Quentin Willson's guide to Checking Your Cooling System
An engine that’s been boiled or frozen is usually scrap. That’s why checking the coolant level regularly and making sure you’ve got enough antifreeze is absolutely essential. Look under the bonnet for the radiator expansion tank, usually made out of off-white plastic, and check it every week. Make sure the engine is cool, before you remove the filler cap, and check the quantity and quality of the coolant. If it has a distinct chemical-like colour that usually means your antifreeze concentrate is ok. The level should be just below the filler neck (some cars have High and Low marks on the outside of the expansion tank) but if you can’t see the level at all, you’ll need to top it up. Try and keep a bottle of antifreeze handy and mix it half and half with water. That way you won’t dilute the concentration. And if you need to top up all the time, you could have a water leak or blown head gasket so seek professional advice as a matter of urgency.
If the water is brown and rusty there’s probably no antifreeze in your radiator at all. If there’s a white mayonnaise like gunge in the water, or on the inside of the cap, again, it points to a problem with your head gasket, and you’ll need to get the engine checked by a professional. One important check is to see that the electric engine fan is working. Let the car idle in neutral after a run and listen for the sound of the fan cutting in, it may take some minutes for the temperature to rise enough for the fan to operate. If it’s not working, you’ll overheat in the next traffic jam. So check both the fuse box (your car’s handbook will tell you where the box is and which is the fuse for the fan) and also look at the fan sender unit on the side of the radiator. Usually a round plug with wires attached, it switches the fan off and on automatically, depending on the engine temperature. Be very careful when working around an electric fan as it can start working without warning – even with the ignition off! Pull off the wires and look to see if they’re dirty or corroded. Clean them with sandpaper or a wire brush and then check the fan again. If it still won’t work, you may need a new sender unit. Get your garage to do this as soon as you can.