August 31, 2021

By Thomas Probus, BG Automotive Research Specialist

You’ve likely heard the phrase “blown head gasket.” It sounds ominous. You might know a blown head gasket is an expensive and labor-intensive repair. It may even cause you to imagine dollar signs raining from the sky and piling up, but what exactly does a blown head gasket mean?

What a Head Gasket Does

A car engine consists of two primary parts: the cylinder head where the camshaft, spark plugs, and valves are found, and the engine block where the cylinders and pistons are located. The head gasket is situated between these two large engine components. The number of head gaskets is equal to the number of rows of cylinders in your engine; so, most commonly one or two.

The head gasket has a critical job. It seals in the combustion chamber pressure and prevents coolant and oil from entering the cylinders or leaking from the passageways they flow through. A head gasket must also withstand the strain of two opposing surfaces of the cylinder head and the engine block expanding, rubbing, and shrinking caused by the engine warming up to operating temperature and cooling down when not in use.

6 Warning Signs Your Head Gasket May Be Blown

A blown head gasket requires immediate attention before it does additional damage to the engine. Here are six signs your head gasket may be blown:

1. Engine Overheating

A head gasket failure may be caused by frequent or extreme engine overheating (as the result of a malfunctioning fan, clogged radiator, or coolant leak, etc.). A blown head gasket may also cause the engine to overheat. The flow of the oil and/or coolant is interrupted, and compressed gases can escape the cylinders. Thus, hot combustion gases can leak into the cooling system, or coolant can leak from the system. Either way, the end result is an overheated engine. 

If an overheated vehicle is driven, it can cause bearing damage, cylinder head warping, and cylinder scoring. In this circumstance, the engine will likely seize and result in the need for a costly, complete engine replacement.

2. Loss of Power

An engine is designed to maintain compression inside each cylinder for optimal efficiency and power. The head gasket may fail in such a way it allows the compressed air/fuel to escape from one or more cylinders. This loss of compression causes misfires, a rough-running engine, and a noticeable reduction in power.

3. Oil Contamination

One of the most common signs of head gasket failure is a milky sludge on the underside of the oil filler cap or on the dipstick caused by coolant getting into the oil. This is generally a good indicator of head gasket failure, and is a sure sign your engine needs to be checked by a professional automotive technician. 

When coolant contaminates the engine oil, driving can quickly ruin the engine’s moving parts. Repair often requires complete disassembly of the bottom end of the engine to inspect the bearings, and clear out all contaminated oil.

4. White Smoke

A blown head gasket frequently results in large clouds of white smoke coming from the exhaust. This is caused by coolant leaking around the gasket into the cylinders, where it is turned to steam as part of the combustion process. Less common, but also possible, is a leak from an oil passage to the cylinder which causes a blue-colored smoke.

Either of these gasket failures could allow combustion pressure into the cooling system, or oil system. If a radiator hose suddenly comes loose, or the engine oil dipstick won’t stay inserted, a blown head gasket could be the reason.

5. Coolant or Oil Leaks

Head gaskets seal passageways in the cylinder head and engine block that contain oil or coolant. So, one of the first things to look for would be external oil or coolant leaking between the engine block and cylinder head.

A more subtle sign of head gasket problems may be a slow loss of coolant with no visible leaks. In this case, the leak is internal, and the coolant can leak into the cylinders or the engine oil passages.

6. Cylinder Misfire

Another symptom of a blown head gasket is a cylinder misfire. If the gasket failure is between cylinders, compression pressure is lost. This problem will often cause a steady misfire and a significant loss of power. A cylinder misfire normally generates a check-engine light.

Prevent a Blown Head Gasket

The best way to prevent head gasket failure in your car is regularly scheduled preventive maintenance. This includes keeping the engine coolant at the proper level with the correct coolant to water ratio, and keeping engine oil clean and filled to the proper level. 

If you experience coolant leaks on the ground or garage floor, have it checked out. Also, make it a habit to keep an eye on your temperature gauge; if you notice it running hot, have it immediately checked out by your professional technician at your nearest BG shop!


by Tom Probus
Automotive Research Specialist, BG Proving Ground
ASE Master Technician
ASE Certified for more than 20 years

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