Engine overheating is often the first sign of trouble when a head gasket is leaking. If the gasket loses its ability to seal the cylinder head to the block, coolant can start to weep past the gasket and into the cylinder. the gradual loss of coolant can pass unnoticed for many miles, but eventually the loss of coolant will cause the engine to overheat.
A leaky head gasket is bad news for several reasons: Besides causing the engine to lose coolant and overheat, antifreeze entering the cylinder can wash away the oil that protects the rings and pistons against wear. Antifreeze in the crankcase can also dilute and contaminate the oil, which may lead to bearing wear and failure too.
You can look for several telltale signs that would indicate your engine's head gasket is leaking:
A low coolant level. If the coolant level inside the coolant reservoir or radiator is low, and you can see with no visible leaks on the outside of the engine, radiator, water pump, freeze plugs, hoses or other cooling system components, coolant may be leaking past the head gasket into a cylinder.
Coolant in the crankcase. Check the oil dipstick. Is the level higher than normal? This could be caused by coolant leaking down through a cylinder into the crankcase. When antifreeze mixes with oil, it often gives the oil a milky, yellowish or brownish appearance that can seen on the dipstick.
White smoke in the exhaust. All engines produce a small amount of visible steam in the exhaust when they are first started on a cold morning, but if you see white smoke when the engine is running at normal temperature (and it is not below freezing outside), there could be coolant in the exhaust from a leaky head gasket. The exhaust may also have a sweet smell.
Coolant leaking into a cylinder may also foul the spark plug, causing that cylinder to misfire. On 1996 and newer vehicles with OBD II, this may set a P030X series misfire code and turn on the Check Engine light. If you remove the spark plug in the misfiring cylinder, and the electrodes are fouled with deposits (various colors but not black), the head gasket is probably leaking coolant into the cylinder.
Head gasket leaks can also be confirmed by pressure testing the cooling system. This requires a cooling system pressure tester. The hand held pump and adapter are attached to the radiator filler neck or pressurized coolant reservoir opening. Air is pumped into the system until the pump gauge shows 8 to 15 PSI (apply only as much pressure as the radiator cap is rated for). A good cooling system that is not leaking coolant should hold its rated pressure for 5 minutes or longer with no drop in the pressure reading. However, if the head gasket is leaking, the pressure reading will usually start to drop almost immediately.
NOTE: An internal coolant leak doesn't always mean the head gasket is leaking. Similar leaks can also be caused by hairline cracks in the cylinder head or engine block.
Another method for finding a leaky head gasket is to do a Leak Down Test. Remove the spark plug from the cylinder that is misfiring and pressurize the cylinder with air. If the air leaks into the cooling system, the head gasket is leaking.
There are also special Combustion Leak Detection kits that use a test fluid to find head gasket leaks. Fumes from the radiator are routed into a bottle that contains the test fluid. If the color of the fluid changes from blue to yellow, it indicates a combustion leak.
The least expensive way to fix a leaky head gasket is to add a bottle of cooling system or head gasket sealer to the cooling system. There are many such products available in auto parts stores, and most will temporarily stop or slow a coolant leak. Some products claim to offer a permanent repair. That's a hard claim to prove. At best, the sealer will stop the leak indefinitely (or until you can sell or trade your vehicle). At worst, such products may not completely seal the leak and your engine will still overheat. If this is the case, the only way to fix the problem is to replace the leaky head gasket.
When adding a sealer to the cooling system, follow the instructions on the product to the letter. Most say you should drive your vehicle for 20 to 30 minutes so the sealer can circulate through the engine and find the leak. Most of these products contain chemicals that solidify when exposed to air or combustion gases.
If a sealer won't stop a head gasket coolant leak, your only other repair option is to replace the head gasket. This is a labor-intensive and expensive job if you have it done by a repair shop or car dealer. A typical head gasket repair job can easily cost up to several thousand dollars depending on the engine. The cylinder heads have to be removed so the new gasket(s) can be installed.
If you want to replace the head gasket yourself, get a shop manual or buy a subscription to an online service information data provider (such as AlldataDIY) so you can follow the step-by-step procedures for replacing the head gasket on your engine.
The general procedure for replacing a head gasket usually goes as follows:
Disconnect the battery.
Drain the cooling system.
Remove all of the plumbing, wiring and intake manifold from the top of the engine. Mark everything so you remember where it goes!
Disconnect the exhaust manifold from the cylinder head.
Remove the front engine cover on OHC engines and remove the timing belt or chain. Note the timing marks so the belt or chain can be reinstalled correctly later.
Remove the valve cover from the cylinder head.
If the engine is a pushrod engine, loosen or remove the rocker arms, and remove the pushrods.
Loosen and remove all of the head bolts. Discard the old head bolts and replace with new if the head bolts are one-time use torque-to-yield (TTY) head bolts. Ordinary head bolts can be reused if not damaged.
Remove the cylinder head from the engine. If the head fails to budge, you may have missed one or more head bolts.
Clean and inspect the mating surfaces on the engine deck and cylinder head.
If the head or deck surfaces are not smooth and flat, have one or both resurfaced as needed by a machine shop.