Is it my head gasket?

White smoke from the exhaust and losing coolant with no leak: what it means

Two of the most common searches a worried driver makes, and they are usually the same problem seen from two sides. The coolant you cannot find is the smoke you may not be able to see.

White smoke from the tail pipe is steam

When a head gasket is breached, coolant gets into a cylinder, where the piston and spark plug are. It hits the heat and flashes to steam, like throwing water on a hot iron, and leaves through the exhaust. That is the white smoke. In a bad case you can see water dripping out of the tail pipe too.

A little vapor on a cold morning that disappears within a minute or two of driving is normal condensation, not a head gasket. Steam that keeps coming after the engine is warm, or gets thicker when you rev it, is the one to worry about.

Losing coolant but no leak: where it goes

A blown head gasket always loses coolant. If you keep topping up the radiator or expansion tank and there is no puddle under the car, no wet hose and no crusty residue around the radiator, the coolant is most likely going out the tail pipe as a fine mist.

Sometimes there is not a lot of smoke. It comes out as a mist that you cannot see going 60 mph down the road, and it is hard to look at your own tail pipe at 60 mph. Two ways to catch it: have someone watch the exhaust while you rev a cold engine right after starting it, and measure how much coolant you add. 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.

Engine overheating with white smoke

People assume the blown head gasket caused the overheating. It is usually the other way around. 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 at the right time. On a high mileage engine, over 150,000 miles, a gasket can also wear out on its own under 100 to 200 psi or more of pressure from the pistons.

Exhaust getting into the cooling system will not by itself make an engine run hot. There are two situations where a blown head gasket does cause overheating:

• So much exhaust builds up in the cooling system that an air pocket forms around the thermostat. Hot coolant never touches it, so it never opens, and the engine overheats.
• Coolant is leaving as steam out the tail pipe, the level drops, and a low coolant level overheats any engine.

Why the pressure cap matters

Water boils at 212 degrees F. Under pressure, the boiling point rises to about 250 degrees F, the same idea as a pressure cooker. A cooling system has to hold pressure to avoid boiling over. If the seal on the pressure cap is leaking, or its spring releases too early, that pressure is gone, exactly as it would be with a leaking radiator or hose. Before you blame the head gasket for a boil over, look at the cap.

What to do this week

1. Drive it as little as possible. Every mile, 100 to 200 psi of pressure from the pistons erodes the breach further. Eventually the engine will not start, and an engine that cannot start and run for 15 minutes cannot be repaired with Thermalweld either.
2. Do the block test. Borrow a tester from a parts store, buy about $10 of fluid, and you will know whether combustion gas is in your coolant. Dave shows it here: How to do a block test.
3. Check the oil cap. A milky coffee look means coolant in the oil. It shows up in at most 1 in 10 cases, so clean oil does not clear you, and a milky cap does not disqualify you. See milky oil under the oil cap.
4. Run the qualifier. