Cooling System Service: Common Sense Guidelines for Covering the Variables

Order Reprints
Cooling System Service: Common Sense Guidelines for Covering the Variables

From the heart of the engine’s cooling system — the water pump — to the choice of hoses and thermostats, quality counts, from the quality of the part to the quality and thoroughness of the service. In essence, this article is a refresher regarding engine cooling system service, preventative maintenance and problem solving. Topics include water pump issues, trapped air, hose concerns, the importance of selecting the correct type of coolant and more.

Generally speaking, a 50/50 mix of water and “antifreeze” is the recommended norm. However, when mixing concentrated antifreeze with water, avoid using tap water, as this may contain excess calcium or other materials which can promote corrosion. Use only distilled/deionized water for the mix. Granted, some coolants are available as 50/50 pre-mix that eliminates the need to add water.

While cooling system flush and refills typically are not performed as often as they should be, urge your customers to purchase this service in order to maintain the efficiency of the corrosion inhibitors that are part of the coolant formulation. Using only water, or allowing a mix to remain in the system past its prime, can erode aluminum surfaces and allow rust buildup in cast iron passages.

As we know, the color of various antifreeze liquids has proliferated over the years, with colors changed to help identify the type of antifreeze. OEMs have used green, pink, yellow, gold, orange and blue, again in order to “color code” for easier identification.

The color is used to identify the type of antifreeze. IAT (inorganic acid technology) is green. OAT (organic acid technology) is orange, red, green, pink or blue. HOAT (hybrid organic acid technology) is usually orange, gold or yellow, but may be found in green, pink, blue or red.

Formulations vary in terms of corrosion inhibitors and the degree to which they conduct ECD, or electromechanical degradation (higher ECD creates an electrical charge similar to a galvanic battery; this can degrade rubber and metal surfaces). This subject can lead to a very lengthy discussion on formulations/allowable mixing of antifreeze types. Play it safe and use the type of antifreeze recommended by the engine/car manufacturer.

As we all know (or should know), any coolant that features ethylene glycol is very toxic if ingested, leading to a variety of ills including painful kidney failure. Because ethylene glycol, even when diluted in a 50/50 mix, is “sweet” to the smell and taste, this can lure an animal or a human infant to taste and/or drink from an open container or puddle spill. A safer alternative is propylene glycol antifreeze, which is readily available from makers such as Peak and Amsoil, to name only two examples.

Have you ever wondered, since propylene glycol is less toxic than ethylene glycol, why the car makers wouldn’t simply specify propylene glycol in all engines? According to an industry source, it simply has to do with longevity, reportedly because propylene glycol doesn’t last as long as ethylene glycol. Plus, propylene glycol antifreeze is more expensive. In a nutshell, those are the reasons.


All coolant hoses are not created equal.

For severe heat applications such as turbocharged engines, an upgrade to consider is the use of high-heat-rated silicone coolant hose. These may be rated up to 500 degrees Fahrenheit and are suitable for coolant hoses that are located in close proximity to turbochargers and turbo air tubes.

Aside from obvious damage such as exterior abrasions or pinholes, ozone damage over time can dry out the exterior, leading to cracks at bend areas and eventual failure. Most commonly, hoses tend to degrade internally. If hard and brittle feeling or soft and mushy when squeezed, it’s overdue for replacement.

Thermostat housings

As we all know, engine designs differ from make to make, model to model and model year to model year.

While inspecting and/or servicing the cooling system, take the time to inspect the thermostat housing. Some are made of “plastic” and are prone to cracking, while others are made of cast aluminum alloy or steel, which may be prone to corrosion, pitting and/or cracking.

An example is the metal housing/upper radiator hose connection on the Navistar 7.3L diesel engine featured in 2003 and older Ford light trucks. These housings are notorious for rusting/rotting. If severe rust is evident, replace the housing. If ignored, this is a severe coolant leak just waiting to happen.


Thermostats feature a wax-filled copper housing that pushes the thermostat open against spring pressure. As coolant temperature rises to a specific point (based on the heat rating), the wax melts and expands, pushing against a small piston, which opens the coolant path in the thermostat. When the coolant temperature begins to drop, the wax contracts, allowing spring pressure to close the thermostat.

If the engine has been overheated at any point, the tension of the spring may be compromised, so always replace a thermostat if you suspect a previous overheating issue. Even a brief overheating condition can destroy the thermostat’s ability to function properly. Even if the thermostat appears to be functioning, if exposed to excess heat, it may not be reliable. The best move is to simply replace it to avoid future issues.

