Understanding coolants

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Cavitation erosion or liner pitting is caused by air in the cooling system, and is aggravated by liner vibration. Bubbles implode violently, removing a little metal as they do so.

Although it’s often called antifreeze, engine coolant has other jobs to do. And different types of coolants do their jobs in different ways. Here’s the lowdown on what they’re made of, how they work, and what types are best suited to what applications and maintenance schedules.

According to Jim Roberts, Shell’s technical service specialist, coolant has three important functions: First, it must transfer combustion heat from a vehicle’s engine to its radiator, where the heat is given off to the atmosphere; second, coolant must provide adequate protection from freezing and boiling; and third, it must protect engine and cooling system metals from rust and corrosion.

While water alone could perform the heat-transfer function, it is subject to boiling and freezing, and actually promotes system rust and corrosion. Therefore, modern coolants are composed of only 2 to 4 percent water before dilution to manufacturers’ specifications (coolants are available in concentrated form or premixed with water).

About 91 to 94 percent of coolant is glycol – almost always ethylene glycol (EG). This performs the heat transfer and boiling/freezing protection functions, while 4 to 5 percent of coolant is additives that protect against corrosion and liner cavitation, or pitting caused by air bubbles on liners violently imploding and removing metal. Major differences among coolant types are centered on these additives, how they work and how they must be maintained.

There are four major types of commercial-grade coolants, as recognized by the Technology & Maintenance Council (TMC). They are: Conventional, low-silicate, or Type A. These derive from passenger-car technology, says Carmen Ulabarro, coolant marketing specialist for ChevronTexaco Products Co. “They require an initial dose of supplemental coolant additive (SCA), and additional SCA treatments every 15,000 to 20,000 miles when used in heavy-duty diesel applications,” she says. Test strips often are used to determine SCA service intervals. SCAs help prevent corrosion and pitting by laying down a thin, protective – but temporary – layer of silicates on metal surfaces.

Typically, these coolants are green in color, according to TMC’s guidelines in Recommended Practice (RP) 351. But it should be noted that not all manufacturers abide by these guidelines, and color cannot be used as a positive identifier.

Supplier TMC Type Brand Name Color Website
ChevronTexaco A Chevron SupremeAntifreeze/Coolant Green www.chevrontexaco.com
A TexacoAntifreeze/Coolant Green
B Chevron Heavy-Duty Coolant Purple/Pink
B Texaco Heavy-Duty Coolant Purple/Pink
D Delo ELC Red
D Texaco ELC Red
Citgo A Antifreeze/Coolant Green www.citgo.com
ConocoPhillips A Ready To Use Antifreeze/Coolant Yellow/Green www.phillips66.com/brands
A 76 Pit StopAntifreeze/Coolant Yellow/Green
A 76 UniversalAntifreeze/Coolant Yellow/Green
Evans C NPG+ Light Brown www.evanscooling.com
ExxonMobil B Permazone Green www.exxonmobil.com
Fleetguard B ES Compleat EG Blue www.fleetguard.com
C ES Compleat PG Blue
B Fleetcool EG Pink
A Universal LS-EG Green
C Universal PG Purple
Old World B Fleet Charge Pink www.peakantifreeze.com
D Final Charge Red
Prestone B Heavy Duty Antifreeze/Coolant Purple www.prestone.com
Recochem A H.D. Antifreeze/Coolant 15-754 Green www.recochem.com
D Extended LifeDiesel Coolant 15-734 Red
Shell A Shellzone Antifreeze Green www.shell-lubricants.com
B Diesel Ready Purple
B Fully FormulatedPhosphate-Free Purple
D Rotella ELC Red
Zerex D Extreme Red www.valvoline.com/zerex

Fully formulated ethylene glycol, or Type B. These are similar to conventional coolants, except that the initial SCA dose is included in the mix. SCAs still must be added at service intervals determined by using test strips. According to TMC, these coolants should be a purple/pink color.

Either conventional or fully formulated coolants can function as an extended-service-interval (ESI) coolant, with the use of time-release SCAs that prolong resistance to corrosion and liner cavitation, thereby extending service intervals. TMC’s RP 338A defines an ESI coolant as one that can go beyond 100,000 miles between service intervals, i.e., changing coolant or replacing depleted chemicals.

Fully formulated propylene glycol (PG), or Type C. Here, the additives are not necessarily different, but the base is propylene glycol instead of ethylene glycol. These coolants are more expensive, but have the advantage of not being immediately toxic to humans or animals, as are ethylene glycol-based coolants. It should be noted, however, that any coolant that’s been run in an engine is apt to contain heavy metals and should be considered toxic.

Another characteristic of this type of coolant is that, from a freeze-protection standpoint, it’s more tolerant of over-concentration. While ethylene glycol’s freeze point begins to increase if it’s mixed much more than 60/40 with water, propylene glycol’s freeze point continues to drop with additional concentration.

Additionally, a conventional hydrometer cannot be used to test PG-based coolants. A more expensive instrument, called a refractometer, precisely measures light refraction through a drop of coolant sample, and gives highly accurate freeze-point readings for both EG- and PG-based coolants.

