How to remedy a Mustang’s overheating issue

Hagerty Drivers Club member Paul V. Romero writes: My ’67 Mustang with its original 289 V-8 runs hotter than normal during the summer months. I live in Santa Fe, New Mexico, where it doesn’t get much above 90 degrees. I have replaced all the seals and hoses, and I’ve installed a four-core aluminum radiator. I’ve considered replacing the original fan with a couple of electric fans or putting fans in front of the radiator, but there’s not a lot of room. I would like to keep the original look of the engine.

This is a companion discussion topic for the original entry at https://www.hagerty.com/articles-videos/articles/2019/02/18/how-to-remedy-a-mustangs-overheating-issue

It’s unfortunate that Hagerty employs numerous experts on many different subjects, but then does its members a disservice on important topics. So yes all the things mentioned in the article are correct, BUT these would apply to any car. Why is the 289 MUSTANG overheating???

The simple answer is the design of the cooling system, specifically the radiator. An aluminum radiator will help, but the problem is a bit deeper than this. When ford designed the mustangs radiator, they made the mistake of having both the inlet and outlet of the radiator on the same side. So you are not getting the cross flow and utilizing the whole radiator. Most of the water is moving top to bottom just on the right side (passenger side), because that is where the water is fed into, and pulled from.
All the things mentioned in the article will be beneficial, but if there was a stock looking radiator that possibly (internally) fed the opposite side of the radiator, and utilized all the surface area, that would be a more permanent fix.
So see if you can get a radiator that feeds into the drivers side top of the radiator, then you just have to look at the auto parts store inventory of hoses for a preformed hose that goes over to that side. ( I hate the flex universal hoses)
I hope this helps, good luck.

In addition to what bullitt65 stated, you might find that getting a radiator with another deeper core will help with the cooling as well. Ford had a major problem with their 390’s overheating all the time. Their solution? Put in an overflow tank to capture the overheated coolant rather than build a deeper core to handle the V8 need for more cooling. My brother’s 66 T bird overheated all the time and he didn’t want to spring for a better radiator so when he was stuck in traffic I told him to turn on the heater. He wasn’t very happy with the idea on a hot day but it kept him off the side of the road more than a few times.

I didn’t see where the thermostat was replaced. That would be a good starting point.

Someone needs to publish a remedy for a Jaguar XJS V12 overheating engine, I had a beautiful 1986 and never could solve over heating issues so I got rid of it, sold it!

It seems to be a little-known fact that many Mustang overheating problems are actually caused by a defective or incorrectly-adjusted distributor vacuum advance unit. Before spending a lot of time and money on the cooling system I suggest using a good timing light to verify the engine’s proper timing through the entire range, and hook up a Mighty-Vac to the vacuum advance unit to observe whether the points plate moves properly. The unit is adjustable to factory specs with an Allen wrench in the vacuum hose nipple; even new units need to be dialled-in.

My 69 rebuilt 302 Mustang engine does not overheat but the the top of the motor on the passenger side heats up to around 220 causing vapor lock, hesitation in heavy traffic and hard start right after parking. The rest of the engine, radiator,thermostat and hoses range in the area of 180 temp. I am thinking this it is due to the heat from the exhaust manifold under the head and if so any ideas as to how to remedy this?

I worked for Ford and bought a new 1965 GT with a 289-4V which yielded an impressive 28 MPG highway showroom. This engine ran so cool that I installed a perforated shield in front of the radiator and often removed the fan in the winter months.
Sorry I can’t offer any help with your overheating issue!

I believe the pictured car does not have a 289 V8, but rather a straight six. Probably 200 cubic inch. Back in the seventies during a drag race I blew up the heater core in my '66 289 GT Fastback. Maybe it was clogged a little.

the best way to solve the mustang over heating problem is sell it and buy a mopar:+1:

1 Like

Might be better off getting rid of the stock radiator fan and shroud and replacing them with a dual electric fan set up attached to the back of the radiator.
Down here in Arizona it gets hotter some months than most other parts of the US. When I first got out Shelby GT350 with the Winsor 352 engine it had the stock cooling setup. Proved to be totally inadequate on 90+ degree days - running at 215-220 degrees in light traffic.
Upgraded to dual Derale fans - one that goes on upon startup and another that goes on at 170 degrees - and I can drive in heavy traffic and my Shelby rarely goes much above 190 degrees even on 90-100 degree days.

