A common problem with older cars is that they either run too hot or too cold. Before trying to fix a problem that may not exist, you should first try to make sure that you are accurately measuring engine temperature. Cars would have either been equipped with a factory temperature gauge on the dashboard or an idiot light. It is easy to add an aftermarket gauge to supplement the warning lamp.

An engine's radiator is simply a water to air heat exchanger and it's job is to reject heat generated by the engine to the surrounding air. Heat transfer is a function of flows of air and coolant and the difference in temperature between the two. Increasing flow (air and/or coolant) and increasing the temperature difference both increase the rate at which heat is rejected from the engine.

Because an increasing temperature difference between air and coolant increases heat transfer, if an engine does not run overheat with a 160°F thermostat, it most certainly will not overheat with a 195°F thermostat. Engines tend to run more efficiently with hotter temperatures so it is better to use a winter thermostat year-round.

Often, a 160°F thermostat will be used to combat problems such as carburetor percolation or engine knock. For percolation, a better solution would be to use a better insulating gasket between the carburetor and intake manifold. For engine knock, a better solution would be to remove rust and scale from the engine's internals with a good cooling system flush.

NOTE: To avoid burns and injury, never, ever attempt to remove a radiator cap while the engine is hot!


Automotive Cooling System


In the old days, cooling systems typically ran water in the summer and a mixture of methanol and water in the winter. Methanol (methyl alcohol) was a problem with the early vented radiator caps because it would boil away with use and would have to be constantly replenished to ensure that it had a low freezing point. While a 35% (vol) concentration of methanol has a freeze point of -15°F/-26°C, it also gives off toxic fumes and has a flash point of 95°F/35°C. The flash point is the temperature at which it will generate a combustible fuel mixture.

Ethylene glycol is added to water to both raise engine coolant's boiling point and lower its freezing point. Ethylene glycol and water mixtures (aqueous ethylene glycol) has a lower freezing point than either pure water or pure ethylene glycol and it has as eutectic temperature of about -69°C/-92°F at about 68% ethylene glycol & 32% water by volume. A common concentration of automotive coolant is 50% ethylene glycol by volume, which provides a freeze point of -34.2°F/-36.8°C. Below this temperature, the ethylene glycol solution with be a slush rather than a solid.

Water is an excellent heat transfer fluid as it has a specific heat capacity about twice that of ethylene glycol so that pure liquid water is twice the ability of pure liquid ethylene glycol to transfer heat. Using 50% ethylene glycol coolant requires about 25% more flow to carry away the same amount of heat as pure water.

Ethylene glycol generally does not degrade with use. The stabilizers and corrosion inhibitors are gradually consumed over time which is why the coolant must be periodically replaced. The old "green" coolants typically have a lifespan of 2-3 years while newer long-life coolants (OAT -Organic Acid Technology, HOAT - Hybrid Organic Acid Technology) typically last 5 years. Do not mix coolant types unless the product you are adding specially states that it is compatible with the coolant in your system.

It is better to use distilled water in the cooling system rather than tap water because tap water contains impurities such as chlorine and minerals.  Chlorine is used for disinfection but can contribute to corrosion.  Naturally occurring minerals in tap water can become scale in the cooling system.

ETHYLENE GLYCOL POISONING.Ethylene glycol tastes sweet but is very toxic to humans and animals. Clean up all spills to prevent ingestion!

In a closed container, all coolants produce vapor pressure. For coolants comprised of a mixture of more than one liquid, the combined vapor pressure is a result of the sum of the partial pressures of each component. The pressure built-up in an engine's cooling system is a function of temperature and the relative amounts of component liquids in the coolant.

Adding ethylene glycol to water is an excellent way to reduce the freeze point of the coolant while modestly increasing the coolant's boiling point. A concentration of 50% (vol) ethylene glycol in water is a good compromise between improved freeze & boiling points and reduced heat transfer ability. The following table (based on calculated partial pressures with Antoine constants) compares the boiling points of 50% (vol) ethylene glycol and 100% water for various pressures:


Temperature °F:
50% Ethylene Glycol
Temperature °F:
100% Water
0.0 226.1 212.0
1.0 229.6 215.1
2.0 232.9 218.3
3.0 236.0 221.4
4.0 239.0 224.3
5.0 241.9 227.1
6.0 244.7 229.7
7.0 247.3 232.3
8.0 249.9 234.8
9.0 252.3 237.2
10.0 254.7 239.5
11.0 257.0 241.7
12.0 259.2 243.8
13.0 261.4 245.9
14.0 263.5 248.0
15.0 265.5 249.9
16.0 267.5 251.9
17.0 269.4 253.7
18.0 271.3 255.5
19.0 273.1 257.3
20.0 274.9 259.0


According to Stant's FAQ, a thermostat typically open at ±2°F of its rated temperature and reaches full open at 15 - 20°F above its rated temperature. Using a 195°F thermostat, for example, means that it should be fully open around 210 - 215°F. A 4 lb pressure cap raises the boiling point of the coolant to 224°F with 100% water or 239°F with 50% ethylene glycol.

Using a higher pressure cap will prevent the engine from boiling over at higher temperatures but the higher pressure will unnecessarily stress the cooling system IF the engine overheats. A higher pressure radiator cap will NOT cause the engine to run cooler.

Be very careful about using a pressure cap that has a higher pressure rating than that recommended by the vehicle manufacturer.

Coolant expands as it becomes warmer. Modern cars have sealed coolant systems so that it is completely full of coolant and the expanding coolant spills into the expansion tank. When it cools, a vacuum breaker valve in the valve cap allows the fluid (or air) to return to the cooling system. Coolant expansion tanks are typically translucent to show their coolant levels and are marked to show the full hot and full cold levels.

