Auto Repair For Dummies, 2nd Edition
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To keep your cooling system cool, check the fluid level in the system and, if necessary, add water and coolant (see below for how to add fluid directly to a radiator). It’s dangerous to remove the pressure cap from the radiator or coolant recovery system reservoir while the engine is still warm. Because it’s hard to tell just how hot things are inside the engine, follow these guidelines when removing a pressure cap:

  • Never remove the cap from a radiator or coolant recovery system reservoir when the engine is hot. Adding cold water to a hot engine can crack the engine block.

  • If your engine overheats on the highway, get to the side of the road, turn off the ignition, and then wait 15 to 20 minutes for things to cool down. If you can stay safely away from traffic, you can lift the hood to help the heat escape but you should leave the pressure cap alone.

  • If you’re parked where traffic is zooming by, or if you’re concerned about your security if you leave your vehicle, you’re better off just waiting in the car until the engine cools. If it heats up again when you start driving, get to the nearest service station or a place where you can safely park, get out, and deal with the situation yourself.

Of course, if the engine is completely cold, you face no risk at all, so get into the habit of checking your coolant level at least once a month in the morning before you warm up the engine.

To remove the pressure cap safely, follow these steps after the engine is cool:

A coolant recovery reservoir (a) and a cap being removed safely from a radiator (b).

A coolant recovery reservoir (a) and a cap being removed safely from a radiator (b)
  1. If your system has a safety pressure cap, lift the lever on the safety cap to allow the pressure to escape.

    To keep from burning your hand, place a cloth over the cap after you raise the lever. Then turn the cap counterclockwise to remove it.

    If your vehicle doesn’t have a safety cap, place a cloth over the cap and turn it counterclockwise just to its first stop. Turning to the first stop allows some of the pressure to escape, but if you see liquid or a great deal of steam escaping, retighten the cap and wait for things to cool down. If nothing escapes, continue turning the cap counterclockwise to remove it.

  2. Tilt the cap as you remove it so that the opening points away from you.

    If there’s still enough heat and pressure to spray hot stuff around, it lands on the engine or inside the hood, where it can do no harm.

  3. Replace the cap by screwing it on clockwise. (If you have a safety pressure cap, push the lever down again.)

Don’t remove the cap on the radiator unless your vehicle has no plastic coolant recovery reservoir, as shown in the figure above.

Radiator safety caps cost very little, so if you don’t have one, buy one! Almost every service station stocks them, but they’re cheaper in auto supply stores. Check your owner’s manual for the amount of psi (pounds per square inch) of pressure in your system and look for the proper number of psi on the new cap. These safety caps are well worth the money.

How to add liquid to a coolant recovery system

If your car has a coolant recovery system, you can check the level of liquid on the side of the plastic reservoir. You just open the cap on the reservoir to check whether the coolant looks as though it needs changing or to add water and coolant.

Many vehicles have a pressurized coolant recovery system called an expansion tank that makes opening the radiator unnecessary. These systems are considered “sealed” because the safety pressure cap is on the recovery reservoir rather than on the radiator.

If you overfill the system, the extra liquid gets hot, expands, and flows out of the overflow pipe. That may not seem too terrible, but because coolant is toxic, it can harm animals or children, who love its sweet taste.

If you don’t have coolant on hand and you just need to add a little liquid to the cooling system, plain old tap water will do. But try to maintain a good coolant level by adding a similar amount of straight coolant the next time you add liquid to the system.

You will probably never need to open the cap on the radiator, but if you have to open the cap for any reason, make sure to fill the radiator to the top with a 50/50 mixture of coolant and water before replacing the cap.

This addition bleeds the system by forcing any air that may have gotten into the system into the reservoir and out through its overflow pipe when the engine heats up. Follow these steps when adding liquid to the coolant recovery system:

Adding cold liquid to an engine that’s hot can crack the engine block because the hot metal contracts sharply when the cold liquid hits it.

If the liquid level is low, add equal parts coolant and water to the reservoir until the level reaches the “MAX” line on the side of the container.

How to check and add liquid to a radiator

If you don’t have a pressurized coolant recovery system, you have to add liquid directly to the radiator. If you must add liquid to the radiator if the engine is still warm, always do so slowly with the engine running. This way, the cold liquid joins the stream of hot water that’s circulating through the system rather than falling all at once into the system when you start the engine again.

To add liquid to your radiator, follow these steps:
  1. Open the radiator cap.

    Place a cloth over the cap and turn it counterclockwise just to its first stop. Turning to the first stop allows some of the pressure to escape, but if you see liquid or a great deal of steam escaping, retighten the cap and wait for things to cool down. If nothing escapes, continue turning the cap counterclockwise to remove it.

  2. Look into the radiator fill hole to see how high the liquid level is inside.

    If you’re unsure about what the liquid level should be, just make sure that it covers the radiator tubes that are visible when you look down the hole, or that it reaches to within a couple of inches below the cap.

  3. Add water and coolant, or pre-diluted coolant, as necessary.

    Under normal conditions, a 50/50 mix of water and coolant is preferred for most vehicles. If the day is extremely hot or cold, a higher proportion of coolant/antifreeze may be necessary.

  4. Replace the cap by screwing it on clockwise.

    If you have a safety pressure cap, push the lever down.

About This Article

This article is from the book:

About the book author:

Deanna Sclar is a freelance writer and auto repair expert. She has appeared on hundreds of radio and TV shows, including NBC's Today show and the NBCNightly News. Sclar lectures internationally on the ecological impact of vehicles.

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