restoration series number 2: Coolant Maintenance

by Bjorn Brandstedt

An efficient cooling system is vital to the health of the internal combustion engine. This article applies to the diesel engine coolant maintenance and will also explain the installation and benefits of a coolant filter for the M35A2C military 2½-ton 6×6 truck equipped with the LD465-1A (-1C or -1D) multifuel engine.

Figure 1. When it comes to troubleshooting the engine cooling system, the dash mounted engine temperature gauge has limited usefulness. This illustration shows problems that will not give you a faulty temperature reading, but need to be tended to rather quickly.

The cooling fluid circulates out of sight and does its job of transporting heat from the engine to the radiator with only one control valve, the thermostat and one monitoring device, the dash mounted engine temperature gauge. Many cooling system problems can occur without an immediate indication on the temperature gauge. It will not tell us, right away, about such things as low coolant level, defective radiator cap, leaky radiator, low coolant flow rate (caused by failing water pump or restrictions in the water passages or just a loose fan belt) or poor heat transfer (caused by build-up of deposits on the water jacket walls, bent fan blade, air in the system or just debris blocking the air flow through the radiator). In short, the cooling system needs your frequent personal care and inspection over and beyond the occasional glance at the gauge.

In normal operation the thermostat will open and close to keep the coolant temperature at or near 185 degrees F. If it has to open completely, then there is no more control of the temperature and the engine is at the mercy of the fan and the efficiency of the radiator to keep from overheating. For liquid cooled engines, it is very important that the coolant itself is at peak efficiency. The goal of this article then, is to enable you to maintain the coolant so that the thermostat never has to operate fully open.

It is true that substitutes can be used in an emergency. The manual TM 9-2320-356-BD Battlefield Damage Assessment and Repair states that any liquid, except gasoline or jet fuels, may be used, but for normal operating conditions, the coolant must meet the following basic requirements:
1)    Provide heat transfer at all times at high and low temperatures
2)    Provide freeze protection during cold weather operations
3)    Offer protection against rust and corrosion within the cooling system
4)     Be compatible with hoses and seals
5)    Reduce formation of foam (microscopic bubbles that may reduce heat transfer properties and cause cavitation erosion)
6)    Prevent the formation of scale deposits that may reduce the heat transfer efficiency within the cooling system

Ethylene glycol mixed with water and inhibitors meets these requirements and is the only antifreeze approved by the US Army for use in the multifuel engine cooling system (TM 750-254 Cooling Systems: Tactical Vehicles, 1972). Since water is a basic ingredient, it is also always okay to add water, if needed.

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When shopping for ethylene glycol antifreeze, select a low silicate formula designed for heavy-duty diesel cooling systems and look for the ASTM specification D-4985 on the label. There are probably a hundred different brands available commercially, most of which are not ideal for your multifuel/diesel engine and are not intended to be maintained using inhibitors. However, Prestone’s “Heavy Duty” (black jug), Motorcraft’s “Premium,” Texaco’s “Heavy Duty,” and Zerex’s “5/100” (white jug) are examples of suitable coolants. “Fully-Formulated” ethylene glycol antifreeze is also available, which may be used if you prefer. Fleetguard’s “Complete” is blue in color and will not require an initial pre-charge of additives. The fully-formulated coolants are diluted 50/50 with water just like the conventional coolants. Zerex’s “Pre Charged” (black jug) is another fully-formulated coolant concentrate.


Figure 2. The proportions of ethylene glycol to water not only determines the amount of freeze protection, but also the boiling point of the coolant. Using antifreeze in the hot summer weather is beneficial as can be seen here.

Don’t use more than about 65% concentrated ethylene glycol in your coolant mixture. Pure antifreeze actually provides less protection, freezing up the system at only +16°F. Furthermore, it is less effective at transferring heat when fully concentrated. The military recommends a 60% to 40% solution (see Figure 2). The extra ethylene glycol lowers the freezing point to -62°F and also increases the boiling temperature by about 4° (compared to the 50-50 mix), which is beneficial in hot weather.

