Bottom Line
Cooling systems - part 2
Last issue, we reviewed the cooling system. We looked at what can go wrong and what has been done over the years to make cooling systems more reliable.

by Paul Abelson, technical editor

This month, we’ll look at maintenance procedures to ensure your engine will not overheat, regardless of how hard they work. Last issue, we reviewed the cooling system. We found the need to manage almost a half million Btu of thermal energy unused by the vehicle and not lost through the exhaust. We looked at the components of the cooling system, from the passageways inside the engine to the clamps that hold hoses to various fittings on the radiator, engine and in-cab heaters. We looked at what can go wrong with the cooling system, and what has been done over the years to make cooling systems more reliable. Even though a great deal of progress has been made, cooling systems still require attention. The concept of “fill and forget” is still not a reality.

This month, we’ll look at maintenance procedures to ensure your engines will not overheat, regardless of how hard they work. We’ll look at the physical side, the hoses, clamps and radiator. We’ll examine the chemical side, the anti-freeze, additives and water. We’ll review some common maintenance errors. Then we’ll finish with a look at the latest developments in coolant chemistry.

Maintaining the hardware

The first step in any cooling system maintenance program is to make sure all the hardware is in good shape. There’s no sense in filling your radiator with pre-mix, only to have it seep out.

Be on the lookout for signs of coolant in and around your engine compartment. Green (or orange or red or purple, depending on the manufacturer and the chemical system) pools under the engine or radiator are obvious. Colored streaks around fittings are less so. Sometimes you won’t see any signs, but you’ll smell the overly sweet aroma of glycol. All are signs of coolant loss. On average, according to Darrell Hicks of Penray, my advisor on this subject, Class 8 trucks lose about a drop of coolant per mile after they are one year old. That translates to as much as a gallon a month. For most trucks, losses are two thirds that, but even eight gallons a year is significant, especially when it is preventable.

How to prevent coolant loss

1 Check your radiator for leaks, especially where the tanks meet the tubes that make up the radiator core, and where the inlet and outlet hoses connect to the tanks. Those are subject to cracking over time, and where hoses work loose.

2 Check your heat exchangers in the cab the same way. Fittings on the firewall are easy to check; those in the sleeper are harder to get to, but should be looked at twice a year. Keep an eye out for glycol drips in between checks.

While you’re checking radiators and heat exchangers, look for debris such as bugs, leaves, shards of paper or plastic – anything that could obstruct airflow. Use an air hose to blow debris out, from back to front.

3 Check hose condition. Hoses deteriorate in one of two ways. Some dry and crack from ozone and heat, getting hard and brittle to the touch. They often show longitudinal cracks. Others are attacked chemically from the inside. The rubber softens or crumbles below the reinforcing cords making them soft and mushy to the touch. When either of these conditions is found, change hoses. Check all coolant hoses, not just those going to the radiator. Hoses take coolant to engine and transmission oil coolers and in-cab heat exchangers. You may have premium silicon hoses at the front of the engine, but hoses running to sleepers may be ordinary rubber.

4 Use the proper clamps. Be sure they’re fitted properly. The best hose clamps have springs that compensate for changes in temperature, so clamping torque remains constant. Never use clamps with perforated surfaces against the hose. Rubber, and especially silicone, can extrude through the perforations, damaging the hose and reducing clamping force. Always use temperature-compensating, non-extruding constant torque hose clamps. A new polymer hose clamp, made by Gates Rubber, slips over the hose, then contracts when a portion is hit with a heat gun. A properly sized band will provide even clamping.

Make sure a hose is placed far enough past the bead on the fitting. The entire clamp should bear on the hose 1/4 inch from the end of the hose, and far enough from the bead to allow the hose to expand without damage.

5 The thermostat is a valve that regulates coolant flow to the radiator. When closed, coolant circulates inside the engine for quick warm-up, and to the heater-defroster for safety and comfort. Once the coolant is warm, it opens to allow coolant to the radiator. Thermostats fail in the open position, to prevent overheating the engine. If they fail or open early, the engine may over-cool. Check the thermostat in hot water. Use a thermometer to check the temperature at which the thermostat opens. The exact procedure is outlined in TMC Recommended Practice RP-313C, “Checklist for Cooling System Maintenance.”

6 Check the radiator cap. Inspect it visually for sludge or goo, and physical damage. Pressure test the cap. Sludge and goo are signs of chemical imbalance in the coolant, which we will address later. Physical damage, such as cracks in the fiber seal or mating surfaces, can prevent the cap from holding pressure or retaining coolant.

Cooling systems are designed to work under pressure. The higher the pressure, the higher the boiling point. When pressure caps open, they allow coolant to flow from the header tank to a coolant recovery tank. That is where hot coolant goes when it expands. When the engine cools, the coolant will be siphoned back into the engine.

Many drivers see the recovery tank nearly empty when cool. They then fill it to either the top or to the higher of the two marks on the tank, figuring they are for “maximum” and “minimum.” They are actually for “hot” and “cold.” The higher mark is where the hot coolant will be at operating temperature. The lower mark is where coolant should be for a cold engine. A great deal of coolant is lost when recovery tanks are overfilled, and blown out when coolant expands.

At one fleet, four of 11 pressure caps were found fractured at the outer seal, and another six opened at pressure below rated. Caps should be inspected and tested when preparing for winter and summer, if not at every PM.

7 The fan, shroud and fan drive mechanism bring air through the radiator to absorb heat from the coolant. Air dissipates heat into the environment. This is a system. The fan draws air into the radiator while the truck is stationary or moving slowly. It consumes large amounts of horsepower, so it should be turned off when not needed. The fan clutch does that. The shroud directs the air to the fan for the most efficient airflow.

