Is it my head gasket?

Blown head gasket symptoms: the four things a technician checks first

Dave Whitmer has repaired more than 1,100 blown head gaskets in driveways, on video. Before he pours anything into an engine he asks about four things. If none of the four is true, it may not be a blown head gasket at all.

Sign 1: you keep adding coolant

Every blown head gasket loses coolant. If your radiator or expansion tank keeps dropping and you cannot find a puddle, a wet hose or a leaking radiator, the coolant is most likely leaving through the tail pipe as a mist.

How much matters. Guess how much you would add after 100 miles: a cup, a quart, half a gallon? Anything under a quart per 100 miles is not a major breach in the gasket. More than that means a larger breach, and every breach grows with every mile you drive on it.

Sign 2: white smoke from the tail pipe

Coolant that gets past the head gasket into a cylinder turns to steam the moment it meets the heat, like throwing water on a hot iron, and leaves through the exhaust. That is the white smoke people mean when they say blown head gasket.

Not every engine makes a cloud. Often it comes out as a fine mist that you cannot see going 60 mph down the road, and you cannot exactly watch your own tail pipe at 60 mph. If you are losing coolant and see no smoke, it is probably still going out the tail pipe as mist. One way to make it easier to see: rev the engine when it is cold, right after you first start it.

Read more: White smoke from the exhaust and losing coolant with no leak.

Sign 3: coolant in the oil

This is the symptom everyone knows and the one that shows up least. In Dave's experience coolant gets into the oil in at most 1 in 10 blown head gaskets. Most never show it, so oil that looks normal does not rule out a blown head gasket.

The check takes ten seconds. Pull the oil fill cap. If there is coolant in the oil the cap has a milky coffee look. If it just looks like oil, you do not have coolant in the oil.

If the cap is milky, look at the dipstick next. At the full mark, you just change the oil after the repair. An inch or two over the full mark, you should still be alright. Five inches or more over the mark points to a bigger problem, possibly a cracked block, so talk to tech support before you spend anything.

Read more: Milky oil under the oil cap.

Sign 4: rough running, misfire, check engine light

With a blown head gasket, coolant is getting into at least one cylinder, where the piston and spark plug are. Gas and water do not mix, so that cylinder misfires and the engine runs rough, and a check engine light may come on. It still has to start and run. An engine that turns over but will not start, or will not turn over at all, is a different conversation, covered on the how to test page.

What about overheating?

Overheating is usually the cause of a blown head gasket, not the symptom. Most head gaskets blow because something in the cooling system failed first: a stuck thermostat, a weak water pump, a clogged radiator, or a fan that does not come on when it should. On a high mileage engine, over 150,000 miles, a head gasket can also just wear out from 100 to 200 psi or more of pressure from the pistons, with no overheating at all.

Exhaust getting into the cooling system does not usually make an engine overheat. There are two exceptions. If so much exhaust builds up that an air pocket forms around the thermostat, hot coolant never touches it, it never opens, and the engine overheats. And if coolant is leaving as steam out the tail pipe, the level drops, and low coolant overheats any engine.

Bubbles in the coolant reservoir

Bubbles rising in the reservoir with the engine running are combustion gases being pushed into the cooling system by the pistons. That is the same thing a block test looks for, which is why the block test is the cheapest way to settle the question.

Not sure? Do the $10 test

If a mechanic, a neighbor or a brother in law said blown head gasket without a block test or a compression test, question the diagnosis before you spend money on it. Most auto parts stores will lend you a block tester; the test fluid costs about $10. A shop charges around $35 to $50 for a block test, and a compression test runs near $100 or more.

Dave shows the whole thing on video: How to do a block test.