A 50-50 mixture should be ideal for most of us since the coolant will only start to form ice crystals at -34°F and will not freeze solid unless the temperature drops below -52°F. Use distilled water to eliminate the calcium and magnesium commonly present in tap or well water, as these minerals may be detrimental to the operation of the cooling system. The minerals may be deposited on water passage walls as “scale.” The scale blocks the heat transfer and the engine will run hotter as a result. According to one source (Carquest), a 1/16″ layer of scale will reduce the heat transfer by 40%. Scale buildup can lead to hot spots on cylinder walls leading to piston scuff and further damage. Note that this kind of temperature increase would not show up on the engine temperature gauge, which only measures the temperature of the coolant.

Water can be used alone, but does not provide any corrosive protection and has a lower boiling point than the glycol/water mixture. Water with chlorine content is not suitable for use in your radiator. When adding fluid, mix it to the correct proportions if possible, before adding. Only add fluid when the engine is cool (thermostat closed) so that you don’t risk cracking the block.

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Figure 3. Inhibitors in the coolant are used up even if the vehicle is not used and it is important to regularly check the level of corrosion protection. Special test strips (see text) are used to determine the acidity, freeze protection and anti cavitation chemistry of the coolant. PS Magazine,  December 1974

In high compression diesel engines, the severe vibrations set up by the detonating fuel may cause microscopic bubbles to form in the coolant along the cylinder linings. When these bubbles collapse, erosion of the metal takes place. This kind of erosion is referred to as “cavitation.” Diesel engines with thin “wet” sleeve cylinder linings are particularly susceptible to this kind of damage.

The off-the-shelf coolants or water have very little protection against cavitation and scale formation. For these, and other reasons, supplemental coolant additives (SCA) or diesel coolant additives (DCA) as they are also called, are used in diesel engine coolants. The military manuals refer to them as “inhibitors.”
The inhibitors supplied in the “fully-formulated” cooling fluids will only last for a year at the most and will need additives when the initial charge has been depleted. The LD-465 series multifuel engines do not have wet sleeve linings, so the primary reason for using additives in the coolant for the multifuel engine is to provide a rust inhibitor and to prevent scale deposits from forming along the passages. The anti-foaming ingredients of the SCA’s also help improve the ability of the coolant to transfer heat more efficiently. Properly maintained, the SCA’s will also neutralize acids as they form in the cooling system. Insufficient amount of inhibitors or no inhibitors at all invites the formation of rust, scale and sludge and reduces the overall performance of the cooling system of the multifuel engine.

The amount of SCA in the coolant should be about 3%. For the LD-465-1A (-1C or -1D) engine with its 8-gallon capacity cooling system, two pints of SCA is needed as an initial “charge” (when first adding fresh antifreeze and water). The SCA may be in liquid or powdery form or the coolant filter may be pre charged with it. If the SCA is in powder form, dissolve it in hot water before adding. The powder, if poured directly into the radiator, may clog some passages of the cooling system. An SCA should be used with plain water coolant in a mixture of 1 pint to every 10 quarts of water (about 5%).

The Army provides a cooling system test kit (NSN 6630-00-011-5039) designed to test the rust inhibitor of the fluid. The test for the reserve alkalinity and freeze protection of the coolant is performed monthly (Figure 3).

Figure 4. A 2-way test is provided by “Cooltrak” test strips. After dipping the strip in the coolant, the color change of the pads indicate pH (acidity or “reserve alkalinity”) and amount of freeze protection. The test strips are sensitive to humidity and must be kept sealed in the jar when not used.

There are commercial test kits for monitoring the effectiveness of the inhibitors in the coolant. These strips are dipped into the coolant and the colors of the pads on the strips are compared to reference color patches supplied. A low pH value indicates acidity and if it is below 8.5 there is risk of corrosive action. The value should be from 9 to 11 for our coolant. It is important to use the right test strip for a particular coolant or the results will be meaningless. You may have to shop around for these, since there is very little demand and major auto parts dealers may not stock them. The test strips have an expiration date, so don’t buy more than you can use.

