160,00 to 200,000 kilometres is a long, long time to drive a car or light truck. Few mechanisms in a typical engine can operate that long without some kind of service, and almost none of the fluids. The exception is long-life engine coolants. Most major O.E.’s offer an extended life coolant, and in the aftermarket, the major companies all have their own brands, often using G.M.’s original title, “Dex-Cool”. What makes it different from conventional anti-freeze? Both conventional and long-life products are glycol-based and must be mixed with water to offer decent protection. The difference is in the additive packages. Conventional coolants use multiple chemistries: nitrites, nitrates, borates, phosphates and amines are used in various combinations to fight corrosion, prevent precipitation of abrasive solids, maintain good heat transfer properties and protect seals and gaskets. Long life formulations normally replace these additives with another chemistry based on carboxylates, which are sometimes referred to as “organic acids”, although this shouldn’t be confused with acids which destroy a coolant’s corrosion protection properties.
An added benefit of carboxylate additive packages in long life products is the elimination of silicates. Silicates (yes, it’s the same stuff that makes up sand and many industrial abrasives) are excellent aluminium corrosion inhibitors, but the suffer from a serious problem if used by themselves: they don’t dissolve easily in coolant. Silicate stabiliser chemicals are usually added to help keep the silicate additives in solution, but off-brand low-priced coolants often omit the stabilisers, leading to plugging and premature water pump seal wear as the silicates drop out of solution. Another issue caused by silicates is shelf life. In time, silicates in solution form gels, limiting the shelf life of conventional products to about 18 months. While this is rarely a problem in a shop environment, consumers who top-up their vehicles from rarely used jugs at home may be contributing to premature failures. It’s another argument for repairing leakers, even if the loss rate is small. Long life coolants, without silicates, have typical shelf lives of eight years or more. Silicates and phosphates (they’re important in maintaining pH in some formulations..see the box below) are also contributors to the “hard water scale” that lines abused systems. The additives used in conventional coolants are at the heart of much confusion about long life products, namely topping up with conventional coolants. Up to ten percent by volume is allowed by many manufacturers, but adding silicates or phosphates makes little sense unless it’s an emergency and there’s no alternative.
Is compatibility an issue?
According to the American Society for Testing Materials, which establishes standardised performance testing for automotive coolants, long-life products are suitable for five year, 150,000 mile drain intervals. While there’s no question about the effectiveness of carboxylate chemistry in keeping long-life coolants effective over the long haul in newer engines, General Motors Service Bulletin #53-62-02 from November 1995 warns against using extended life antifreeze in pre-1993 GM cars, after which it was the factory fill. The reason is possible corrosion of radiator copper soldered joints. Another caveat regarding long life products involves vehicles that are driven infrequently. Remember the “organic acid” definition of the carboxylate chemistry in long life coolants? In some cases where a vehicle sits for an extended period, bacteria can consume the carboxylates, reducing their effectiveness. Engines that are run frequently reach temperatures that kill the bacteria, but collector cars or vehicles in long-term storage may need a shorter drain interval. In every case where conventional coolants are replaced with long life products, thorough flushing of the system is required. No trace of old silicate/phosphate additives should be left in the system. Some sources claim that the only sure way to convert to long life is complete system disassembly and acid cleaning of all parts to remove scale. Major manufacturers generally disagree, and recommend that thorough flushing is adequate, since the long life formulations are fully compatible and there’s little old coolant left in a well-flushed system. One thing is certain: Long life coolants should always be used in vehicles that were factory filled with them.
What is pH?
Every swimming pool owner knows the importance of maintaining the proper pH in the chlorine solution that we call “water”. Human blood needs the correct pH too, but what exactly is “pH”? It’s a measure of the acidity or alkalinity of a solution; pH is the chemist’s shorthand for “potency of hydrogen’, since hydrogen ions were once thought to be the sole contributor to the acidity of a solution. It’s measured on a scale from one to fourteen, with a perfectly neutral solution measuring seven. Acidic solutions are less than seven, and alkaline solutions read over seven. Inside vehicle cooling systems, pH control is crucial to prevent corrosion. Alkaline readings from about 9 to 11 are preferred, and if the pH drops below 8.5, the formula may become unstable, leading to corrosion. Phosphate additives help stabilise pH. In long life formulations, carboxylates help keep pH stable (in general, pH stabilisers are called ‘buffers’). With modern cooling systems and formulas, colour and clarity are no indication that the additives are doing their job. Testing with a pH meter or test strips is a much more precise way to judge if the coolant needs replacing ahead of the normal drain interval.