(Editor`s note: Part 1, “Diesel Cooling System Service,” appeared in the May 1999 issue.)

When I`m instructing a new group of technicians, I enjoy the reactions I get when I ask, “What kind of engine coolant do you use?” Today you will find a variety of coolants and additives from which to choose.

Diesel coolant must have special properties and additives. How does a diesel engine differ from a gasoline engine? The first thing that comes to mind is engine compression. To burn diesel fuel, we need about three times the compression. This puts a tremendous stress on the cylinder walls, which move in and out as the engine compresses, fires, and then exhausts. This ends up as a vibration of the cylinder walls, where the normal antifreeze coolant mixture cannot follow. A cavitation of the coolant results. Because of the high compression, the coolant gets pushed away from the cylinder walls on the compression-firing strokes with such speed that it cannot return with the exhaust-intake strokes. Air bubbles develop along the cylinder walls and collapse with a force similar to hammer blows. In an untreated cooling system, these blows against the cylinder walls may exceed 15,000 psi. Enough of these ham-mer blows can cause pitted, cracked cylinder liners and blown head gaskets. Of course, the hotter the motor, the worse this cavitation is and the greater the chance that the motor will fail during pump testing–or even worse, at “the big one.”

The cooling system additive nitrite minimizes this hammering effect. A common industry name for this additive is SCA (supplemental coolant additive), and it is marketed in the following ways:

•as a gel-containing coolant/antifreeze concentrate and SCA that needs to be mixed with water (it looks like an ordinary automotive coolant),

•premixed with “pure water,” and

•as an SCA added to the normal automotive coolant/antifreeze and water mix.

If the motor is equipped with a coolant filter, as an option, the filter can be “precharged” with the proper nitrite additives. These filters are available as new or replacement precharged that would be installed after flushing, service precharged (sometimes called a maintenance filter), or no precharge (sometimes known as a blank filter).

Some manufacturers market a “need release” filter. As the nitrite is used up, the filter releases the proper amount of additives to keep the system in balance.

Each diesel manufacturer recommends different levels of nitrite and other additives. Some of the jobs assigned to the additive are to protect against mineral scale, oil fouling, deposits, rust, and general corrosion. It is very important that you use the right additive for your motor. Be leery of the “universal coolant/antifreeze,” even though it says that it is approved by your original equipment manufacturer (OEM). The formula may not have any SCA.

The nitrite and other additives do get depleted. Proper maintenance entails checking the nitrite levels during your regular PM (oil changes) interval. You can do this by using test strips supplied by your motor manufacturer. The test strips look like the litmus paper used in chemistry class. Two, three, or four tests can be done on each strip. Check for the following: (1) the correct nitrite level, (2) the pH level (acid), (3) the percent of glycol (freeze point), and (4) the presence of a wrong (different manufacturer) additive.

Again, you have to use the right strips for the additive and manufacturer being tested. The strips do have a shelf life; make sure the date on the bottle is not expired. They must also be airtight and kept dry. Some manufacturers offer individually wrapped strips for the low-volume shops. Additional additive (SCA) will have to be added at every PM to maintain the proper level.

If the above is too difficult for you, buy a coolant analysis kit from your manufacturer, and send in a sample. The manufacturer will even suggest the type of service (if any) you will need.

Since too much coolant additive is as bad as too little, I recommend that if your driver-operators are topping off the system when the coolant is low that they be supplied with the proper coolant-water-additive (SCA) mix. We cannot expect the average guy to mix the supplied concentrate coolant 50-50 before filling; somehow the solution always seems to be on the rich side. So with premixing and creating a one-step top-off procedure, the proper balance is more likely to be maintained at a proper level for ideal cooling system protection.


What is the biggest selling point of having a coolant system maintenance schedule in place? Internal motor damage will occur without the correct coolant, water, and chemical mixture.

As I mentioned before, the additives get used up, so make sure to flush out the system every two years. A good flush machine is invaluable for the time it saves, the thorough job it does, and the coolant it saves as a result of the refill feature.

There are some five-year coolants on the market. They may cost twice as much, but they might also save you a flush job. Just don`t mix the two-year stuff with these coolants!


I am sometimes asked about coolant recycling machines. In my shop, I consider the price of coolant vs. the price of repair and the downtime if the apparatus should go down because of mechanical failure; the more expensive “fresh” coolant and additives win every time. If recycling is used, be aware that OEMs may approve only certain methods of recycling, and a base booster must be used to replenish agents that were used up and are missing in the recycled coolant.

When mixing straight coolant antifreeze with water, use a good quality of water. Water containing excessive materials will damage the cooling system quickly. If using tap water, have it analyzed. Water must be within the following parameters: less than 40 ppm chloride, less than 100 ppm sulfate, and less than 170 ppm hardness (calcium/ magnesium). If your tap water does not meet these criteria, use coolant premixed with deionized, distilled, or demineralized water.

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A gallon of concentrate coolant/antifreeze with additives (SCA).

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An SCA additive that can be mixed with regular coolant/antifreeze to boost the SCA level.

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Precharged coolant filters with the right amount of SCA. The right filter must be used with the right amount of SCA when changing the oil filter at the PM interval.

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The SCA test kit includes test strips that will tell you the nitrite level, the pH level, and the percentage of glycol when dipped into the coolant.

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This premixed coolant/antifreeze is a good idea when adding coolant in the field.

Terry Eckert, a 16-year veteran of the fire service, is a firefighter and head of apparatus maintenance in the Darien-Woodridge (IL) Fire District and th echief engineer of the Westmont (IL) Fire Department. He has 25 years of experience as a vehicle technician. He is an ASE-certified master automobile technician and master heavy truck technician and an EVT Level 3 master technician. He also has ASE certification in advanced level engine performance. Eckert is a member of numerous professional associations, including the National Association of Emergency Vehicle Technicians (NAEVT) and the Illinois Fire Apparatus Mechanics Association. He is a member of the EVT Certification Commission, where he serves on the Validation Committee and had chaired the E-3 section, and the NFPA Technical Committee on Emergency Vehicle Technician Professional Qulifications. He was the 1997 recipient of the NAEVT Certificate of Achievement Award.

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