65 Comet overheating


I have a 65 Comet Caliente overheating when I am in city traffic or idling for a while. Since purchasing the car, I have installed a new 160 degree thermostat, had the radiator serviced and still have a problem. What can I do now to correct the overheating? The car originally did not have a fan shroud and still does not. Thanks, Joe


Have you checked the radiator cap? An old one may not be able to maintain enough pressure in the cooling system. I usually replace mine every coolant change.

Ed B.


It has a new 13# radiator cap on it as suggested by the radiator shop.


Hello,when your rad was serviced was the water rusty?If so you might consider the water pump.Do to the age of this flying machine the impeller that moves the water thru the system may have rusted and is no longer doing its job. S-King


Question : What are the overheating symptoms ?

Blowing off coolant ?

High temp reading ?

If the car is blowing coolant past the rad cap, then you do have a physical cooling problem.

But if you are only showing a high temp reading, I’d look at the temp sensor first particularly if it’s the original unit. Changing that out is a ten minute job compared to tearing water pumps etc; down.


The water was not rusty because a previous shop supposedly cleaned it out but it never changed anything. The second shop said the fins were 40% blocked and cleared them.


The problem is two-fold. When I drive on the highway, it is OK. In slow traffic, the temp guage gradually gets hotter and hotter. When I stop, I can hear it boiling and radiator fluid escapes from the overflow tube. Shoul I change the temp sensor out just to eliminate that as a potential problem? Joe


If I recall correctly, that engine had a clutch fan on it, is it still the original? If it is, I think it is time to change. I would also go for a water pump.

Make sure there are no leaks, especially around the freeze plugs on the block.


The car has 92000 miles on it and I believe it is all original. Are the temp sensor, clutch fan, and water pump difficult to change myself?


Have the radiator temperature read with an infra-red thermometer. That can tell you if the cooling system is getting too hot. It is normal for a cooling system to safely operate at 220F degrees. Yes, after the thermostat opens, the coolant does get hotter than 170F degrees, and its temperature goes to 220F degrees. This is normal. Changing to a cooler thermostat does nothing beneficial. In fact, it can increase carbon deposits in the cylinders and the exhaust.

Make sure that the idle rpm is where it should be. An idle too low can turn the fan too slowly for good cooling.


Well if it’s boiling over you have a physical problem. But easy things first, I’d leave the sensor alone since it seems to reflect reality.

First thing, does the heater work ? If not or is heating poorly you could have either a blocked ot airlocked heater matrix. Make sure you have no airlocks. If the heater is working okay then you can eliminate that.

Next thing, remove the stat altogether and eliminate that, just because it’s new doesn’t mean it’s okay, see how the car behaves with the stat out and test the stat to make sure it’s functioning correctly.

If both these things seem okay, that water pump could be the culprit as already mentioned by S-King, your next port of call I’d think.


Difficult no, water pump uncomfortable yes.


Is there a fan clutch? If so it may be the problem. If there is no fan shroud, how close to the radiator is the leading edge of the fan blade? I light a cigarette to blow smoke into the grill of a car with your problems. When idleing hot the smoke should be ‘briskly’ pulled away.


Is that supposed to be helpful ?

If you read the poster’s comments I’d day he can forget about a temp sensor problem.


The neat thing about these older cars is that you can look directly into the top tank of the radiator and see if the coolant is flowing. Warm the engine up to operating temperature without the radiator cap on, then rev the engine a little while looking in the top tank. You should be able to see the coolant flowing. If you can’t, then you need a water pump or the thermostat is stuck.

Normally, if the radiator cap or water pump is bad, the overheating occurs at higher speeds, not idle. These old cars are prone to running hot if left idling for any length of time, even overheating. They did than when they were new. One solution was a clutch fan. If the clutch wears out, then the fan does not work as efficiently. With the engine up to operating temperature, shut it down and then try to turn the fan. If it turns easily, then you need a new clutch.

Not all those older cars had a fan clutch in them though.

If this car was ever in California during the 70’s, then it may have a NOX kit on it. The most popular NOX kit blocked your vacuum advance, that made the engine run hotter. But it came with a thermostat device that would allow the vacuum advance to turn on when the engine got too hot. If it has one and the thing isn’t working right, that would contribute to your problem and I would recommend that you bypass it and just run the vacuum line from the manifold or carburetor base to the distributor. BTW, check that the vacuum advance is working properly.

Last thing would be look into non stock modifications. Like I said, most of the cars of the 50s and 60s were prone to overheating while idling for extended periods of time or in heavy stop and go traffic, especially a traffic jam. Some of the mods to consider would be a coolant recovery system or an auxiliary electric fan.


I have installed a new 160 degree thermostat

I used to do this many years ago on my older cars as well before realizing it was a misguided attempt at getting a cooler running engine. Colder running engines are less efficient and are prone to sludging problems. Most people do this as a means to reduce intake air temps and get more performance but it leads to longer term problems IMO. The engine should run very well at around 195 deg F with everything in working order.

The first things to do are to confirm that the engine is sound and all cooling components are working as expected.

When you had the radiator serviced, what exactly did they do to it? The days of flow testing radiators seems to have disappeared due to the cost versus just replacing it and being positive about the outcome. How do the tubes look when viewed from the filler neck? are there a lot of deposits on them?

Have you checked some of the basics like verifying the ignition timing, carb mods the prior owner may have done, wrong rad for your application, etc. Is it basically a stock setup?


The way I read it, the car runs fine with air blowing across the radiator while driving. But, when in slow traffic, it overheats. The car has no fan shroud, so I think this is the starting point. The fan is not pulling air through the radiator properly. A fan shroud will improve this. Is this a car in stock finish, or has it been modded in any way, like with larger engine, or a V8 swap? If so, the radiator may be too small for the job. But, I’d start with a fan shroud. That may help narrow it down. Here’s one at JC Whitney :http://www.jcwhitney.com/CHROME-PLATED-UNIVERSAL-FAN-SHROUDS/GP_2000845_N_111+10201+600001646_10101.jcw


I agree with bustedknuckles about the shroud . . . and add this . . . why not get a new radiator from JC Whitney when you order the shroud . . . $130 for the radiator and $30 for the shroud. If this '65 is a keeper, spend a few bucks before you toast the motor. The cooling system on these cars is pretty simple, and if it cools on the highway, mechanical doesn’t seem to be the problem(like timing or dragging brakes). The radiator and shroud will do a lot . . . also the thermostat and radiator cap should be replaced. Only thing left would be hoses and water pump. Nothing worse than toasting the motor in traffic due to overheating, and although not really cheap solutions, much better than replacing the motor. A car of this year could have rust in the system . . . rusted blades on the water pump (not pumping enough coolant) . . . air in the system . . . lots of things. By process of elimination, I’d go from cheap and easy to hard and expensive until I solved it. New (correct) thermostat and radiator cap and flush of the system, fill with new coolant. Then radiator and shroud . . . then water pump . . . then new hoses . . . lastly the heater core, not too expensive but can be a pain to change on some cars. Good luck and have fun with this old Merc. Rocketman


Good, no shroud. Check those fins between the tubes. If ANY are corroded away, change the radiator.


All of the responses have addressed water, and air flow; but, is extra heat being added to the coolant by exhaust gases going into the coolant?
This could happen through the head gasket(s). A repair shop can use a gas analyser to “sniff” for exhaust gases in the coolant.