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What if Your Car Coolant Is a Milky Color?

by Rob Wagner

A milky color in an automobile’s coolant could be the sign the engine has blown a head gasket, and may lead to catastrophic failure of the engine. The odd milky, gray or chocolately coloring is one of several symptoms the engine is in deep trouble. Although it is possible another source contaminated the coolant, car owners should err on the side of caution and immediately take their vehicle to a mechanic for inspection.

Head Gasket

The automobile engine’s head gasket is a copper or steel seal fitted between the cylinder heads and the engine block. It the engine remains sealed to allow the spark plugs' spark to explode in the combustion chamber under pressure. The head gasket also keeps the coolant and motor oil separated from the chambers and serves as a passageway for the liquids to flow. Neither liquid is mixed. A blown head gasket occurs during overheating when the engine runs at an excessive temperature due to loss of coolant, or blockage in the coolant hoses or radiator. Some engines are prone to head gasket failure due to manufacturing flaws, but this is generally rare.

Mixing Liquids

Most motor vehicles have coolants that feature a bright lime, green or orange color. Newer models have a plastic reservoir in the engine compartment either at the front of the engine near the radiator or against the firewall. Without removing the cap, a vehicle owner can simply look at the reservoir to observe its level and color. Older vehicles may not have a reservoir. This requires the owner to remove the pressurized radiator cap when the engine is off and cold to inspect the fluid level and color. A milky, gray or dark color is apparent if the coolant has leaked into the combustion chambers and missed with the motor oil. The odd color in the coolant is diluted oil. By mixing the two liquids, they become diluted and fail to do their job, which is for the coolant to cool the engine and the oil to lubricate the parts.

Accompanying Symptoms

The casual car owner who rarely looks under the hood of his car may not notice the coolant’s color. However, other warning signs become apparent if the head gasket needs attention before severe damage occurs. Perhaps the most obvious symptom is the engine overheating. The coolant can’t do its job if it’s mixing with oil and its cooling properties are compromised. The exhaust exhibiting a gray or white color is another sign of coolant leaking into the combustion chambers. The dashboard’s temperature gauge needle will rise above the mid-level mark. Most passenger cars and light-duty pickup trucks operate with the temperature gauge needle at the mid-level more or a fraction above it. An overheating engine will send the needle well above its standard position, if not pegged at the highest point. Leaking coolant is another symptom. The engine also may be frequently stalling, stuttering or jolt. This is usually a sign of abnormal combustion and the engine is close to failure.

Bottom Line

By the time an automobile owner observes a milky, gray or chocolatey color in the coolant, the odds are the head gasket has already lost its seal and the engine suffered serious damage. A foreign liquid may have been introduced into the coolant, causing the change in color and may not affect the engine’s cooling, but this is unlikely.

About the Author

Rob Wagner is a journalist with over 35 years experience reporting and editing for newspapers and magazines. His experience ranges from legal affairs reporting to covering the Middle East. He served stints as a newspaper and magazine editor in Saudi Arabia and the United Arab Emirates. Wagner attended California State University, Los Angeles, and has a degree in journalism.

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