We all know the value of anti-freeze/coolant. Anti-freeze is essential for preventing casting cracks when you park the vehicle in freezing weather. By contrast, the coolant properties raise the boiling point of the solution, making our modern engines tolerate higher operating temperatures, which can provide more complete combustion of fuel and cleaner tailpipe emissions. Higher pressure radiator caps also help raise the boiling point.
Every liquid cooled engine parked at below freezing temperatures requires anti-freeze. Specifications call for anti-freeze/coolant that is compatible with engine and cooling system metals. We follow these requirements to extend engine life and preserve the engine's castings, seals and gaskets, heater core and other vital cooling system components.
In addition to the type of anti-freeze/coolant, there is the manufacturers' recommendation about the concentration or "specific gravity" of the anti-freeze mixture. For cooling in summer and reasonable anti-freeze protection in the winter, most manufacturers settle for the traditional minus-34 degrees F anti-freeze protection as a year-round mixture. A 50/50 mix of pure (straight) anti-freeze and distilled water will usually provide this degree of anti-freeze protection. (See the label on the container.)
Some environments require even more antifreeze protection. However, most products limit the maximum anti-freeze protection to something like minus-60 degrees F or a maximum percentage like 70% antifreeze and 30% distilled water.
A closed thermostat (new Cummins thermostat shown at left) and too much anti-freeze/coolant are a recipe for excessive pressure in the cooling system. The right amount of anti-freeze/coolant will raise the boiling point of the coolant. Too much anti-freeze/coolant can actually cause boil over and coolant loss, damage to the radiator or heater core, and a reduction in anti-freeze protection...You wouldn't want to overheat—or freeze and crack—this Cummins 5.9L inline six cylinder diesel's head or cylinder block by running either too little or too much anti-freeze/coolant!
Warning: Do not attempt to increase the concentration of antifreeze for a temperature lower than advised on the label. Running a stronger concentration of anti-freeze than this will not provide better anti-freeze protection. In fact, with too much anti-freeze/coolant, the freezing protection decreases. Overly high concentrations or pure anti-freeze may lead to cracking a casting in freezing temperatures. As for boil over, the boiling point actually drops with too much anti-freeze concentration. The system may boil over—either during normal warm-up phase of the engine or at normal engine operating conditions!
Anti-freeze is designed to mix with distilled water. If you run straight anti-freeze, there is a likelihood of high cooling system pressure during warm-up with the thermostat closed. The engine may also boil over within normal operating temperatures. In freezing weather, you can crack the block, a head or other castings by running either too much anti-freeze in solution or pure anti-freeze!
Pure anti-freeze is not to be confused with "pre-mixed" anti-freeze coolant. "Pre-mix" is typically distilled water and anti-freeze mixed before packaging at a 50/50 ratio. This "pre-mixed" anti-freeze coolant is usually good for minus-34 degrees F protection in the winter and a boiling point of 260-plus degrees F in the summer—with the right pressure cap on the radiator or system.
Boiling point increases with the use of a specific radiator cap pressure, usually 17 PSI or so for most modern engines. If the cap pressure is lower than the recommended OEM cap, the boiling point will drop accordingly. For this reason, it is important that your radiator cap is in top condition and holding proper pressure. Understand that a vintage vehicle with a much lower pressure radiator/cooling system cap will have a lower boiling point than 260 degrees F, even with 50/50 mix of anti-freeze/coolant.
So, make sure your engine's cooling system is protected against both boil over and freezing. But don't use more anti-freeze than the mixture for the lowest recommended temperature protection on the anti-freeze/coolant container. (Typically, this mixture limit is indicated on the container's label.) Know whether the anti-freeze is pre-mix or pure anti-freeze. Make sure you allow the coolant to mix thoroughly before reading the protection level with an anti-freeze hydrometer or specific gravity tester.
Too little anti-freeze/coolant is dangerous and leaves the engine unprotected against cold freezing. Too much anti-freeze/coolant can also lower protection against both overheating (boil over) and cold freezing. Anti-freeze requires the right amount of water to work properly. Read labels carefully.
As a footnote, we're talking about the anti-freeze/coolant in the radiator and overflow tank. Always check the anti-freeze at the radiator after the engine has circulated coolant thoroughly, including through the heater core; to avoid severe skin and eye burns, remove the radiator cap only after the engine has cooled down completely! Loosen the cap slowly, stop at the first notch, and release all pressure before removing the cap.
Prestone or equivalent tester like the one at left can be purchased for $5 or so at any auto supply. If you follow directions, this hydrometer can be accurate and a quick test for anti-freeze protection. At right is a Stant cooling system pressure tester for the radiator/cooling system and also for testing cap pressure. A Stant diagnostic tool kit like this has been in my tool set since 1981, and it still works great. The cooling system pressure tester has a variety of uses and is an excellent troubleshooting tool.
Mix the anti-freeze solution in the overflow bottle to the same mixture you have in the radiator. After several complete heat-up and cool down cycles, the anti-freeze/coolant in the engine, radiator and overflow bottle should reach a uniform mixture. At that point, measure specific gravity with the anti-freeze hydrometer to get an accurate read on the protection level. Test the radiator cap's holding pressure with a pressure tester if available. If in doubt, install a new radiator cap.
Periodically, test anti-freeze/coolant protection at both the radiator filler neck (engine completely cooled down first!) and at the overflow bottle. If the cooling system has been transferring coolant back and forth—cycling from cold to hot and back to cold over a long time—a quick hydrometer test at the overflow tank can be accurate.