Is it my head gasket?

How to tell if your head gasket is blown, and how to test it for about $10

You do not need a lift or a scan tool. Four questions you can answer in your driveway, one test you can borrow from a parts store, and you will know more than most of the people who have been guessing for you.

Four questions before any test

1. Did it overheat? Most head gaskets blow because a cooling system part failed first: a stuck thermostat, a weak water pump, a clogged radiator, a fan that did not come on. If it overheated, find out why, because that part is still in the car.
2. Are you adding coolant? A blown head gasket always loses coolant. If you are not adding any, ask yourself why you think it is the head gasket.
3. Is there white smoke or water from the tail pipe? That smoke is steam, coolant hitting the heat in a cylinder. It can be a cloud or a mist you only notice when you rev a cold engine.
4. Is there coolant in the oil? Pull the oil fill cap. A milky coffee look means coolant. Only about 1 in 10 blown head gaskets show this, so clean oil does not clear you.

If the engine has not overheated, is not losing coolant, has no steam from the tail pipe and no water in the oil, it might not be a blown head gasket. That is worth knowing before anyone sells you a repair.

The block test: about $10 and a few minutes

A block test checks whether combustion gases are getting into your cooling system, which is exactly what a breached head gasket lets happen. A tester sits on the radiator or expansion tank with a blue test fluid in it; exhaust gas pulled through the fluid turns it yellow.

• Auto parts stores such as O'Reilly will usually lend you the tester. You buy the test fluid, about $10.
• A shop will do the same test for around $35 to $50.
• A compression test costs somewhere near $100 or more and tells you which cylinder is affected.

Watch Dave do it start to finish: How to do a block test.

Somebody told you it is the head gasket. Did they test it?

The most common way people arrive at "blown head gasket" is that a mechanic, a neighbor or a brother in law said so. Ask one question: did they do a block test or a compression test? If yes, you probably do have a blown head gasket. If no, the engine may have been misdiagnosed, and a $10 test is cheaper than any repair based on a guess.

How bad is it? Three quick measures

Coolant per 100 miles

Guess how much you would add after 100 miles. A cup or a quart is not a major breach. More than that means a bigger breach, and every breach grows with every mile you drive on it.

The dipstick

If the oil cap is milky, check the dipstick. At the full mark, you simply change the oil after the repair. An inch or two over, you should be alright. Five inches or more over the mark can mean other damage, possibly a cracked block, so get tech support on the phone before you do anything.

Does it start and run?

An engine with a blown head gasket usually starts and runs rough, because coolant in a cylinder causes a misfire. If it turns over but will not start, check the battery first; a jump sometimes gives it enough to catch. If it will not turn over at all, it may be hydrolocked: coolant has filled a cylinder and the piston cannot move. Do not keep cranking it. Pull the spark plugs and turn the key, and the coolant will come out of the plug holes.

What the test result means for you

A positive block test on an engine that starts and runs, with clean oil or only a little coolant in it, is the profile Thermalweld was built for. A positive test on an engine that will not start, or shows five inches of coolant on the dipstick, is the kind we turn away. The qualifier below sorts that out in about a minute.