Rick's Free Auto Repair Advice

Blown Head Gasket Symptoms

Blown head gasket symptoms, testing and replacement cost

Head gaskets can fail for many reasons; warped cylinder head from engine overheating, crack in the cylinder head, gasket degradation due to coolant neglect, prolonged periods of detonation, engine misfires, and over-boost on turbo equipped engines. There are several methods to diagnose a blown head gasket and I’ll detail them here. I’ll also discuss head gasket replacement costs and head gasket sealers. First, let’s discuss the symptoms of a failed head gasket

Symptoms of blown head gasket

White smoke coming from the exhaust indicating a gasket breach between the cylinder and the cooling passage
Coolant in the oil indicating a gasket breach between a cooling passage and an oil passage.
Coolant in the oil often causes the oil to turn a milky brown color, almost like a milkshake.
Oil in the radiator and coolant reservoir indicating a gasket breach between an oil passage and a cooling passage or a cylinder and oil passage
Over pressurization indicating a gasket breach between the cylinder and coolant passage where combustion gasses are pushed into the cooling system. This causes bubbling in the coolant reservoir and causes the radiator hoses to become hard and coolant to vent from the reservoir
Overheating indicating that hot combustion gasses from the breached head gasket are superheating the coolant

head gasket leak

Typical head gasket

Before jumping to the conclusion that your engine has a blown head gasket, ask yourself these questions:

Does your engine have a turbocharger and oil cooler?

Because modern turbochargers run at up to 300,000 RPM and are in constant contact with hot exhaust, car makers circulate coolant around the bearings. In addition to coolant, the bearings are lubricated with engine oil. If the seals fail, coolant can enter the combustion chamber, where it burns and creates white smoke in the exhaust.

Turbocharged engines also have an oil cooler to remove the high heat. The engine oil cooler receives coolant from the radiator and circulates the coolant around an oil radiator. If you’ve neglected coolant maintenance, the oil radiator can corrode and coolant can enter the oil and vice versa.

Does your vehicle have an automatic transmission?

Automatic transmissions generate a lot of heat and that heat gets removed by way of a transmission cooler located inside the radiator. If you’ve neglected coolant changes, corrosion can eat through the transmission cooler resulting in transmission fluid entering the coolant and coolant entering the transmission.

Head gasket failure modes

A head gasket can fail between a coolant and an oil passage. The result? Coolant in the coil or oil in the coolant.

The gasket can fail between a coolant passage and a cylinder. The result? Overheating and over-pressurization in the cooling system.

A gasket can fail between two cylinders. The result? Rough idle due to loss of compression. High fuel consumption due to lower compression. Low power due to low compression.

Diagnose possible head gasket failure

Test for exhaust gas in the coolant

If you have over-pressurization and/or overheating, you may have a head gasket breach between a cylinder and a coolant passage. Each compression cycle pumps exhaust into the cooling system causing it to over-pressurize and overheat.

Perform a chemical test on the coolant reservoir. combustion gas tester Purchase a combustion gas test kit from any auto parts store. Start the engine and let it heat up. Fill the tester with the test fluid and apply the suction bulb to the tester. Place the tester in the coolant reservoir and suck air (not liquid coolant) into the tester. If the blue fluid turns yellow, combustion gasses are in the coolant.

To isolate the actual cylinder that’s leaking, disconnect either the spark plug or the fuel injector for a cylinder. Start the engine and run it for a few minutes to purge the disabled cylinder. Perform the combustion gas test again. If the fluid turns yellow, the disconnected cylinder is NOT the cause of the exhaust leak. Move on to each additional cylinder, making sure you run the engine between each test to purge the cylinders. If the fluid remains blue during one of the disabled cylinder tests, that confirms the disabled cylinder is the cause of the leak because it is not producing exhaust gas.

combustion gas test

Test for coolant in the cylinder

Remove the spark plugs and keep them in order. Examine the removed spark plugs for signs of coolant staining or wetness. If you don’t see either, drape a clean white cloth over the spark plug holes and crank the engine. If coolant is in the cylinder the cranking operation will spit the coolant onto the cloth.

Perform a compression test

A low compression reading between two adjoining cylinders is a sign of a gasket failure between the cylinders.

Does head gasket sealer work?

Head gasket sealer works less than 50% of the time. First, it only works when you have a gasket breach between a coolant passage and an oil passage OR a coolant passage and a cylinder. It does not work if the gasket failure is between two cylinders or if the cylinder heat is warped due to engine overheating.

Modern head gasket sealers work with heat to melt the sealing material out of suspension at the hottest spots, so they rarely work with leaks between a cooling passage and an oil passage. If you have coolant in the oil or oil in the coolant, your chances of fixing the problem with head gasket sealer are next to 0%.

However, if you have a very small leak between a cylinder and a cooling passage, gasket sealer may work. Before you spend money on a sealer, read the directions online. Some formulas require you to run the engine for at least 20-minutes for the product to work. If your cooling system won’t hold pressure for 20-minutes, don’t even try the product. Also, if the leak is so bad that you can’t keep the engine running without causing engine damage, don’t bother.

Some leak sealers require you to flush the coolant and refill with water before adding the sealer. Draining the radiator won’t remove the bulk of the coolant. If you can’t drain the entire engine, don’t even try those products.

Other sealers require you to fill the system with water, add the sealer, run the engine and then drain the engine and leave the cooling system open to the air for 24-hours. Finally, some sealers work simply by adding it to your existing coolant and then running the engine. In my experience, those products work best — when they work at all.

Head gasket replacement cost

Head gasket replacement is costly because the tech must drain the cooling system and remove the intake and exhaust manifolds before they can remove the cylinder head. In some cases on low mileage engines, the tech can simply remove the cylinder head and swap in the new gasket. But in other cases, the shop must send the cylinder head to a machine shop for milling and the tech must prepare the block surface to receive the new gasket.

The typical head gasket replacement cost for a 4-cylinder engine is around $1,500. The replacement cost for a V-6 or V-8 engine is around $2,400.

©, 2020 Rick Muscoplat

Posted on by Rick Muscoplat


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