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Hendon Publishing

Cooling System Maintenance

In the May-June issue, we discussed the basics of coolant chemical makeup. This time, we will go into a little more detail about the cooling system and how the fleet manager can eliminate coolant-related problems.

The internal combustion engine converts chemical energy into mechanical energy. Because it is only about 25% efficient, 75% of the energy contained in each gallon of fuel is lost as heat. The role of the cooling system is to dissipate much of that heat.

Water is the primary component of coolant. The ability of water to absorb and emit heat with out changing in state or composition is the reason we put up with its properties that are less than ideal.

One of the less ideal properties of water is that it is the universal solvent. It dissolves and absorbs more different materials than anything else. This is one reason there is disagreement about whether it is better to use distilled, deionized, or tap water.

Since water is a solvent, in most forms, water contains impurities. When it comes in contact with a soluble substance, that substance ends up as an impurity in the water. The chemical action of the water both dissolves and deposits substances that it contains.

Temperature change can act as an enabler for this action. If you have a closed system with water only, you will eventually find the water achieving what is known as chemical equilibrium. This does not mean the water stops dissolving and depositing substances, but it does mean that the rate of dissolving matches the rate of deposition.

If you use distilled water in your cooling system, the solvency of that water will attack the water-soluble components of the system. These components include those made of cast iron and aluminum. The propensity of water to dissolve certain substances is known as the Langelier Saturation Index, LSI. Tap water with impurities of a certain kind and of a certain level will have an LSI of zero. Distilled water has an LSI of less than zero. This is why some authorities recommend deionized over distilled water.

Tap water is another story. If you have it tested and if it has less than 100 ppm total hardness—50 ppm chlorides, 50 ppm sulfates and 250 ppm total dissolved solids—you may be alright. However, do not use tap water without testing it first. The problems associated with distilled water are minor compared with the risk of using untested and therefore possibly mineral-rich tap water.

I have been using distilled water exclusively for several years and have seen no cooling problems from it whatsoever. Deionized water is great but only if used from a sealed container because of the tendency of water to self-ionize.

An important property of coolant is its pH. This refers to the acidity of the coolant. The pH scale is from 1-14 with the low end being acidic, the high end basic and 7 being neutral. The desired pH depends on the type of coolant you have. IAT (Green EG) should be in the 9-10.5 range. OAT (Dex-Cool) should be in the 7.8-8.8 range. HOAT (Ford and DaimlerChrysler G05) should be about 8.0. Allowing the pH to drop below these ranges can lead to corrosion.

Another concern is cavitation. The depletion of coolant additives over time tends to weaken the coolant’s ability to avoid cavitation damage. Cavitation is caused by the pressure differences inherent in a system that has moving fluid containing dissolved gasses. In areas of low pressure, dissolved gases can come out of solution, and then when these bubbles enter an area of higher pressure, they can implode against components and have an effect similar to a sand blaster.

By keeping air out of the system, you can minimize the amount of dissolved gases. Also, maintaining the proper levels of anti-cavitation additives is critical to the function of the cooling system. The proper way to do this is to flush the cooling system according to manufacturer recommendations.

In some cases, if air is allowed to enter a system that contains aluminum components, the action of coolant, heat and air will lead to oxidation of the aluminum. Anyone familiar with an automotive shop has seen aluminum oxide. It is what the abrasive wheels on your bench grinder are made of.

Allowing this oxide to form in the cooling system is about equivalent to taking a handful of abrasive grit and pouring into the radiator to be circulated though the system where it can attack seals and erode components. This often can be seen as tiny black granules similar in appearance to poppy seeds. If you see this in your coolant, it might take heroic efforts of flushing to completely remove it.

An often-neglected component of the cooling system is the radiator cap. The cap seals the top of the radiator neck to prevent coolant loss to the outside or the entrance of air. It seals the bottom of the radiator neck to provide pressure for the system to operate above 100 C. The expansion of the coolant when heated lifts this seal off its seat and allows coolant to flow into the recovery bottle.

The cap also has a check valve that allows the contraction of the coolant to draw coolant out of the recovery bottle and back into the radiator as the engine cools. Testing the cap by conventional means only checks the second of these seals, but all three are critical to the function of the system. Replace the cap at each cooling system service.

Most OEM’s have gone to a spring-type hose clamp rather than a screw type. The reason for this is that spring clamps maintain tension through heating and cooling cycles, whereas screw clamps cannot. If the hose joint leaks past a spring clamp, the problem is likely with the hose, not the clamp. You may be able to stop the leak by replacing the spring clamp with a screw clamp, but the fix is temporary and might lead to a catastrophic loss of coolant if the hose fails.

The thermostat should be thought of as a maintenance item. If a cooling system service is done, replacing the thermostat should be considered. Thermostat failure can lead to almost instantaneous overheating or to over fueling because of low engine temperatures.

Belts will usually show deterioration, but an often-neglected component is the bearing in either the idler or tensioner. It is not unusual to see these bearings last 100,000 miles, but anything beyond that can be asking for trouble. To check them, the belt must be removed and the bearing spun to check for play or noise.

I have an idler bearing from an ambulance on my desk that I use as an object lesson. Because we remove the belt at each PMI over 100,000 miles, we found this one making noise. This allowed us to replace it before it failed. Because the bearing didn’t fail, the belt didn’t fail, which means the water pump didn’t stop, which means the engine didn’t overheat, which means the rig didn’t end up on the side of the road, which means the patient made it to the hospital, which means he survived, which means his family didn’t sue for $2.2 million. Ah, the benefits of preventative maintenance!

Your cooling system is just that—a system. This dependability of your cooling system depends on all the components working properly and together. Failure of one component can compromise the ability of the cooling system to do its job.

Kevin Roberts is the president of Roberts Repair in Rhinelander, WI. The company has specialized in emergency vehicle maintenance since 1989. He is an ASE Certified Master Automobile and Master Truck Technician. He can be reached at

Published in Police Fleet Manager, Jul/Aug 2006

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