2-way “Cooltrak” brand test strips (Figure 4) provides an indication of the pH value of the antifreeze and the amount of freeze protection for ethylene glycol/water coolant. 3-way “Fleetguard” brand test strips test for nitrite/molybdate concentration, the ingredients that prevent cavitation erosion.

The level of freeze protection may also be checked by using the specific gravity as a guide. The FM 9-207 Operation and Maintenance of Ordnance Material in Cold Weather, 1978) shows how to use a refractometer to accurately tell what the glycol/water mixture is. By the way, the same instrument is used to measure the specific gravity (indication of charge) of lead acid batteries. There are commercially available low cost plastic hydrometers, some with trapped colored balls for a quick check of the level of freeze protection provided by the coolant (Figure 5).

Like engine oil, the antifreeze does “wear out” and must be replaced, perhaps, as often as every two years. Using additives will extend the life of the coolant, but it must be monitored and will eventually have to be replaced. The additives are used up even if the vehicle is not used. Contaminated coolant is not as efficient as clean, filtered fluid, it should be clear and greenish in color. If it is muddy looking, with bits of junk (scales, rust, etc) mixed in, it must be replaced.

If you experience overheating and have a contaminated coolant, flushing and cleaning of the cooling system is in order. Refer to TM 750-254 for detailed information and how to perform these procedures. The following explains how to replace the coolant.

Five out of the eight gallons can be drained by opening the petcock in the lower radiator tank. The remaining three gallons of coolant is trapped by the closed thermostat inside the engine, in the oil cooler, water pump housing and in the personnel heater core (and about 1 pint in the coolant filter if so equipped).
So, how does one remove the coolant from the engine? Well, there are drain plugs. The drain plugs are difficult to access and, if accessible, may be very hard to remove, so a different procedure will be used (that involves gradually diluting the coolant with water). Note that distilled water is very inexpensive and should be used at all times.

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You will need 21 gallons of water, 4 gallons of antifreeze and 2 pints of SCA. If you are using a “Fully-Formulated” antifreeze, the 2 pints of SCA is not needed at this time. Proceed as follows:

Figure 5. A low cost method to test for freeze point of the coolant is to use this little device. The colored balls are of different densities and the number of balls that sink indicates the amount of ethylene glycol in the coolant mixture

1)    Find a 5-gallon bucket and place it under the radiator drain. Open the drain petcock and the radiator cap (to let air in). About 5 gallons will be removed from the radiator. Keep in mind that ethylene glycol is toxic to people and animals and that there is a need to dispose of it properly.
2)    Close the petcock and fill radiator with water. Save the empty water jugs and fill them with the drained coolant. Replace the radiator cap.
3)    At this time you want to mix the water with the coolant still in the engine. Do so by running the engine until the thermostat opens and circulates/mixes the fluids. The best way is probably to drive the vehicle around the block a few times, but you can also warm up the engine by covering the radiator and run it at high idle for perhaps 20 minutes. Let the engine run for about 10 minutes at operating temperature. Shut it down and let it cool off.

Repeat the drain/refill/mixing process four more times. It will take 20 gallons of water. The fifth time the radiator is filled, use the 4 gallons of antifreeze plus about 1 gallon of water plus 2 pints of SCA. Using this procedure, only about 1% of the original coolant still remains. After the first drain, 3 gallons remain. After the second drain, 1.12 gallons remain. After the third drain, 0.42 gallons remain, and when you put the 5 gallons of water in the radiator for the forth time, it is mixed with 0.16 gallons of the original coolant. When that is drained and the antifreeze is finally added, it is mixed with only 0.06 gallons (about 1 cupful) of the original coolant (see Figures 6-7).

Run the engine again at operating temperature. This will leave a 50/50 mixture of antifreeze/water in your radiator. The procedure may be extended over a period of days, weather permitting and it will replace virtually all the coolant without you having to worry about twisting off stubborn drain plugs.

