# What Does Coolant Do, and Why Isn’t Plain Water Enough? | Otto Explains | AutoTronix **Section:** Otto Explains **Canonical:** https://autotronix.om/otto/coolant-explained **Language:** English **Arabic version:** https://autotronix.om/ar/otto/coolant-explained --- ## Short Answer Your radiator needs coolant rather than plain water because coolant does more than carry heat. Mixed with water as the manufacturer specifies, it raises the boiling point of the liquid in the pressurised cooling system, and its additives protect the engine, radiator and water pump against corrosion and deposits. Plain water is an emergency top-up only, to be replaced with the correct coolant as soon as possible. Use the coolant type your car’s manufacturer specifies rather than mixing types, and have it replaced on the manufacturer’s schedule, because its protective additives wear out with time. Never open the radiator or expansion-tank cap while the engine is hot. --- ## What does coolant actually do? The liquid in your radiator is coolant: water mixed with an antifreeze concentrate, usually glycol-based, that contains additives called corrosion inhibitors. Many manufacturers specify a 50% mix, and some supply it ready-mixed. Coolant has two jobs that plain water can’t do on its own. First, the cooling system runs under pressure to stop the liquid boiling as the engine gets hot, and coolant raises the boiling point further than water alone. Second, its inhibitors protect the metal inside the engine, radiator and water pump against corrosion. Many modern engines have aluminium parts, and their manufacturers say these must be protected by the glycol-based coolant they specify. The name antifreeze matters little in Oman, but the protection does: Land Rover, for example, says the antifreeze content must be kept up all year round, not just in cold weather, so that the coolant keeps protecting against corrosion. --- ## Can I just use water in the radiator? Only in an emergency, to get the car somewhere it can be put right. Plain water gives none of coolant’s corrosion protection, and manufacturers tell owners not to top up with water alone. Land Rover, for example, allows clean water only in an emergency when the right coolant isn’t available, and asks for the correct mix to be restored at the earliest opportunity. There is no safe distance or time for driving on water that applies to every car. If you have to use it, use clean fresh water, never salt water, then have the cooling system checked and the water replaced with the correct coolant as soon as possible. A top-up is not a repair either: a level that keeps falling has a cause, which [Otto’s guide to a dropping coolant level](https://autotronix.om/otto/coolant-level-dropping) explains. --- ## How can the wrong water or coolant harm the engine? The water in the mix matters as well as the coolant. Manufacturers specify soft, demineralised or deionised water, and Kia says hard water should never be mixed into the coolant filled at the factory, because minerals in the water leave deposits in the engine and cooling system. The water pump is one of the parts that suffers. One water pump maker lists the wrong coolant, the wrong mixing ratio, no coolant at all and contaminated water among the causes of early pump damage: overheating, deposits, corrosion and cavitation damage to the pump’s parts, which can end in a leaking seal. The right coolant, mixed as the manufacturer specifies, is what prevents this. --- ## Can different types of coolant be mixed? Not unless they are confirmed to be compatible. Coolants are made with different chemistries, many of them don’t mix, and mixing incompatible types can form a gel or deposits that block the cooling system and damage its parts. Car makers specify the coolant their engines need. Mazda, for example, warns that on engines marked for its specified coolant, using another may seriously damage the engine and cooling system, and Honda treats a different make only as a temporary measure, because continued use can cause corrosion. Your owner’s manual decides which coolant your car takes, and a workshop can confirm it from your car’s specification before topping up or refilling. --- ## When should coolant be replaced? When your car’s manufacturer says. Coolant’s protective additives wear out with time, so manufacturers set a replacement interval, and Land Rover says antifreeze must be renewed regularly regardless of the distance travelled, because old coolant can let the radiator and engine parts corrode. Intervals differ between makes, models and coolant types, so there is no single figure that suits every car: follow the service schedule for your vehicle. Between changes, look at the level in the expansion tank when the engine is cold. It should sit between the minimum and maximum marks and stay steady. A level that falls noticeably, or needs topping up often, points to a leak or overheating and needs an inspection. --- ## Why should you never open the radiator cap on a hot engine? Because the cooling system is under pressure. Opening the radiator cap or the expansion-tank cap while the engine is hot can release scalding coolant and steam with enough force to cause serious injury, and manufacturers warn that this can happen even after you switch the engine off. Check or top up only when the engine is completely cool; a radiator specialist advises waiting a few hours after a drive, because the coolant can still be very hot under a cap that looks safe. If the temperature gauge climbs while you are driving, don’t open anything: Otto explains [what to do when a car overheats](https://autotronix.om/otto/car-overheating). Coolant is also poisonous, so keep it away from children, and wipe up spills, which can damage paint. --- ## Who should replace coolant and check the cooling system? Car makers recommend having coolant replaced by a professional workshop. AutoTronix Express replaces engine coolant as part of its fluid replacement service, using the specification and concentration your vehicle needs, and its checks cover the coolant’s condition, its boiling and freezing protection and its corrosion-inhibitor level. If the level keeps falling, the engine runs hotter than usual or you can see a leak, that is a cooling-system fault for AutoTronix Garage, which diagnoses and repairs radiator leaks, water pump failure, thermostat faults and cooling fan problems. Before a long journey, include the coolant level in your pre-trip checks. --- ## Common Questions **Is it normal for the coolant level to change?** A small change between hot and cold is normal, because coolant expands as it heats up, which is why the level is checked with the engine cold. A level that keeps falling, or needs frequent top-ups, is not normal, and the cooling system should be inspected. **Does coolant matter in Oman, where it never freezes?** Yes. Freezing is only one of its jobs. Coolant raises the boiling point of the liquid in the cooling system and protects the engine and radiator against corrosion, which is why Land Rover, for one, says the antifreeze content must be kept up all year round. **What is the difference between concentrate and ready-mixed coolant?** Concentrate has to be diluted with the right water to the manufacturer’s ratio. Kia, for example, says the mix should contain no more than 60% and no less than 35% antifreeze. Ready-mixed coolant is already diluted, so Honda says not to add water or straight antifreeze to it. **Should I add a rust inhibitor or other coolant additive?** Honda advises against it, because added products may not be compatible with the coolant or with the engine’s parts, and vehicle makers don’t recommend coolant additives. Use the coolant your manufacturer specifies and replace it on schedule instead. --- ## Related Otto Explanations - What Should I Check Before a Long Drive in Oman?: https://autotronix.om/otto/long-drive-car-check-oman --- ## Related Services - Fluid Replacement Services: https://autotronix.om/express/fluid-replacement - Cooling System Repairs: https://autotronix.om/garage/cooling-system-repairs --- ## Book a Service WhatsApp: +968 9966 7600 https://api.whatsapp.com/send/?phone=96899667600&type=phone_number&app_absent=0 Location: Mabellah Industrial, Street 7, Muscat — Saturday to Thursday, 9 AM to 9 PM