My 66 289 overheated for years when you stopped the car
After I had the engine rebuilt It got worse
I had the radiator updated
Mounted a fan shroud
replaced the fan with a flexible fan
added overflow tank

1 Like

“For the water-flow issue, verify that the thermostat is opening by feeling the lower radiator hose and making sure it’s hot when the vehicle is warmed up.”

Why would Mr. Siegel suggest checking the lower radiator hose? I always checked the upper radiator hose directly downstream from the thermostat itself. When the thermostat opened, the temperature of the hose would quickly change from warm to very hot. If the radiator worked properly, the lower hose should have felt hot, but not as hot as the upper hose.

I guess I have been checking thermostats wrong all these years!

1 Like

I have a 1966 289 and the inlet to the radiator is on the passenger side and the outlet to the pump is on the drivers side which is correct for the 66. Ford must have made a change from 1965. But, my engine still overheats.

In response to a comment above about uneven heating of the engine. It is possible to get air trapped in the engine when you are filling it with coolant unless you get lucky or vent the engine when filling it. Per the Ford Manual, one should remove the top heater hose when filling the system so that any trapped air escapes. For years, I never use to do this until last year when I had uneven heating and could even hear “bubbling” in the engine when it was hot. I now always vent the engine when filling it.

Mustangs are just like most old cars, in the sense that 50-some years of rust and gunk accumulation within the walls of the cooling system inhibits coolant circulation and heat transfer from the block to the coolant. That rust crust is an insulator that keeps heat within the block rather than moving it into the coolant for transfer to the atmosphere at the radiator. The best cure includes simply pulling the freeze plugs and rodding out the coolant passages by poking around in there with coathangers, then flushing out everything . It is a supremely messy job, but infinitely more effective than a a power flush alone. We did this on our '54 Studebaker and ever since it runs steadily low in the heat range, whether creeping along on parade duty or spending several hours humming along at 70 mph on an Interstate.

Living in a place that regularly sees temps over 100 I tried every thing to get my 65 Fastback with A/C to run cool. Aluminum radiator, 4 row radiator, electric fans, you name it. What finally worked was the four row radiator, the most powerful electric fan that would fit and Evans waterless coolant - finally normal temps even when it was 105 in city traffic.

We drove our 1967 289 mustang for 11 years in sunny Sacramento. When everything is right, the car won’t over heat. Many people who live in mild climate think they don’t need antifreeze. Engine will produce rust and crap, and plug the radiator. Use antifreeze. I have had the lower hose collapse under acceleration, but don’t remember what car or truck it was. Some came with a coil spring inside to keep it from doing that. I would make sure the block was flushed very well and de-rusted. A rusted up block will plug up your lovely new radiator. New water pump, new radiator, carb rebuild, correct timing, and make sure the exhaust is not obstructed. If after all this it still runs hot, check you gauge.

On the rust issue, I had a Mazda pickup. Drove it for 238000. It was tired so I purchased a rebuilt engine. My truck soon began over heating. We found the radiator was plugged up with rust and crap. New radiator, and all was well, for a while, then it began over heating again. Three radiators later, it was pretty much cleaned up. That rebuilt engine had never seen antifreeze. I should have had my own engine rebuilt as I keep it correct with antifreeze and it had never over heated.


An uncommon but possible reason for the Ford V8 to overheat is an improperly installed head gasket (backward). The large hole must be to the rear for proper circulation. I have found this only 3 times in a 50 year career.

My 1966 Mustang had a 289 with 2 barrel carburetor. It had a factory air conditioning. It would overheat when negotiating hills, mostly driven in the moderate weather of Southern California. My dad believed the air conditioner condenser, which was mounted in front of the radiator, blocked air flow and contributed to the overheating of the engine.