Engine overheats

I have a 1969 cougar xr7 with a 351 Windsor engine. This car sit in my aunts garage for about 10 years. She got another car to drive so the cougar sit and was never started. The car is all original with 34,000 miles. I have finished the restoration and it looks and runs great. But…the engine will overheat if I run it at freeway speed. I have replaced the following…

  1. Radiator with a 3 core
  2. Water pump
  3. Thermostat
  4. All the hoses
  5. Spark plugs
  6. Spark plug wiring
  7. Ignition coil
  8. Points and condenser with a solid state
  9. Fuel pump
  10. Radiator cap
  11. 50/50 coolant
  12. Fan with a 6 blade flex
    The engine starts and runs great.
    Does anyone know why it will overheat?

Does it actually spew coolant, or does the gauge just indicate a HOT temp? Could be your sender is bad, and needs replaced.
(BTW, welcome to CCCF…smarter folk than I will be along shortly to give you better information)

The coolant has boiled over.
The temperature gauge and sender are good.
There are no leaks in cooling system.
The coolant level is good.

You have to allow about 2" of air space int he top of the radiator tank for water expansion. If you fill it completely it will force very hot water out of the tank. Many people think this indicates overheating.

When it runs hot on the freeway but not at idle it indicates that the radiator is not flowing well. I have have had a new radiator do this. When they took the top tank off it was obvious that about half the tubes were blocked by a very poor solder job.

I learned about the coolant level and have kept it about 2 inches below the cap seal.

However, the engine temperature gauge still slowly creeps up to the red mark and has boiled over a couple of times. Fortunately I was able to water down the radiator and cool the engine.

The radiator is new but that doesn’t mean it is flowing well. Based on your experience I should get it checked. How was your radiator checked so that they ended up removing the top to find the blocked tubes? Did your cooling problems end after the radiator was repaired?

If the radiator is good is it possible that a faulty head gasket or cracked head could be the problem? The engine runs so good it’s hard to imagine that would the problem.


Over heating caused by a bad head gasket is caused by hot exhaust gas getting into the cooling jacket. You will see lots of small bubbles at the radiator filler neck. It will also over heat at idle with this problem and usually you will have coolant in the oil.

I took my radiator to a old school radiator shop and they ran water in the top and watched how much came out the bottom. Based on what they saw and about 40 years of experience, they were able to tell me that the radiator was not flowing properly. They then unsoldered the top and took a look and you could see that there was solder filling in lots of tubes. They took both top and bottom tanks off and cleaned out the tubes. Worked perfectly after that. It was a brand new radiator…

When a car overheats at idle but no on the highway, you suspect too little air. When it over heats on the highway but not at idle, you suspect too little coolant flow. At highway speeds a car doesn’t need a fan at all. At 60 you are putting close to 15,000 CFM through the radiator. At idle or around town you have to have a fan to keep it cool if the weather is hot.

Thanks for the information. Sounds like I need to have my radiator checked.

Unless there is a blockage in the water passages?

Just because everything is new doesn’t mean it’s good. There’s a lot of junk being sold these days. Could be a bad cap or thermostat.

A blown head gasket will often result in steamy exhaust that smells like coolant. As was said, be sure to check your oil. If it looks like emulsion you’ve got water in there. Bearings don’t like water. If you run it with water in the oil you’ll be needing that overhaul. Otherwise I, too, suspect cooling system issues, not engine issues.

Is the thermostat on the right side of its gasket ? if its on the wrong side, it will nop open correctly, and thus overheat

It will not work properly if it is installed backwards.

I think we might have overlooked one of the other things it could be. In your new bottom radiator hose, is there a spring installed inside it? If not, the hose will suck itself shut at higher speeds and this will prevent coolant flow. If you can squeeze the bottom hose almost shut with your hand, then you don’t have the spring installed.

Actually you don’t need a spring in the lower radiator hose. Those were installed at the factory to enable the vacuum fill of the cooling system that they did to ensure no air was trapped in the system. It’s not actually needed in a properly functioning cooling system:

Very interesting read…but, I’ll keep the spring in mine just in case. :unamused:

This concerns me…

  1. Fan with a 6 blade flex

Are you sure it is not a fixed blade fan with a clutch as used on AC cars? Flex fans are for performance in that they are light weight and are designed to not create much drag, great for 1/4 mile but terrible in traffic. Post a couple of pics if you can.

Here is my response to the latest issues posted…

  1. The thermostat is installed correctly and working.
  2. The lower radiator hose does not have a spring inside. My cooling problem occurs at higher prolonged speeds. If the hose does collapse that could be worth replacing. Any other thoughts?
  3. I removed the 4 blade fixed fan that did not have a clutch and replaced it with a 6 blade flex fan. The car has an under the dash Ford AC unit. The AC does not blow cold air so I am not using it. Will repair when I get the engine running cooler. Don…what pics do want to see?
  4. As far as I can tell there is no coolant in the oil …with regard to the head gasket.

If your car had factory AC it would be in dash and you would have a 24" wide rad. The four blade fixed will cool much better than a flex. Can you feel cool spots on the rad fins? That would indicate lack of flow. show a pic of rad / fan and area.

Sounds like cavitation to me when pump rotates at higher RPM.

45 year old car that sat for 10 years prior to being put back on the road, my bet is the water jackets in the block and the heads are full of crud/corrosion which is preventing proper heat transfer when the car is running at freeway speeds.
Even though the car is very low milage it has had liquid (who knows how well it was maintained) sitting in the water jackets contributing to the crud building up in the motor. I am sure that any of you out there that has torn down one of these old motors has seen how much crap/corrosion builds up over the years.

That was my thought as well.