The Causes of Low Engine Coolant Levels

by Laurie BrennerUpdated July 13, 2023
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Low Engine Coolant Levels

The average car has an operating temperature of around 190 to 225 degrees Fahrenheit. Your vehicle’s coolant system helps to keep engine heat at these recommended levels for your car’s optimum operation. This system includes multiple components: a radiator, a water pump, an overflow reservoir, thermostat, temperature sensor, hoses and channels in the engine itself to remove the heat. The thermostat opens at a pre-designated engine temperature during operation to allow the coolant to circulate.

Coolant Symptoms

Several common causes can reduce your car’s coolant levels. First, take a note of when the engine overheats. If your engine overheats while the car is in motion, you most likely have a coolant flow issue. If it overheats while sitting parked or at a traffic light, your fan is most likely malfunctioning.

Be cautioned, low coolant levels put your car at risk of engine overheating which causes engine damage, corrosion and seizing. So, make sure to inspect the coolant level on a regular basis. If the coolant level in the white plastic reservoir is below the line marked "minimum" or "min," pour coolant in the overflow reservoir until it reaches the "maximum" or "max" line.

The most obvious sign of a coolant leak is a warning light that will appear on your dash when coolant system problems are detected in your car’s engine. The next are visibly low levels of coolant in the coolant reservoir. Other signs include external coolant leaks like colored puddles under the car, and burning smell when the coolant drips on a hot engine. The worse case scenario is white smoke coming out of the exhaust system. This indicates that you have a head gasket problem. All of these symptoms are explained below.

Coolant Reservoir

Check the coolant reservoir for cracks and holes. Coolant reservoirs are made from plastic and can be easily cracked. If the coolant reservoir has a crack allow the car to cool down. Remove the reservoir. Thoroughly wash and clean it. Patch the crack with a plastic welding material or epoxy. Allow the patching material to dry and replace the coolant reservoir.

Overflow Reservoir

Most overflow reservoirs come equipped with a small tube that allows the coolant to escape if the engine gets hot enough and the coolant expands past the full mark on the overflow reservoir. This can happen on high-temperature days when you also run your air conditioning. Because of this, you need to check the reservoir level periodically to ensure coolant is between the minimum and maximum marks on the reservoir. Use the recommended mix of coolant and water for your vehicle and fill to this location. Vehicle owners driving in climates with extreme temperatures should regularly check to make sure they have enough coolant.

Radiator Leaks

The radiator itself is prone to damage from stones and road debris and from general aging. Small pinhole leaks in the radiator will cause coolant fluids level to drop. Pinhole leaks may not be noticeable during operation, as the fluid will leak engine oil when the car is running under load. But after operation, the radiator will give off a hot sweet smell and show signs of leaking underneath. Pinhole leaks will grow if not fixed. Replace the radiator or have it repaired at a radiator repair shop.

Leaking or Blown Head Gasket

A leaking or blown head gasket will cause water to leak into the areas of the block where it was not designed to go. This will cause the coolant fluid levels to drop without any noticeable leaks under the coolant reservoir, hoses or radiator. To check for a faulty head gasket, pull the oil dipstick. Look at the color of the oil at the end of the dipstick. If it's milky in appearance, or water bubbles appear, this is a good indication of a blown head gasket.

Hoses and Connections

Check all connections where hoses connect to the radiator and the engine. Make note if you see any fluid around these areas. If the hose has a pinhole sized leak it is sometimes possible to see the coolant dripping from the hose. Some bad hoses may not show leaks until the engine is running.

Other signs of bad hoses included blisters and bulges, soft spots, hardening, and cracks. Perform a quick fix by cleaning the hose with a rag and wrapping the damaged area tightly with duct tape. ‌But‌, this is only a temporary fix. Replace all damaged hoses and clamps with new hoses. Include radiator hose inspection during oil changes or tuneups to prevent problems with the cooling system in the future.

Radiator Cap

A malfunctioning radiator cap is an important engine part and could be the cause of fluid leaks in the coolant system. Never take a radiator cap off after the car was just shut off, as the system is under pressure. Most cars do not have a temperature gauge here, so wait until the radiator is cool to the touch before removing the cap and emptying the contents of the sealant into the radiator. Run the engine for fifteen to twenty minutes.

Replaceable caps are available from auto parts stores for a small cost. Pinhole sized leaks in the radiator can also be temporarily repaired with a radiator sealant product such as Alum-A-Seal or Bar's Leak.

Use a coolant additive (radiator flush) when you normally add coolant to your radiator. This will help to prevent a radiator clog, but may not fix a problem that is already present. You can also make coolant at home by mixing antifreeze and water, however this may be less efficient for some vehicles.

When to Take the Vehicle to the Mechanic

Take your car to the mechanic if you are unable to diagnose the coolant loss problem. If the coolant reservoir is continually reading at a low level and you cannot locate the source of the problem, then there is an internal leak somewhere in the cooling system that requires professional attention.

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