Engine cooling system servicing


Nearly all modern vehicles made in the last 60 years have very similar cooling systems. Fortunately, modern engine cooling systems are reliable and easy to service and diagnose when issues occur.

If you’re experiencing a car that is overheating, take a look at our explanation of an automotive cooling system and book into Grease Monkey Autowerx today!

Pouring fresh coolant into a car radiator.

Components of a modern car engine cooling system

A modern automotive cooling system contains a variety of important components all working together to create a system based on maintaining the ideal operating temperatures.


A radiator is a type of heat exchanger with a coolant inlet and outlet that allows excess heat to be removed from the coolant and transferred to the atmosphere. This results in lower liquid temperatures on the outlet. Most radiators have a similar construction with a tank on each end, connected by thin tubes that carry the coolant. On the outside of these tubes many smaller fins are soldered to increase the surface area. This increases the cooling capacity of the radiator.

Radiator cap (pressure cap)

All liquids used as a coolant will boil at higher temperatures under pressure. For example, water boils at 100 degrees C at sea level, and at higher pressures (let’s use a 1.1 bar or 16.7 psi radiator cap as an example) water will boil at 122 degrees celsius. By way of two one-way valves, the function of a radiator cap is to vent excess pressure from the coolant expanding as it heats up. It also allows liquid back into the system as the system cools back down after shutting off the engine.


Engines are not efficient when they are cold, requiring a standard operating range somewhere between 80-105 degrees c to perform optimally (more on this later). The function of the thermostat is to redirect coolant back into the engine until a standard operating temperature is reached. At this time, the valve in the thermostat will open, allowing coolant to circulate through the entire cooling system.

Water Pump

Early cars used much taller radiators with inlets and outlets at the top and bottom to circulate coolant through convection. However, with the trend of lower cars, we’re now seeing shorter and wider radiators that require a pump to circulate water enough to be effective. Most water pumps are driven from the motor crank by way of a drive belt of some kind and pump speed is related to engine speed.

Overflow/expansion tank

Some vehicles will have a recovery reservoir to catch coolant that is evacuated from the coolant system through the radiator cap. These small tanks will have a system where the radiator cap can pull coolant back into the radiator as the system cools back down.

Some makes and models will not have a pressure cap on the radiator and will instead have an expansion tank. This pressure sealed vessel will have an allowance for air in the system that can be compressed as the coolant system heats up. It will only vent when the system is outside normal operating temperatures or pressures. Be sure to check your manual or ask the staff at Grease Monkey Autowerx if you’re unsure.

Temperature sensor

As engines do not function correctly while cold, and may even risk damage if ran for extended periods, a coolant temperature sensor will report back to the ECU how warm the engine is. While cold, the ECU will allow more fuel into the engine to help with emissions, causing items like catalytic converters to warm up quicker. he downside is that running an engine too rich for too long can wash excess fuel into your oil over time, causing damage to bearings and journals through inadequate lubrication.

Pressure sensor

Coolant pressure sensors play an important part in diagnosing potential issues with radiator caps or blown head gaskets as well as coolant system leaks. This information is either fed back to the ECU or read manually using a gauge.

Cooling fans

If the thermostat is open and the radiator passively dissipating heat is not enough to keep temperatures below the coolant boiling temperature, your vehicle will have fans to pull extra air though the radiator. Often this is the case at low speeds, in start/stop traffic or when towing.

  • Hub driven fans: Many older vehicles and industrial equipment will have a large fan bolted directly to the engine crank. This fan is locked to the engine speed and thus is constantly rotating whenever the engine is running. This saves electrical and mechanical complexity as well as manufacturing costs. If a vehicle with an older mechanical fan is stuck in traffic and overheating, apart from revving the engine higher in neutral there are not a lot of options for bringing down coolant temperatures.
  • Electric fans: Most modern implementations of cooling fans are switched on and off by solenoids, controlled by the ECU. This negates the horsepower draw of running a big fan directly from the crank and allows the fan and engine speed to be controlled independently.

Coolant additives

Water is typically the best coolant for a few reasons. It has the potential to store a massive amount of heat energy, it flows well and has an increased boiling point under pressure. However, the weakness of water as a coolant is the freezing point. At temperatures below 0 degrees, water will turn into ice and expand. This expansion can cause severe damage to engine blocks, hoses, and water pumps. Therefore, an additive is used -in most cases Ethylene glycol or Propylene glycol. This coolant additive is mixed in with water at a mix of 40:60 additive to water (freeze protection down to ~25 below 0) or up to 70:30 additive to water ratio for extreme environments, giving protection down to ~60 below 0. In most cases, additives will have corrosion inhibitors to prevent premature wear and tear.

Checking the fluid level on the expansion tank.

Issues caused by running too hot

Generally a hotter engine burns fuel economically and runs more efficiently. An overheating engine however, can suffer from a variety of issues, including a warped head, blown head gaskets, cracked cylinder bores or seized bearings. Engine lubricating oils will start to break down and oxidise at ~109 degrees c, while high performance synthetic oils will be okay up to around 130 degrees c. There is a fine operating window where the temperature is optimal.

Items like bearings are engineered to take thermal expansion into account when being made and designed to have proper clearances within operating temperatures. At too high a temperature it is possible for a bearing to be jammed. This can result in rod knock, spun bearings and expensive engine rebuilds.

Issues caused by running too cold.

As mentioned before, running an engine too cold can cause issues too. Engine oil requires being brought up to temperature to lubricate engine components correctly, with cold engine oil attracting moisture that is carried through the oiling system. This will damage bearing surfaces and cause excessive wear and tear on an engine. Once oil gets up to normal temperature this moisture often boils off and is removed from the engine. A cold engine will run extra fuel as the ECU will try to warm up exhaust emission components such as catalytic converters to operating temperatures. If the engine stays in this mode, fuel economy will drop significantly. Running too much fuel will cause problems with diluted oil over time. Fuel in oil results in improper lubrication of bearings and wear surfaces.

Common cooling system issues:

  • Punctured radiators - Radiators are often made of aluminium or brass. These are metals that transfer heat very well, however they are typically soft. An impact with road debris like a stick, rock or wildlife can easily puncture a radiator.
  • Stuck thermostats - thermostats are typically made with a wax pellet that expands and contracts with the temperature. If this gets stuck open or closed, the engine may overheat or may never reach operating temperatures, resulting in very poor fuel economy.
  • Old/split hoses - Coolant leaks from radiator and heater hoses are common on older cars. Fortunately replacement hoses are usually a quick and easy fix.
  • Head gaskets - if a car has overheated at some point or another, this may result in a leaking head gasket. There are several types of head gasket failure, all of them with their own symptoms and things to check.
  • Old coolant - Antifreeze coolant additives also contain corrosion inhibitors. These eventually become ‘used-up’ and require the cooling system to be flushed and refilled using appropriate additives. Each manufacturer has slightly different service interval recommendations.
Radiator cap

What to do if you’re having cooling issues.

As you can see, the job of a cooling system is to carefully balance the temperature of your engine to maximise your vehicle’s lifespan. Fortunately, most cooling issues are easy to diagnose with the right tools.

Contact us!

If you’re in Moorabbin, Cheltenham, Clayton or Malvern East and need suspension, steering, mechanical work, a vehicle inspection or new tyres and wheels, come visit us at Grease Monkey Autowerx. Booking in is easy and we have experienced and qualified mechanics to provide you with an excellent and comprehensive service.

Come into our shop at 28/684-700 Frankston - Dandenong Rd, Carrum Downs VIC 3201 or call us on 03 7037 6211 for a professional and trusted experience.

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