The radiator cover was very effective in controlling the engine temperature during the “mixing” process explained above. A digital thermometer was temporarily installed with the thermocouple tip mounted in a spare plug near the thermostat. Fully covered, the temperature stabilized at about 187° and with the flap on the radiator cover rolled up, the temperature dropped to about 182° (ambient temperature about 75° and truck stationary with an engine rpm of 1,400).

So far, we have striven to make our cooling system as efficient as possible, but under certain weather and/or operating conditions, it may actually be too efficient. At idle and/or at low ambient air temperatures, the engine simply does not reach its normal operating temperature. The thermostat remains closed.

Part of the winterization kit for the deuce (and other vehicles) is a radiator cover that is installed in front of the grill (Figure 8). Being part of the winterization kit makes it an item that we perhaps rarely consider; why would we need it when the temperature barely gets below freezing and some of us may not even drive the vehicle in the winter? Well, for the multifuel engine, it is really needed more than you might think.

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For low speed, minimum loads, like wrecker crane operation, pipeline, tanker pump and earth-boring operations, for example, the engine may not ever reach operating temperature. My particular vehicle is equipped with a hydraulically operated crane, which needs the engine to power the hydraulic pump. The truck can run at 1,200-1,500 rpm for an hour or more and never reach operating temperature. For this condition, the radiator cover is ideal.

Figure 6. This photo shows the last batch to be added to the cooling system when the add/dilute/drain method is used as described in the text, four gallons of antifreeze, about one gallon of water and two pints of inhibitor. The result is a 50-50 mixture with a 3% charge of inhibitors (also called SCA’s or DCA’s)

The readability of the dash-mounted engine temperature gauge may leave something to be desired when looking for small changes in temperature. The author hooked up a digital thermometer and was able to follow the smallest variations in temperature changes. Even with the cover fully closed and the airflow within the engine compartment reduced to almost nothing, it still takes a good 15 minutes before the thermostat begins to open at 180° when the truck is used in this “low load” application.

Radiator shutters have been in use for more than half a century, often with a manually controlled operating mechanism. Thermostatically controlled shutters on over-the-road trucks are very effective in maintaining the proper temperature of the diesel engine. There is no reason why one should not use the radiator cover on his/her vehicles more often than just in the winter.

The first time I removed and cleaned the radiator on my deuce, I found, to my surprise, a cupful of sediment accumulated in the lower radiator hose. After a lot of flushing using a regular garden hose, the radiator and hose were cleared of this potentially harmful stuff.

Core sand, scale and rust are abrasive and will shorten the life of the water pump and seals. Fine rust particles may even form a scale on the walls of the narrow water tubes of the radiator and reduce its efficiency.

Installing a filter to remove this sediment suddenly appears to be very desirable. The coolant filter is a low-cost assurance of longer life of the radiator, water pump, and the engine by removing harmful particulate matter, rust, and solids from the fluid. Grit in the coolant is abrasive to the water pump and seals.

A side effect is that it also makes the operator aware of the cooling system in a new way. He/she sees the filter canister and is reminded of the importance or even presence of the cooling system. Most over-the-road diesel trucks have a coolant filter and it is possible that most of us caretakers of vintage military trucks with big diesel/multifuel engines have never even heard of this kind of filter. After all, it isn’t mentioned in the manuals and our personal automobiles lack this device. Figure 9 shows the coolant filter mounted on the engine side of the right hand headlight support panel. This location is ideal; it is close to the connection points on the engine and below the top tank of the radiator.

Three screws are all that’s needed to “hang” the filter base. Use a “Unibit” to drill large holes in sheet metal (Figure 10). It is perfect for this job and automatically deburs the holes. The filter base has two connections for the hoses, the inlet and the outlet. The inlet is connected to the engine side of the thermostat housing and the outlet is routed to the water pump return side.

Figure 7. Here, one of the two required pints of DC4A inhibitor is added.