Older vehicles have a large upper tank in the radiator that allow for the expansion of the coolant. As such, the normal coolant level in the radiator is never up to the neck when the engine is cold but is at some level above the radiator tubes. If the radiator is completely filled with coolant when cold, excess coolant will be spilled through the radiator cap onto the ground as it becomes hot. Enough coolant to keep the tops of the tubes submerged with cold engine operation is sufficient for these older systems. Make a note of the cold coolant level for when you may need to top up at a later time. You can verify the hot and cold coolant levels by leaving the radiator cap off and allowing the engine to reach operating temperature while idling in your shop. Do NOT remove the radiator cap when the engine is hot!

Many vehicle owners add their own overflow tanks to older vehicles (a 24 oz / 710 ml beer can works well) if a modern expansion tank cannot be retrofitted. It is not necessary or recommended to completely fill the radiator of an older vehicle with a retrofitted expansion tank. The beer can is only there to contain any spillage.

Do not add 100% ethylene glycol to top off the cooling system as this will increase its concentration in the cooling system and reduce its cooling ability. Either use commercially available premixed coolant or mix your own. Obviously, for a 50% (vol) concentration, mix equal volumes of each.

The factory gauges often just had lines (and no numbers) for temperature graduations. Whether your car has instrumentation or a warning lamp, it is not very hard to calibrate it. This is easily done by putting a thermometer in a idling engine. I like the digital thermometers with a meat probe and you can even use them for cooking after calibrating your instrumentation. With the thermometer placed in the neck of the radiator, start the engine and let it idle. As the temperature gauge's needle passes a line on its face, record the temperature displayed by the thermometer. For those of you with cars having closed cooling systems with expansion/overflow tanks, you may need to syphon some coolant out to prevent the radiator from overflowing when hot. It would be wise to do a flush & fill with fresh antifreeze (ie, ethylene glycol) with a strength of 50% and not more than 68% (by volume).

When the flow increases substantially from the minimum observed from a cold engine, make a note of this temperature as it is the temperature at which the thermostat opens and the temperature gauge's reading. To increase the engine's temperature, block off the radiator with some cardboard, preferably the same size as the rad's cooling area to ensure minimal airflow through the rad. Be careful as the coolant will be very hot and you may need to siphon more coolant off to prevent the rad from overflowing. If your coolant has an insufficient ethylene glycol concentration, the coolant may start to boil.

With the front of the rad blocked off, the engine's temperature will continue to rise. Do not allow the engine to become hotter than the last graduation of the temperature gauge or when the temperature warning light comes on. Remove the cardboard from the from the rad and let the engine temperature stabilize at its normal operating temperature and then shut it down.

A hot running engine is can be caused by a variety of problems:

  • Defective thermostat
  • Scale & rust accumulation
  • Blocked hoses
  • Incorrect ignition timing
  • Cooling System Leaks
  • Wrong Coolant Concentration
  • Bad cooling fan
  • Bad water pump
  • Defective fan/water pump belt

The above calibration will determine at what temperature the thermostat opens. It is possible that, even though it may open at its rated temperature, it may not fully open.

Especially with older cars that have had multiple owners, there may be some accumulation of scale in the radiator and within the engine. This accumulation could block off the passages within the radiator which can severely downgrade the radiator's ability to transfer heat. Rust and scale on the internal surfaces of engine will reduce the heat transfer to the cooling water. The only way to remove both rust and scale is with an cooling system flush that contains an acid safe to the metals used in the engine. A good commercial flush is Gunk Super Radiator Flush, PN C2124C. You can also use citric acid (available through DIY wine shops) and oxalic acid (wood bleach) - both are safe on cast iron, brass, and aluminum used in older automotive engines. A chelating solution (like Evapo-Rust or Metal Rescue) will also remove iron rust from automotive cooling systems.

It is important to keep the coolant from boiling within the engine. The coolant strength should be kept at a concentration of 50% (vol) and any leaks must be repaired. A pressure tester will help determine how well the cooling system is sealed.

Since I've been using the green antifreeze, I try to do a flush & fill every 3 years.  In October of 2015, I did another citric acid cleaning and noticed that my water pump was leaking at the shaft.  This image shows my water pump housing in my slant six after I ran an 8.1% citric acid solution (1 kg citric acid in 12.3L) for a few days. I drained out part of the solution when I removed the rad in order to replace the pump.  I couldn't finish replacing the water pump right away so the water pump remained half-immersed in the acid solution for a few days.

Water Pump Housing After Citric Acid Flush 15-10-08


Because the inside of my engine got rusty during the water pump replacement, I put the citric solution back in and ran it a for a few more days before filling it with fresh coolant.  After less than a month, my new water pump developed a leak at the shaft, so I had to replace it once again.  As you can see, the interior of my engine now looks freshly cast. I'm not impressed with the quality of these imported water pumps but this 2nd pump is still leak-free.


Water Pump Housing After Citric Acid Flush 15-11-07


I've looked inside the cooling systems of my other newer cars (with aluminum engines) and their interiors look like new. If you keep up with regular flushes (removing old coolant & replace with fresh coolant), the coolant's corrosion inhibitors should keep the engine corrosion-free and reduce the need for chemical cleaning.

A cold running engine is generally due to a bad or missing thermostat. It would be too cold or take too long to warm up because it doesn't fully close. There is nothing to be gained by running the engine too cold because the engine will run inefficiently. It could also potentially suffer from excessive wear because internal components such as pistons will not expand to their fully warm sizes.