Dave Stribling
August 4, 2016

Running Hot

I have a 1965 convertible with a 1974 302 engine that’s been rebuilt with C4 auto transmission behind it. It used to run at three-quarters of the way to hot on my temp gauge. I recently installed a new under-dash air conditioner. I have a Blue Code six-blade fan with a 3 1/2-inch-deep shroud. The radiator is a three-row made for the 302 engine. I have flushed it and have no blockages with a new thermostat opening at 170 degrees. Now, if I drive it 5 or 6 miles on a hot, 90-degree day, the gauge moves almost to the hot mark. The fan blades are about 1 inch from the backside of radiator. I considered adding an electric fan, but I don’t have room. What do you think could be causing the overheating?

David Smith
Via the Internet

It sounds like you have a blockage somewhere in the system. The fact that adding the condenser on the front, which restricts the airflow through the radiator even more, tells me either the radiator is clogged or there is not enough airflow.

You didn’t say what kind of driving you were doing, such as commuter traffic or on the highway. The Flex-a-lite Blue Code fan is only working at low speeds when there isn't enough airflow over the radiator to cool the engine. At moderate speeds, it isn’t doing anything, and you didn’t say how well it works when the temperatures are mild. A 20-degree drop in the outside air temperature may make a difference in how efficiently the radiator transfers the heat to the fins (a sign of a clogged radiator). Finally, the 170-degree thermostat doesn’t make the engine run cooler. It just opens up earlier.

My first check would be the radiator. Some people recommend running it until it is warmed up, turning the engine off, and then feeling the upper and lower radiator hoses to check for a big temperature difference. If the upper hose is really hot, and the lower one is just warm, you have a blockage in the radiator. You can also purchase an inexpensive touch-free infrared thermometer and do the same thing without touching the hot parts. Please be really careful when dealing with hot engine components.

When the system is cold, you can remove the radiator cap and drain the coolant. It should come out with a good amount of pressure. Note: Home-flushing systems won't do much on a severely blocked radiator. You may have to take it out and have the fluid boiled out at a radiator shop. If it flows out fine, simply fill the radiator back up (only use distilled water, not the garden hose). Even if it flows well, scale buildup in the tubes might be preventing good heat transfer.

The thermostat may be partially opening and causing a blockage. Alternatively, a bubble may have built up behind the thermostat that is insulating the thermostat and not allowing it to open up. This is actually common on the small-block Ford engine because Ford mounts the thermostat vertically. Some manufacturers now add a small vent to the thermostat to vent out this air pocket. If your thermostat doesn't have one, you can drill a small hole in the side located on the upper side of the thermostat, and this will help burp the system.

It is possible the engine block itself is clogged somewhere (crud building up in the bottom of the block), and it is not allowing the pump to send coolant everywhere it needs to go. I bought a 1967 Cougar once that the kid put a brand new radiator in, and by the time I got halfway home from California to Indiana, all the crud from the engine ended up in the bottom of the new radiator. Start with the radiator because adding the condenser made your problem worse. I suspect a flow problem through the radiator.

The 1965-1966 Mustangs with A/C are really working hard. The radiators are undersized, and adding the condenser only makes it worse. An infrared thermometer can help locate hot spots or flow issues with a misbehaving cooling system. They are getting pretty reasonable at the tool outlet stores.
Vendors are now putting a small vent in the thermostat (the small brass button above the center post) to burp air trapped behind the thermostat. The air acts like an insulator and prevent the thermostat from opening at the correct temperature.