How to Check Fluid Levels on Your Car

Translucent plastics have made checking the levels of various fluids in cars much easier and safer. On all but the oldest cars you should be able to check most by just looking at the reservoir bottle so there is no risk of inadvertently introducing contaminants or undoing something where the contents are under pressure. However, make sure you identify the correct reservoir: you don’t want to top up the brake system with washer fluid or vice versa. If levels drop substantially between services in these items they should be professionally checked.


The most obvious reservoir is usually the expansion tank for the cooling system. Since the 1970s cars have been built with sealed cooling systems, where instead of venting to the air they have a large container into which coolant can pass when it heats up and expands, then be drawn from as it cools. These days the tank is made of clear plastic with levels marked for minimum and maximum, sometimes both hot and cold. Never remove the cap from the tank or the radiator when the engine is hot or you may be scalded by the escaping steam and coolant, which can be hotter than boiling water.

Check Fluid Levels How to Check Fluid Levels on Your Car

The coolant is a mixture of water and anti-freeze, which does much more than stop it freezing. It contains additives to stop sludge forming, prevent corrosion, to stop different metals reacting with each other and to raise the boiling point. If it needs topping up a little, you can use plain water but it is best to use an anti-freeze mixture, though check the handbook for the type of anti-freeze required.

Coolant shouldn’t need much topping up and if it does, you should investigate the cause. First, check that the soft plastic or rubber washer in the expansion tank cap is clean and in good condition and if your car has a radiator cap, check that too. Next look for leaks: you may see dripping coolant but it is more likely you’ll see the telltale white marks where coolant that has leaked under pressure has dried. Also look for mayonnaise deposits in the oil, as explained in the section above on oil.

If you can find no leaks, or have doubts about the condition of the expansion tank or radiator caps, buy a new cap but make sure you get the right one for your car. These caps are cheap so it is worth seeing if this fixes a minor drop in levels before you seek professional help, though if it doesn’t solve the problem remember to tell the garage you have replaced it so they don’t renew it. If the level keeps dropping, or drops fast, you’ll have to get it professionally checked. A garage technician not only knows what to look for but has the facilities to be able to see all over the engine. Repairing a leaking system, even if you need a new radiator, is much cheaper than fixing an overheated engine.

Check Fluid Levels 1 How to Check Fluid Levels on Your Car

Anti-freeze explained

Like so much else, anti-freeze is not as simple as it once was. Traditional anti-freeze, which required replacing every two years or 30,000 miles, was called inorganic acid technology (IAT) but new formulae were developed to extend its life to five years or 150,000 miles. Organic acid technology (OAT) was introduced in the 1990s followed by hybrid organic acid technology (HOAT), which is a mixture of IAT and OAT. There’s also a nitrate organic acid technology anti-freeze but at the time of writing no car manufacturer used it. You don’t need to know the technology behind this, just that mixing them isn’t recommended because it shortens the life of the coolant and reduces its protective qualities. So just see what set of initials are applied to your car’s anti-freeze in the handbook and what percentage of anti-freeze solution to water is required. It is best to mix anti-freeze with deionized water, though some stores stock ready mixed anti-freeze for topping up.

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