A crucial multi layered metal and/or graphite composite that makes a water-tight seal between two halves of an engine, the head gasket’s job is two-fold: to seal combustion chambers that reach pressures in excess of 200 pounds per square inch and extreme temperatures of nearly 3,000 degrees Fahrenheit; and to separate coolant passages that hold almost 20 pounds per square inch of pressure (with temperatures over 220 degrees) and oil passages that typically run 30-60 pounds per square inch. If the head gasket leaks from one passage into another even a smidgeon, it is toast and must be replaced immediately; and the engine will soon be toast, too, if it’s run with a leaky head gasket for very long!
The most common head gasket failure: coolant can get sucked improperly into the combustion chamber when the piston is on its down stroke pulling in gas and air in preparation for the combustion process. With a leaky head gasket, when the piston comes back up to Top Dead Center (TDC), the fuel ignites from the spark, burning the misplaced coolant at the same time. This is what causes excessive white smoke to come from the tail pipe. When there’s an excess, the vehicle runs rough; too much more, and the engine can stop in its tracks as the result of hydro-lock. But that’s not all that can happen. A connecting rod may be bent, causing internal engine failure. When the piston comes to TDC, it pushes 200 lbs of pressure into the cooling system, causing it to boil over from excessive air pressure and boil out due to the extreme heat from the combustion chamber.
With a blown head gasket, oil can run into the coolant system and vice versa. The mixing and sloshing of the two creates a ‘chocolate milkshake’-looking effect. This is bad! The coolant in the oil system will ruin the bearings in the engine, sometimes immediately. Left unaddressed, the cost to repair or replace this kind of engine damage goes well beyond simple head gasket replacement.
Because of the extreme conditions this gasket is under every time you turn on your engine, it’s just a matter of time before it will fail.
Common failures are the result of high mileage, old age, failure to maintain the cooling system by changing your antifreeze regularly, running the engine hard, and engine or gasket design flaws. (Subaru is infamous for gasket design flaws.)
Early warning signs of a failing head gasket include overheating, loss of coolant, excessive white smoke from the tail pipe, or a rough-running engine when you start the vehicle, especially if it’s been sitting overnight. The rough running will generally last for just a few seconds. On engines with a severely blown head gasket, the engine will run rough all the time, coolant may spew from the radiator, and the engine may hydro-lock.
Again, a leaking head gasket will often leak engine coolant into the oil, causing a distinctive ‘chocolate milk’-like appearance that you can recognize when checking or changing your oil. The oil level might be higher than normal, too.
Failed head gaskets can emit a white, sweet-smelling smoke from the tail pipe. If you suspect that your head gasket is leaking, get it in right away. Do not delay!