All modern automobile engines are “water-cooled”, which means they use circulating fluid to carry heat from the engine to a radiator where it is cooled and circulated back. Coolant is about 50% water and the balance is made up of a variety of chemicals and additives.
These lower the freezing point, improve the heat absorption capacity, resist corrosion, and sometimes contain compounds that seal up porous castings or plug tiny leaks.
Most manufacturers recommend replacing the coolant as a part of ordinary scheduled maintenance. This varies from vehicle to vehicle and the specific recommendation will be found in the owner’s manual, but the average is about every 3 years or every 50,000 miles.
That can be done by simply draining the system and refilling it. However, a much better result is gained by flushing the cooling system, and that is what is usually recommended.
Cost of a Coolant System Flushing
The average cost for a coolant flush can vary from £60 to £150. Some of the reasons for the variety in the cost are the differing labour rates and the cost of different vehicle-specific coolants.
A significant part of the cost is the quantity of the materials. This is because some vehicles with smaller engines take less than 2 gallons of coolant, while a light truck with a diesel engine might take more than 5 gallons.
Factory coolant costs from £10 to £20 a gallon on average, so it does add up. The coolant necessary for each vehicle is usually a part of the price of the service.
Another factor in knowing how much a coolant flush is whether or not a flushing compound is used. Typically, a compound will be added to the cooling system, run for a few minutes to circulate through the engine and get it up to operating temperature, and then, the old coolant will be removed.
The idea is that it dissolves and suspends old deposits, allowing them to be flushed out more thoroughly. Then, the system is refilled with a new, clean coolant. A flushing compound isn’t always used and isn’t always necessary, but it is part of the cost if it is done.
Why Does Coolant Need to be Serviced?
Over time, the coolant can react with the various surfaces it’s in contact with. Many coolants have specific anti-corrosion additives that act by reacting with and coating the metal surfaces of the engine and radiator and becoming depleted with time.
Coolant compounds can also break down over time from heat, from contact with the atmosphere (in the coolant reservoir), from simple age, etc. Coolant that has lost its valuable properties can allow internal corrosion.
One of the most important features of coolant is its resistance to freezing, and probably the worst-case scenario is the coolant that doesn’t have sufficient resistance to freezing. That is one of the main things that can be tested for during a vehicle inspection.
Water expands when it freezes and has enough force to even crack an engine block; one of the reasons that engine blocks have freeze plugs that can pop out and relieve pressure in that situation.
What Can Go Wrong?
There’s not a lot that can go wrong with a properly done coolant flush. The two things that have to be done right are fairly simple in a competent shop; the correct coolant needs to be used, and care must be taken to not leave trapped air in the cooling system.
With the drain and fill method (which is sometimes necessary), many vehicles need to have the cooling system evacuated with a vacuum device to remove trapped air and then run up to temperature and topped off. This is common enough now that most shops are equipped, but using a flush machine avoids that issue.
There are three main types of coolant used by manufacturers now, and these are generally not compatible. Sometimes they are identified by the different dyes used, but that’s not always a reliable indicator.
Chrysler, for instance, used a pink coolant mix for a time, then switched to a purple mix that is incompatible with the pink. They congeal and can block up the cooling system if mixed.
One unfortunate tendency of the purple mix is that it tends to turn more orangish or pinkish as it ages. Most shops will verify the correct coolant from spec sheets or the owner’s manual if there is any doubt.
Manufacturers make coolant with specific mixes to work best with the metals and gasket materials that the engine uses. A little money can be saved by using an aftermarket universal coolant that meets many different specs, but it’s not always possible to gauge the suitability of a universal coolant mix for a specific engine.
An additive that is designed for one engine might not be suitable for another. One general rule is that the less universal a coolant mix is, the more likely it is to work for an application it is specified for.