These are the same connecting points, which are used for the personnel hot water heater unit and if both are installed on your vehicle, use “tee” fittings to permit coolant to circulate to both the heater and the filter. This kind of filter is referred to as a “by-pass” filter in that only a portion of the coolant is guided through it. The coolant is filtered in much the same way that we replaced the fluid above, a little at a time, but eventually all of it will be filtered.

A good quality heater hose with a 5/8” I.D. is used for the connections. It is advisable to install shut-off cocks in the line to make less of a mess when the time comes to replace the filter element.

Use hose clamps to keep connections tight and wrap a couple of turns of teflon tape over the threads before installing the fittings on the engine and the filter base. There are many choices when it comes to the filter element itself, made by Wix, Baldwin and others. The filter may be “charged” with inhibitors, which are gradually dispersed into the coolant. The filter used by the author was a NAPA #4070 and does not contain any inhibitors. The filter mounting bracket from NAPA is # FIL4019.

After installation of the filter base and the spin-on filter element, start the engine and momentarily loosen up the return line to the water pump to make sure that there is no air trapped in the system. The filter may be filled with distilled water before installing it to reduce the possibility of introducing air into the coolant. Tighten all hose clamps and check for leaks. The filter canister gets warm rather quickly as part of the coolant is now circulated through it. The filter is plumbed into the circuit on the “engine” side (like the personnel hot water heater) and filters coolant at any temperature with the thermostat closed or open. The filter should be replaced every 200 to 300 hours of operation.


Figure 8. The radiator cover may appear out of place in the hot summer sun, but is actually needed for the multifuel engine to reach operating temperature when it is used in low load modes. These operations include running the winch, the fuel water pump (tankers), pole setting/boring operations or in a recovering operation (wrecker) and the author’s hydraulic crane

The U.S. Army PS magazine is an excellent source for preventive maintenance information and the importance of the care of your vehicle’s cooling system is reflected in the frequent notes regarding this subject. Every year, at least one section of the magazine is devoted to testing and maintaining the coolant, radiator, fan, fan belts and the hoses making up this very important part of your engine. Issue 199, (1969 series) has 23 pages on cooling systems for our (now) vintage military vehicles.

This article covers the ethylene glycol/water type only, since it is the coolant specified by the U.S. Army for the multifuel/diesel engine and most likely used in your MV (along with rust inhibitors). There is a lot of new technology involved in the coolants available today—“extended life coolant,” “non toxic” propylene glycol, “fully-formulated,” “pre-charged,” and so on. Cooling fluid comes in a number of colors including green, orange, yellow, and blue depending on formulation and application. They should not be mixed unless the directions tell you that it is okay.

Besides the obvious advantages of installing it, the highly visible coolant filter is a reminder of the need to look after the cooling system. The addition of this filter will also help in keeping the engine temperature where it belongs and will extend the life of the water pump and other components of the multifuel engine cooling system.

Perhaps the most compelling reason for taking good care of the cooling system is the fact that so many problems—including an overheated engine—are not indicated on the “engine” temperature gauge.


TM 750-254 Cooling Systems: Tactical Vehicles, (1972)
FM 9-207 Operation and Maintenance of Ordnance Material in Cold Weather (0 to -65 deg F) (1978)
TB 750-651 Use of Antifreeze solutions, Antifreeze extender, Cleaning compounds and test kit in engine cooling systems (1997)
TB 43-0239 Maintenance in the Desert (1981)

Figure 9. Here the coolant filter is shown after installation. The connections are close by, the input to the filter from the engine side of the thermostat and the output at the water pump. The filter was installed in less than one hour and is a low-cost insurance for a longer life for the water pump and the radiator. The connection points are the same as those used for the hot water personnel heater so if both are installed, a T-type fitting is needed at each location

Figure 10. Three holes must be drilled to mount the coolant filter. As usual, the “unibit” is superior for drilling sheet metal. A magnet was used to pick up shavings from the operation. 

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