Good Coolant Chemistry Saves Wet-Sleeve Engines

Walt Moore, Senior Editor

What would you guess is the most frequent type of diesel-engine failure caused by a poorly maintained cooling system? It's the perforation of cylinders in wet-sleeve engines, says Ed Eaton, chief engineer for Amalgatech, a laboratory in Phoenix, Ariz., specializing in research, development and analysis of engine coolants.

In a "wet-sleeve" engine, pistons operate in steel cylinders (also called sleeves or liners) that are sealed into upper and lower rings within the open structure of the block. The sleeves are "wet," because they are exposed to the engine's coolant, which is good for efficient heat transfer, but not so good for the sleeve if coolant chemistry is out of whack.

As the diesel works, combustion forces set up a natural vibration in the sleeves, causing the sleeve walls to first pull rapidly away from the coolant, creating a low-pressure area in which the surrounding coolant boils and forms tiny air bubbles. Then, as the sleeve springs back, it slams into the bubbles with a force estimated at up to 60,000 psi, causing the bubbles to collapse, or implode, violently.

This bubble-bursting action against the sleeve is called "cavitation," because it can create tiny cavities in the sleeve wall if the coolant inadequately protects the metal. Left unchecked, this "pitting" process can eventually perforate the sleeve wall, allowing coolant into the cylinder, where its incompressible nature can crack blocks, break pistons and bend rods.

Good coolant cures cavitation

Although cavitation can be devastating, it's also easy to prevent, simply by maintaining a film of cavitation-resistant native oxides on the sleeve's outer wall. The diesel engine's coolant is responsible for keeping this protective film in good repair, but to do so, it must in turn be adequately maintained. This means, essentially, keeping the coolant's additives in proper concentration.

For many coolants, a "supplemental coolant additive" (SCA) package not only has compounds that help the cooling system resist corrosion, scaling and attack by acids, but also contains nitrite, which mends the sleeve's protective-oxide film when cavitation tries to wear it away. Some SCA packages also contain molybdate, intended to enhance nitrite's healing action and thus allow lower levels of nitrite, which, in excessive concentrations, may contribute to solder corrosion.

If you still use "conventional coolant," a heavy-duty, low-silicate antifreeze with an ASTM D4985 specification, then remember that this antifreeze contains no supplemental coolant additives. At initial fill, along with mixing it with 50 percent water, you must add an SCA package, typically at a ratio of one pint to four gallons of coolant.

A better alternative, says Eaton, is to use "fully formulated" antifreeze that meets ASTM D6210 specifications. These antifreezes already contain SCA components and thus require no initial treatment. In fact, all makers of vehicles with heavy-duty diesels now require coolant meeting this standard.

If your diesel's cooling system is filled with an "extended-life coolant" (ELC), then it contains an antifreeze formulation of ethylene glycol and special organic (carboxylate) acid additives, which replace the compounds in a typical SCA package. All heavy-duty-diesel manufacturers at the present time, however, says Eaton, require that ELC formulations also contain nitrite or the nitrite-molybdate combination.

Yet another coolant family uses "hybrid technology" and contains benzoate and inorganic inhibitors, which offer extended service and improved compatibility. Hybrids intended for use in heavy-duty diesels also contain nitrite and are maintained similarly to other coolants meeting the ASTM D6210 specification.

You're the chemist

Although we offer here general guidelines for maintaining coolant quality, always defer to the engine or vehicle maker's recommendation for specific procedures.

With conventional coolant, for example, your approach may be to add an SCA package at regular intervals, then to change the coolant every two years to prevent saturation with dissolved solids. Do, however, periodically test the coolant's freeze-point.

A variation is to periodically test this coolant (at two months, 250 hours or 20,000 miles) and to add make-up SCA only when needed. A fluid-analysis laboratory can test for you, or you can use test strips designed to measure nitrite and freeze point ("2-way"), or nitrite, molybdate and freeze point ("3-way"). These paper strips react to coolant by changing colors, which can be evaluated with a supplied chart. In theory, if the nitrite or nitrite/molybdate concentration is okay, then the rest of the SCA package is adequate.

Fully formulated antifreeze is designed to be maintained by a coolant filter that releases measured amounts of an SCA package over time, or, on an "as-needed" basis, which is the best way, says Eaton, to maintain optimum chemical protection. Properly maintained, he says, these coolants may last 600,000 miles or more in truck operations. Periodic test-strip sampling is advised, along with full laboratory analysis at extended mileage or hours.

If your diesel uses extended-life coolant, vehicle makers require the addition of an "extender package" at some interval. But, says Eaton, don't ignore the coolant. Periodically test its freeze point and visually inspect it, and get a laboratory analysis annually. At maintenance intervals, if it's orange and clear and has adequate glycol, then it's probably in good shape.