2001 Mustang V6 - Can’t figure out why it’s overheating

I have a 2001 mustang that was overheating. I say WAS because the old engine had overheated one too many times and lost compression in 2 cylinders. I just replaced the engine with a low mileage one, however the car still runs hot.

Car just had a tested engine with 60k on it put in. (So the possibility of a blown head gasket can be out ruled)

-oil and coolant levels are good

-the 60k engine had a brand new water pump, 180 degree thermostat (this is colder than the 192 degree oem one) and coolant temperature sensor put on it before the engine itself was installed.

-Radiator has been flushed and cooling system has been bled properly.

-Looked under the hood while the car was running while in park, and the coolant fans turn on and off automatically a few minutes after the car has reached operating temperature, which indicates that the fan switch and relay are working properly right? (Someone please confirm this statement is correct?)

With all the above being considered, what could possibly be the reason the car is still running warm? As soon as the temp gauge starts going noticeably past the middle, I shut the car off, so i’m not sure if the car would overheat if I allowed it to continue running but I don’t want to find out and ruin my brand new engine I just paid for. So I have to assume the car is overheating because that’s not a risk I’m willing to take

The following are my only hunches as to what could possibly be causing the car to run hot. Any input is appreciated…

1.) A clogged heater core (car heat only works sometimes, seemingly completely randomly as in it works some days and other days it blows cold air instead) - I’m not even sure if a clogged heater core could cause overheating, I have read very mixed answers online about that, could someone also please clarify this?

2.) A bad radiator. (Visually inspecting the current one, it does have some bent fins and black stuff in some of the vents from visually inspecting it and generally looks dirty and not the silver metallic color a new one would be.) HOWEVER, the mechanic said he flushed and tested it and the radiator to him, appeared to
have enough flow. It is possible he made a mistake here though.

Please anyone offer their input or thoughts. Thanks!

Check the color and the odor of the transmission fluid.

A transmission that’s running hot can cause the engine to overheat.



You are correct. However when i look at the fluid, it is normal level and the fluid appears at the “normal” color of the spectrum you posted

Today, I actually put in the 180 degree thermostat, but i didn’t notice a difference in the middle or baseline operating temperature compared to when it had the 192 degree one. Do you know why this is? Also I noticed when i removed the old thermostat it was in backwards. Obviously i put the new one in the right direction. The car has yet to overheat now, but I have not test driven it thoroughly. Could a backwards thermostat cause overheating?

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On some engines, such as the Chrysler 2.2/2.5 4-cylinder engines, the heater core functions as the bypass flow when the thermostat is closed. If the heater core is plugged, it is necessary to drill a hole in the thermostat to provide some bypass flow.

A decent aftermarket radiator costs less than $150 online with tax and shipping. If in doubt, I’d replace. In fact, I always replace the radiator after 15 years or 150,000 miles, since they lose efficiency and become plugged up with minerals, etc. Look for a good brand, such as Denso First Time Fit, rather than one of the “no name” brands that are cheaper.

Absolutely. The side which contains the wax pellet must be facing toward the hot coolant. Otherwise, the thermostat won’t open until much later, i.e. after the engine has already overheated.

Do you know that if specifically 99-04 mustangs have that type of heater core that will cause the engine to overheat if it is plugged? If they don’t then I can rule that out. I sat idling in the car for 30 minutes and drove at highway speeds and the car did not overheat. I’m hoping I found the fix but I don’t want to consider myself in the clear just yet. I’ll take your advice and replace the radiator too