Water pumps

An engine’s water pump (more appropriately referred to as a coolant pump) is the heart of the engine’s cooling system. Many of today’s engines feature water pumps that are mounted to the engine block behind the timing cover. For engines that feature a belt-driven timing system, whenever the engine is due for a timing belt replacement (for example, at 50,000 miles or so), plan to also include a water pump replacement during the belt service. Since the same labor time may be required for either a belt or water pump, it just makes sense to replace both items at the same time.

Instead of replacing only a water pump (when the need arises), consider the related components that work in conjunction with the pump as a system package. This would include the pump, the belt that drives the pump, the pump pulley, belt tensioner and any belt idler pulley(s) that may be involved on a specific engine. If enough time and miles have accumulated to result in a worn/failed pump, the same time and wear cycles have been experienced by the belt and any associated pulleys/tensioners. It simply makes sense to replace the entire system of parts that directly relate to the pump, even if those items show no immediate failure symptoms.

A preventative maintenance approach as part of a pump replacement helps to ensure the reliability of the cooling system. Yes, this requires the customer to spend a few additional dollars, but the elimination of worry and avoidance of potential future issues is well worth the investment.

Be aware that some engines (such as BMW) are equipped with an electric water pump. Pump issues should prompt specific fault codes as follows:

  • 2E81......pump speed deviation/speed outside of specified range
  • 2E82......pump cutoff/over-current
  • 2E83......pump in low pressure mode (possible dry running)
  • 2E84......communications issue
  • 2E85......communication/no input voltage

If the pump and thermostat have been replaced but code 2E83, 2E84, 2E85 continues to store, suspect an issue with the positive power distribution block and/or power connections.

Pressure caps

Cooling systems are traditionally operated under a specific amount of pressure in order to raise the boiling point of the coolant. Consider that for every pound of pressure, the boiling point is raised by about 3 degrees F. The pressure cap is designed to apply and control this pressure, typically in the range of 13 psi to 16 psi (which increases the coolant’s boiling point by about 39 to 48 degrees F).

Naturally, as an engine’s coolant soaks heat, it expands, creating system pressure. When this pressure reaches the cap’s pressure rating, the cap’s valve should open, allowing coolant overflow. This also helps to vent air out of the cooling system. When the radiator cools, a vacuum is created that allows any overflow to move from the overflow tank back to the system.

When coolant expansion occurs at around 200 degrees F, about 16 psi to 18 psi pressure will be generated. However, if the engine overheats because of other factors, pressure could climb as high as 28 psi or so. It’s important to carefully choose the pressure cap, both in terms of quality and pressure rating.

The radiator pressure cap should always be located at the cooling system’s highest location, on the low pressure/suction side (the side where coolant leaves the core, on its way back to the water pump). The reason for locating the cap at the highest point? If the cap opens and vents because of excess pressure, any air in the system will escape first, before any coolant loss.

Cleaning radiator cooling tubes/fins

Inspect the radiator’s fins and tubes for debris such as dirt, mud, leaves, bugs and other road debris. Flushing with a hose and hot water may be needed. If compressed air is used, be careful not to apply excess air pressure, as this can easily damage the cooling fins.

Also look for debris trapped between the radiator and A/C condenser. Any air flow obstruction can reduce the radiator’s ability to release heat, resulting in an overheat issue.

Cooling fans

If the engine is equipped with mechanical fan, fan clutch issues can prevent the fan from operating. Bent, damaged or missing fan blades will not only reduce the fan’s ability to pull air, but can easily result in fan operating vibration (due to imbalance), which will in turn lead to premature wear of the water pump bearings and shaft. If equipped with one or more electric fans, check for fan operation by using your scan tool to command the fan on and off.

If the fan operates in both low speed and high speed when applying battery power directly to the fan motor’s connector but the fan does not operate when commanded or when the temperature peak is achieved during normal operation, suspect fan motor connectors and/or the low and high speed fan relays, and/or the cooling system’s temperature sender.

Bleeding trapped air

Air must be allowed to bleed from the cooling system. Air pockets allow the creation of pressurized and extremely hot steam, which can create isolated hot pockets in the engine block, potentially damaging cylinder head gaskets, warping cylinder heads, distorting cylinder bores, cracking in heads or block, piston ring distortion, and more. Just as we must evacuate air from a brake system hydraulic circuit for proper hydraulic operation, we need to remove air from the engine cooling system.

