Workshop Skills: Engine Coolant
How often should your engine’s coolant be renewed, can you use a different colour coolant and what’s the best means of flushing the system? Find out more with our maintenance and servicing guide to coolant.
Our series on maintenance and servicing moves on to look at engine coolant, a more complicated subject than you’d think.
An engine’s coolant has quite a demanding job. It must be able to cope with freezing temperatures to avoid the risk of turning into a solid, which could cause internal damage to the engine such as fracturing a waterway and causing it to leak. During the summer it needs to maintain the engine’s temperature to ensure it doesn’t get too hot, along with help from the radiator and a mechanical or electric cooling fan. And the coolant also needs to prevent internal corrosion of the engine, which can otherwise result in porous waterways and a build-up of silt that can block passageways and the water pump.
Most classic cars from the mid-1990s or before use a blue- coloured coolant, also generally called antifreeze. As Witham Group’s Technical Director, Mick Kenyon explains: 'Coolants are made up of a combination of finely balanced additives that each have a specific role in the quality of the product. Water acts as the main heattransfer fluid moving heat away from the engine block. Monoethylene glycol (MEG), an organic compound, is added as it also provides heat transfer, but most importantly it lowers the freezing point and raises the boiling point of the water. Without MEG or a similar organic molecule, coolant would freeze too readily and subsequently damage the engine as it expands.'
Water and MEG on their own would be corrosive to metals within the cooling system such as cast iron, aluminium or solder, so rust would quickly block the cooling system and overheating would occur. 'Additives are used to prevent corrosion,' continues Mick. 'In the case of our Qualube Universal Blue, this is an example of an inorganic additive technology product (IAT). This means that the chemicals which protect against corrosion are inorganic in nature, such as borates, silicates and nitrates. Inorganic chemicals are mined from the earth and refined. IAT is a tried and tested classical method for protection.'
Later and more modern engines, such as the Rover K-series that’s used in the MGF and TF, can use a longer-lasting silicatefree, organic acid technology (OAT) based coolant. 'In some cases OAT coolant may be chemically unsuitable for the older classic cars,' says Mick. 'I would recommend a hybrid or IAT over an OAT for classic cars. Qualguard hybrid is a premium product and would be perfectly suitable here as well as across a wide range of other applications.'
There’s also some confusion over the colour of coolant and its lifespan. For instance, blue- coloured coolant is generally regarded as lasting for two years before it needs changing, whereas pink- coloured coolant lasts for six years, but as Mick warns: 'This is broadly true, but colour is purely cosmetic and gives no indication of the technology used or the quality of a product. I would certainly be wary about making decisions based purely on colour.' Unfortunately, the only sure fire way of determining what chemical make-up existing coolant has is by chemical analysis.
RENEWAL INTERVALS
As we have noted, the recommended intervals for changing coolant appear to be around two years for blue coolant and six years for pink – subject to the provisos above! However, it’s worthwhile testing the freezing and cooling capacity of the coolant at every service interval (at least once a year) to check its condition. This can be done with a simple anti-freeze hydrometer that costs from as little as £10 from motor factors and tool suppliers such as Machine Mart.
Whilst checking the coolant, it’s also worthwhile checking over the cooling system of the engine by squeezing hoses to look for perishing, inspecting all connections for traces of leaks (usually shown by residue) and looking around the radiator for similar leaks. Hose clips should be sufficiently tight to prevent leaks, but Jubilee and similar clips must not be over tightened as they can cut
into the rubber and create a leak.
Most coolant hoses are made of rubber and are specifically designed for coolant – other hoses such as a breather hose will deteriorate rapidly and leak or blow off under pressure if used in the cooling system. Steel coolant pipes and similar connections can corrode and leak, often beginning with a tiny hole that only allows coolant to escape when it’s warm and under pressure. 'Quality coolant will protect against steel corrosion,' says Mick. 'However, I would still recommend rubber hoses as being most suitable.'
A pressure cap may be fitted on the top of the radiator or onto an expansion tank. This helps to raise the boiling point by an additional 25°C by increasing the pressure in the cooling system as the engine warms up – the higher the pressure, the higher the boiling point. ‘The boiling point of coolant is dependent on the ratio of concentrate to water that is used,' explains Mick. 'Typically this is a 50-50 mix and will have a boiling point of around 110°C at atmospheric pressure.' Increasing the pressure in the cooling system not only allows the boiling point to be raised, this pressurised system reduces the risk of the coolant expanding and leaking out when it warms up, although there may be an overflow tank to collect the expanding coolant and store it until it is drawn back in when the engine cools down.
The pressure cap generally has a spring and valve on the underside to control the pressure and stop it rising so high it causes damage. However, if the valve has failed, it may allow coolant to leak out when the engine is running normally, resulting in coolant loss and then potential overheating. Renewing the pressure cap is part of the recommended service schedule for some cars, and the condition of the rubber seal should be one of your regular service checks.
