high. Suspect that radiator, I do …

Please let us know if you solve it, best luck.

To all; I solved the problem! The overflow tank had a couple of hairline cracks in it on the bottom. As the car heated up, the increased pressure was enough to make the tank leak and lower the pressure in the cooling system, only a small amount, but the tank wouldn’t leak when the car was ‘cold’ (below a certain temperature). Replaced the tank and refilled the system, temp holds steady …