Some vehicles feature convenient bleeder valves in the cooling circuit. If a bleed valve is present, with the engine cold, open the bleeder and fill the system with the appropriate coolant mix (generally 50/50 coolant/water) until a steady stream of coolant exits the bleeder, and then tighten the bleed valve.

To cite but one example, the Chrysler PT Cruiser features a bleeder that must be opened in order to remove air from the system. If coolant level has dropped, or during a flush and fill, this bleeder must be opened to allow air to escape. Otherwise, you will never be able to remove trapped air. This may need to be repeated several times in order to fully burp the system. After performing one bleed, it’s possible that the car could be driven for a few days with the temp gauge in the normal range, and suddenly the temp rises into the danger zone and immediately pops back down to normal. This is a sign that there’s still an air pocket in the system, requiring another bleed.

Using the PT Cruiser as our example, the thermostat housing (black plastic) features the pressure cap and provides a coolant fill point. This plastic housing bolts to a cast aluminum lower housing which secures to the cylinder head. The aluminum housing features a bleed valve that requires a 10 mm wrench. If the bleed valve has not been serviced for a long period, it’s likely corroded in place and can be snapped off if excess force is applied while attempting to loosen.

If the valve seems stuck, don’t force it. Instead, soak the bleed valve with a thin penetrating oil (such as WD40) and allow to soak overnight. Chances are good that the valve’s threads will then break loose with moderate force.

With the bleed valve open (no need to remove it... simply back off about two or three turns to open the valve’s seat), add coolant mix to the thermostat housing neck until a steady stream of coolant exits the bleeder, then tighten the bleed valve. If only a dribble of fluid is evident, the valve may be contaminated. If this is the case, remove the bleeder and clean it by soaking it in solvent, running a wire through the valve’s orifice and blow with compressed air.

Depending on the engine design, the path for air bleeding will vary. GM’s LS engine format, for example, features “steam” plumbing that connects the front and rear coolant passages of each cylinder head to a central point in the upper radiator hose assembly via a tube network. Be aware that because steam hole plugs are available, some owners may have plugged the front and/or rear steam holes in the heads. Plugging the rear steam ports can result in overheating the rear #7 and #8 cylinders.

Some engines are equipped with an electric water pump (various BMW models, for example). The bleed procedure may be as follows:

  • If the system features one or more bleed screws, open the screw(s).
  • Fill the expansion tank slowly with a 50/50 mix of coolant.
  • Close the bleed screw as coolant begins exit the bleed screw.
  • Once the bleeder on the expansion tank (if equipped) flows coolant and has been closed, fill the expansion tank to the lower edge of the filler neck.
  • Install the coolant pressure cap.
  • Connect a battery charger to the battery or to the jumper cable connections.
  • Turn the ignition to the ON position. If equipped with a START button, push the button but do not depress the brake or clutch pedals. This will turn the ignition on but will not crank the engine.
  • Set the heater temperature to max and set the blower to the low speed setting.
  • Press the accelerator pedal to the floor for 10 seconds.
  • The self-bleeding procedure will begin via the electric water pump and will run for about 12 minutes. During this time, the water pump will alternately turn on and off. Do not remove the cap while the bleed sequence is running.
  • When bleeding is complete, check coolant level and add if needed, and replace the pressure cap.

Common sense

A few guidelines apply to all vehicle cooling systems, regardless of make/model/year, as noted by Gates Corp.

  • Use only distilled or deionized water when mixing with full-strength coolant. Using tap water can lead to mineral deposits and scale.
  • Do not mix dissimilar coolant chemistries.
  • If the water pump or other components show any signs of contamination, the entire system should be flushed completely, including the heater core, radiator and engine block.
  • Most timing-belt-driven water pumps are replaced when the belt and tensioner(s) are being replaced.

If you’re not familiar with a specific engine, don’t assume anything. Refer to the service manual for the correct cooling system bleeding procedure.    ■ 

Related Articles

Complete Brake Service Tips: Service Guidelines to Aid in Your Diagnosis

9th Annual ‘Tips From Techs’: Service Technicians Share Tricks of the Trade

8th Annual ‘Tips From Techs’: Service Technicians Share Tricks of the Trade

You must login or register in order to post a comment.