Although it doesn’t enjoy widespread popularity, “Propylene glycol is sometimes used in environmentally sensitive applications,” says ChevronTexaco’s Ulabarro. PG also is found in the Evans nonaqueous (water-free) cooling system (see “H2No! – Cooling without water,” CCJ, February 1998).

TMC recommends that fully formulated PG coolants be colored blue.

Organic additive technology (sometimes called organic acid technology or OAT), Type D. Instead of laying down a protective blanket, the additives in this type of coolant – chiefly carboxylates – act locally and selectively to pacify corrosion or pitting that’s about to begin. “We’ve got those carboxylates pretty well trained,” jokes Shell’s Roberts.

Often, per engine manufacturers’ coolant specs, nitrites (NOAT) are added as extra insurance against liner pitting and flash rusting. “We recommend nitrited versions because nitrite is easily tested, and most heavy-duty OEMs require the addition of nitrites. Nitrate levels can be tested with strips,” says ChevronTexaco’s Ulabarro. “It’s a belt-and-suspenders approach.” In addition, triazole may be added to some mixes to protect copper components from corrosion, and phosphates can be included to improve water pump life.

With no chemical blanket laid down on engine metal, heat transfer generally is maintained like new and is definitely improved versus silicated coolants. And, since these additives deplete much more slowly than conventional SCAs, they not only can extend service intervals beyond 100,000 miles, but also can extend coolant life by several times those intervals, easily surpassing ESI requirements. Typically, an extender package is required at 300,000 miles, and the coolant can last up to 600,000 miles, greatly reducing downtime and the amount of used coolant that must be discarded. These coolants also are known as extended-life, or ELCs.

In addition to these four basic coolant types, there are hybrid products, which use a combination of conventional (inorganic) and carboxylate technologies. Aside from costing less than OAT technology, a reported advantage of this type of coolant is that it’s relatively tolerant of cross-contamination. For example, “You can mix a hybrid with conventional coolant,” says Darrell Hicks, national OEM manager for The Penray Companies. “But you don’t want to mix an OAT with a conventional.”

That said, there are kits on the market intended to convert conventional coolants to extended-life, OAT technology, but they should not be used with conventional SCAs (see “Presto … without the change-o,” CCJ, August 2004).

Pick your potion
In over-the-road applications, where miles are racked up quickly, ELCs have proven popular. “That’s all we use,” says Will Wasylik, trainer/quality control for Bison Transport, Winnipeg, Manitoba, Canada. “There’s not much reason not to. Of course, if a driver blows a hose and loses coolant on the road, he’s going to pour in whatever he can get his hands on, so it’s possible to lose some of the long-life protection.

“Also, I wish coolant suppliers would standardize on colors,” Wasylik says. “One company sells a blue ELC that looks like washer fluid.” OEMs sometimes put coolant reservoirs near windshield-washer reservoirs, so it would be easy to confuse the fluids. “Washer fluid isn’t going to protect your cooling system, and coolant just smears windshields,” Wasylik says.
While ELCs might seem like a no-brainer for vehicles that see high mileage every year, can you get by with a less expensive, conventional coolant for medium-duty trucks that go only 15,000 to 25,000 miles a year? “You probably can,” answers Bob Flesher, managing director, vehicle maintenance for FedEx Ground, Moon Township, Pa. “It all comes down to maintenance. No matter what coolant you’re running, you need to check its level and condition frequently.”
How about on/off-road applications? It depends who you talk to. “I’m sure there are guys out there who like extended-life coolant,” says George Husack Jr., manager of George Husack Inc., a regional, heavy aggregate hauler based in Schnecksville, Pa. “But we’ve backed away from it. We’ve gone to the fully formulated, green stuff, and we monitor it frequently with test strips. It’s a little cheaper, and we’re comfortable with it. I like the simplicity.”

Don’t send maintenance down the drain
No matter what coolant you choose, here are some cooling system maintenance concerns addressed in a TMC Mini-Tech Session put together by Lou Stumpp, customer service engineer for International Truck & Engine; Bill Stahl, director, OEM service for Cummins; Bob Wessels, program manager for Caterpillar; and Shell’s Roberts.

  • Know what coolant is in your system, and use only that coolant to top off. Although most coolants can handle 10 to 15 percent contamination with the wrong coolant, systems should be labeled conspicuously so technicians and drivers know what’s in them.
  • Check coolant chemistry and condition at least every six months. Look for contamination, and check freeze point, pH or acidity, and degree of corrosion protection. Use a refractometer to check freeze point on all types of coolant.
  • Add conventional SCAs or time-release filters as indicated by testing. Test strips can be used, or any coolant supplier can analyze your coolant for you. If using ELCs, check at 300,000 miles, and add extender if needed.
  • Color is not a reliable indicator of coolant type.
  • When in doubt, follow the vehicle manufacturer’s recommendations for coolant type and maintenance.

Why not use water?
Although water alone could perform the heat-transfer function, it is subject to boiling and freezing, and actually promotes system rust and corrosion. Therefore, modern coolants are composed of only 2 to 4 percent water, with glycol making up most of the rest. The finished product can be a variety of colors.