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Classic/Antique Car Repair/1965 Mustang 302 engine cooling


Hello fellow Expert!

I have a '65 Mustang with a 302 engine out of a '73 Torino (confusing, yeah, but bear with me!) Anyway, the 302 was professionally rebuilt prior to me getting it, and has about 500 miles on it. It's bored .030 over and runs great. I've had the car about a month and have noticed something strange though. I can drive the car around town without any problems, but when I drive it 20+ miles on the freeway, it starts to boil the coolant and blows steam out of the overflow hose. Yesterday, I made sure the coolant was full (at cold) and drove it home from work (~25 miles) in 85 degree weather. By the time I exited the highway, the car was belching steam from the overflow hose. I got home and turned the car off, and the steam continued for several minutes. After the car cooled, I removed the cap from the radiator and I was down about 1/2 gallon of coolant (which I assume had boiled off)

The interesting thing is this- the oil looks perfectly clean, no signs of coolant in the oil. The coolant looks nice and green/clear, as it is supposed to. No white smoke/steam out of the exhaust either, so this says to me it's not a head gasket.

I'm stumped. The car is completely stock, minus a 6-blade fan. If it matters, there is no shroud for the fan, but I can't see that making a huge difference.

Any help you can offer would be great!

Thank you in advance.

Hello Brian,

Obviously, You've got an overheating condition at speed, no great diagnosis there. So, since it's OK at idle, probably the radiator's OK also. Something is either restricting flow at speed or the coolant is being blocked somehow at speed. From looking at your engine (knowing rebored engines run hotter (thinner cylinder walls and such)), for basic things; I'd suggest you try a fan shroud for additional cooling.

As far as possible blocking, you've got those color matching hoses. Are they a good brand or a pretty brand?  Collapsing hoses at speed could be your blockage. Also, make sure there are no air pockets in the system, including heater. Why? air pockets could prevent you from adding a full measure of coolant. Crazy, but I've seen it before in new engines.

Other than these simple things, you can start looking for the odder but known things like a partially opening thermostat. Yes, I know it's new but just because it's new doesn't mean it's good. I'm conservative and try not to use PEP Boys and Auto Zone specials (no offense to them, it's just the category). I pay more but try to use OEM or long time aftermarket brands like NAPA only. Yes, it's more expansive but who wants to risk their $6K plus engine job on a junk part just beause it saved you a few bucks?

Good luck, I think if you follow these thought processes you'll nail your problem and enjoy that Mustang!!


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Robert Russell


Mechanical, Body, Historical, MOPAR, GM, Ford, Packard, 1930-1975


Former Chrysler Tech., Project Manager for Antique and Classic Car Dealership. Owner/Driver of Classic Cars Since 1972

AACA, North GA Mopar Club, Packards International, W.P. Chrysler Club

High School Auto Shop plus Adult Night School

Past/Present Clients
California Car Company, Roswell, GA

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