Check to make sure the fan is undamaged. Nicked edges and broken blades affect airflow and unbalance the fan, stressing bearings. Check the shroud for damage. Cracks and dents can alter airflow and interfere with the fan. For proper sequencing of fan clutches, refer to TMC Recommended Practice RP-316A, “Engine Temperature Control Settings for Liquid Cooled Diesel Engines.

Maintaining the coolant

Water is the best coolant available. It takes a great deal of energy to change water from its liquid state to the gas we know as steam. This ability to absorb extra heat helps it cool other objects. Water also has undesirable properties. When it freezes, it expands, with enough pressure to crack cast iron. It carries minerals in solution. They form scale deposits that insulate parts of the engine creating hot spots and blocking coolant flow. Water also promotes rust. Rust and scale can abrade water pumps and seals, causing failures. De-ionized or distilled water should always be used to mix coolant to prevent scale. Supplemental Coolant Additives (SCAs) protect metals from rust and corrosion, prevent scale formation, prevent coolant from foaming, lubricate the water pump and protect cylinder liners from pitting.

Fully formulated coolant is antifreeze, chemicals and pure water. Ethylene glycol is the most common antifreeze. Propylene glycol is non-toxic, and is growing in popularity, although it is still in fewer than 10 percent of trucks. Mixed 50/50, EG will protect to minus 34 degrees F. PG will protect to minus 27 F. A 60 percent mixture of EG will prevent coolant from freezing down to minus 63 F, about the same as a 70 percent PG mixture.

At every PM, check coolant for clarity and test for SCA concentration using test strips. Different families of chemicals, have different chemistries, so test with strips from your SCA supplier. If concentrations are greater than 6 percent, chemical will precipitate out of solution, forming “green goo” that can harden to block passages, causing overheating. If concentrations fall below 3 percent, the engine will not be protected and pitting could destroy liners.

Heavy-duty diesel engines use filters to keep rust and scale from circulating in the engine. Some filters contain SCA concentrates that are released when the filter is installed. Other systems use uncharged filters and require mixing liquid SCAs. About a decade ago, Penray developed a unique filter with multiple doses of SCAs encased in membranes. Sensitive to SCA concentration, they dissolve when SCAs drop too low, releasing a predetermined amount of chemicals. Called “Need Release,” this system ensures the concentration is neither too high nor too low. Each Need Release filter is good for about one year or 125,000 miles. Changing more frequently will not harm the engine, since the chemical is released only when needed.

Other suppliers have filters that operate on a time-release basis, dissolving the solid SCAs at about the rate they are consumed. Nitrite based chemicals are absolutely essential for protecting cylinder liners from pitting. That’s why it is critical to check SCA concentrations regularly.

Checking coolant chemistry is simple. At every PM, check antifreeze level and SCA concentration. To check SCAs, dip a test strip in the coolant, and then compare color against a table on the package. Then follow the manufacturers’ instructions. To test for freeze point, the best tool is an “optical refractrometer.” Put a drop of coolant on its plate, and look through the eyepiece to read freeze point on a scale. Refractometers are accurate to about 1 F. Hydrometers look like huge eyedroppers. They measure specific gravity using a bulb to draw fluid into a chamber and a float that indicates freeze protection. Accuracy varies from 2 to as much as 10 or 15 F, depending on how well the hydrometer is made. Discount store devices that draw coolant and allow one to five colored balls to float are notoriously inaccurate, and should be avoided for truck use.

If anti-freeze protection is inadequate, it is usually because coolant was lost and replaced with water. Whenever topping off coolant, you should use a pre-mix of 50/50 water and anti-freeze, with an appropriate measure of SCAs added. Many owner-operators carry jugs of pre-mixed coolant to top off between PMs.

Specifications for SCAs and coolants are found in TMC Recommended Practices. Applicable RPs are: RP-319 “General Guidelines and Precautions on Supplemental Coolant Additives,” RP-328 “Fleet Purchasing Specification for Nitrite-Containing Supplemental Coolant Additives,” RP-329 “Fleet Purchasing Specification for Nitrite-Containing Ethylene Glucol Base Coolant” and RP-330 “Fleet Purchasing Specification for Nitrite-Containing Propylent Glycol Base Coolant.”

NOTE: To obtain the books of Recommended Practices mentioned in this and other articles, contact The Technology and Maintenance Council at (703) 838-1763.

Future coolants

It would be great if we didn’t have to check coolants and change them every year or two, or if we didn’t have to change filters at every PM. Perhaps some day we will have a sealed cooling system, but that day is not here yet. There are, however, devices and coolants that offer Extended Service Intervals (ESIs). TMC Recommended Practice RP-338 “Extended Service Interval Coolants” describes several recent technologies. We discussed delayed release SCA systems earlier. Organic Acid Technology-based (OAT) coolants, such as Texaco’s ESI coolant, use carboxylate inhibitors to prevent corrosion and cavitation. They offer coolant life to 600,000 miles, provided the inhibitors are replenished at 200,000 to 300,000 miles. ESI coolants are also marketed under private label by engine manufacturers. Many add nitrate-based protection. The greatest problem with these OAT coolants is they lose their effectiveness when diluted with ordinary coolants. If a truckstop tops off your coolant during a PM with regular EG anti-freeze, after just a few PMs, the OAT coolant will lose its protective properties. That’s why many fleets that have tried it are switching back to regular coolant plus SCAs.

Whether regular or ESI, the best way to avoid coolant problems is to set up a program, check coolant often and don’t mix types of coolant. The manufacturers stand behind their products, but only if you haven’t mixed types, and only if you’ve maintained the required level of SCAs.

Given proper care and attention, your cooling system should give the same long life we’ve come to expect from our engines.

Paul Abelson is Land Line's technical editor.

Aug/Sept Digital Edition