Make your classic car’s cooling system dependable

This week’s installment of The Big Six (the problems in the ignition, fuel delivery, cooling, charging, belts, and ball joints that are likely to strand a vintage car) is dedicated to the cooling system. I should say “the cooling system for cars with water-cooled engines,” because if you own a vintage Porsche, Volkswagen, Citroen 2CV, Corvair, or, heaven help you, a Trabant, your car is air-cooled.

On a vintage water-cooled car, the cooling system consists of the radiator, water pump, thermostat, hoses, fan, belt, and heater core. Newer cars may augment or replace the belt-driven fan with an electrical fan, actuated by a relay and temperature sensor.

old radiator
Really you have no idea what’s lurking inside an old radiator.


The good news about cooling systems in vintage cars is that they’re all metal (except the belt and hoses). Therefore, coolant loss is rarely catastrophic. The metal may corrode and leaks may develop, but you are unlikely to hit a bump, crack a component, and lose most of your coolant, as frequently happens with newer cars and their plastic-laden cooling systems. But the bad news is that, truth be told, most cooling system problems require sucking it up and replacing the radiator before the problems are completely put to bed.

Hot Running: First, it is crucially important that you not drive any car, vintage or otherwise, with the temperature gauge in the red. If that happens, wherever it happens, unless your life is in immediate danger, stop! Do not drive the car! The inconvenience and cost of the tow will pale compared to the cost of a cracked cylinder head.

Verify that the belt that drives the water pump hasn’t snapped or jumped the pulley, then check for coolant leaking from a burst hose, a failed water pump seal, or the heater core.

Next, feel the hose coming from the bottom of the radiator. If it’s stone cold, that means the thermostat isn’t opening. Thermostat replacement is fairly easy and inexpensive.

A word on thermostats: Many folks install a cooler thermostat (one that opens at a lower temperature), thinking it’ll help the engine run cooler. If the radiator is robust, then that’s true, but if an old radiator isn’t adequately exchanging heat (and it usually isn’t), a cooler thermostat is not going to make a difference. The better path is to install the best radiator you can, then select a thermostat that causes the engine to run in the middle of the gauge.

Absent those causes, most hot running problems originate from the fact that, typically, vintage cars didn’t have great cooling systems when they were new. Since then, their performance has only degraded due to corrosion inside the radiator making it less able to shed heat. If a drive during hot weather or up a big hill causes the temperature to rise dramatically, replace the radiator, even if it looks good.

Note that the heater core is essentially a second radiator inside the passenger compartment. The old advice that if a car is running hot you should turn on the heat is usually true. As long as the car isn’t leaking coolant or running in the red, it is often possible to avoid stranding and nurse it along by stopping to let it cool down, turning on the heat, running at night, and being light on the throttle.

Hot Idling: If the car idles hot but cools down when driven, if there’s no electric fan or fan clutch, the diagnosis is the same as hot running – you probably need to replace the radiator. If there’s an electric fan, make sure the fan comes on while idling. If it doesn’t, there could be a problem in the fan, relay, or sensor. Anyone whose car has an electric fan is strongly advised to know how to wire the damned thing directly to the battery if need be.

Maintenance Tips

External inspection: Check the belt. If it’s cracked, replace it. If it’s loose, tighten it. Grab one of the fan blades and rock it fore and aft to test the water pump bearing. If there’s anything other than the tiniest amount of play, the water pump should be replaced. Inspect the hoses, both visually and by squeezing them. They should be firm but pliant. If they’re soft and pillowy, with the hoses ballooning over the clamps, or rock-hard with the clamps cutting into them, they should be replaced immediately. I wouldn’t drive an unfamiliar vintage car more than a few miles without first doing these three things.

Open, inspect, and re-seal: Cars that sit for years are prone to corrosion buildup in every part of the cooling system, as water and coolant react with steel and aluminum. Really you have no idea what’s inside until you open it up. Drain the coolant and remove and inspect the water pump. If it’s corroded, the inside of the radiator might look the same. Then, pull off every hose and, with a pen knife, scrape any corrosion off the coolant necks to help the hoses seal. If you’re trying to be cost-conscious, and the water pump looks fine and has no bearing play, and the hoses are firm and free of cracks and cuts, just put the pump back in with a new gasket, re-use the hoses, and absorb the peace of mind from the inspection process.

corrosion inside a water pump
This water pump was functional, but the corrosion inside was pretty nasty.

Full prophylactic cooling system maintenance: Basically, flush, then replace everything – radiator, water pump, thermostat, fan, belt, and every hose. Make no mistake, that’s the smart move if you plan to road-trip and don’t know how old the car’s cooling components are, and don’t want to get stranded. I have not had good luck with “rodding out radiators” (taking them to a radiator shop for disassembly and cleaning). Any new radiator is better than a 40-year old one, but check enthusiast forums to find out the options for an improved radiator for your make and model. A triple-core radiator that looks similar and occupies the same physical space, or a larger aluminum radiator may be available. Different “tropical” fans that push more air may also be an option. The choice comes down to your desire for originality.

Stay cool.

(Next week: The charging system.)

Rob Siegel has been writing the column The Hack Mechanic™ for BMW CCA Roundel Magazine for 30 years. He is the author of Memoirs of a Hack Mechanic and The Hack MechanicGuide to European Automotive Electrical Systems. Both are available from Bentley Publishers and Amazon. Or you can order personally inscribed copies through Rob’s website: Look for his upcoming book “Ran When Parked: How I road-tripped a decade-dead BMW 2002tii a thousand miles back home, and how you can, too.”

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    can you tell me the required omh’s for a coolant sending unit for a 1931 Hupmobile six cylinder

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