RENEWING COOLANT
The ratio of water to antifreeze varies, depending on the desired freezing capability you need – the greater the proportion of antifreeze, the lower the freezing point. However, Mick warns: 'Don’t go above two parts antifreeze to one part water, as the quality and freeze protection would then be lower.' And Mick offers some useful advice on the water used in a cooling system, saying: 'I would always recommend the use of deionised or distilled water for dilution. This is what comes in the premix product. Antifreeze is designed to tolerate dilution with hard water, but if the water in your region is especially hard then there could be a greater tendency for silt formation.'
We should also mention products such as Motul MoCool or Water Wetter, which are two non-glycol-based additives for use in race engines where the focus is purely on cooling and resistance to freezing in sub-zero temperatures is not really an issue. Witham explains that this kind of product helps the engine to run up to 15°C cooler and improves thermal exchange and engine cooling system efficiency. It can be used diluted with water to a ratio of 20:1 (20 parts water), or with a monoethylene glycol (MEG) based antifreeze.
The starting point for renewing engine coolant is to determine how you are going to drain the old fluid. On older cars there may be several drain points, such as a drain tap on the engine block and others on the bottom of the radiator and heater. If there aren’t any of these, or if they are seized, then releasing a lower coolant hose from the engine and the bottom of the radiator can be just as effective, though the resulting flow of coolant may be harder to catch cleanly. Remember to open any heater valves or switch the heater to hot to ensure the coolant is drained out of that too – and this is also essential for refilling.
Draining the system by releasing drain taps or hoses probably won’t remove all of the old coolant, and certainly won’t remove all of the
silt that may be present. Instead, try flushing fresh water from a hose pipe through the system to help flush out the remaining coolant and any silt. Alternatively, an engine flush could be added to the coolant before draining it, which requires the engine to be run to help break down and loosen any silt.
Flushing the coolant system may not be sufficient if a radiator or heater matrix is really choked with silt, in which case it is worthwhile removing it and attaching the hose pipe to one end at a time and flushing extensively in both directions. The expansion tank can also benefit from being removed and cleaned with a hose pipe and bottle brush. Badly stained plastic tanks can also be cleaned by repeated swilling with brake cleaner and a handful of nuts and bolts.
SWITCHING
Changing to a different type of coolant isn’t entirely straightforward. 'Compatibility between chemicals is a wide-reaching topic,' says Mick. 'Coolant technologies are formulated to give protection to the system in isolation, and a mixture of two different types of technology will typically be of lower quality than either on their own. In extreme cases the additives may be chemically incompatible with one another and a reaction could occur, leading to gel formation or interfering with the corrosion protection of the product.' So flushing the system is essential if you intend to change the type/ colour of coolant.
This also applies if you are switching to a different sort of coolant altogether, such as Evans Waterless Coolant. This is a non-waterbased coolant which must not be mixed with water. It provides the same cooling and protection performance as traditional engine coolant, but doesn’t require the same pressure in the cooling system and provides other advantages too. A dedicated engine flush (called Prep Fluid) should be used to remove all traces of water before adding Evans Waterless Coolant.
When you’re ready to refill the coolant system with fresh coolant, try to see how the air in the coolant system is going to escape. There may be some bleed screws that can be undone, for example. Ideally, these need to be situated at a high point, but they can be extended by attaching a hose to the bleed screw. This is the recommended means of renewing the coolant on the Mk1 Toyota MR2 for example, where there are coolant pipes with bleed screws under the front compartment and hoses are attached to these bleed screws when refilling the coolant system.
Whether there are bleed screws or not, make sure the heater is switched to hot and any pressure caps are removed, then start to refill the system via points such as the radiator and expansion or header tank. The thermostat will restrict the flow of coolant, but it should force its way through and allow the air to escape via the bleed screws and/or the open radiator and expansion tank.
However, the engine may need to be run to help refill the system. When running the engine, watch the temperature gauge, check the heater gets warm and look for air bubbles escaping via the radiator or expansion tank. It may take 10-15 minutes for the engine to warm up to temperature and for all the remaining air to escape. That remaining air can cause air locks, so carefully check all heater hoses are warm. If an air lock is suspected or found, then it needs to be removed, which can be time-consuming. Running the engine at high revs may help to force it out as the water pump will rotate faster, but watch the coolant temperature gauge. Pressurising the system with a pressure tester is a safer means. This involves pumping the coolant system to increase its pressure using a hand pump before releasing a bleed screw to see if any air escapes. It’s usually used for pressure testing the coolant system, but can help to remove an air lock. Budget for around £40 for a universal kit.
In the next instalment of this series, we’ll be looking at gearboxes, overdrives and differentials and the different oils that they require.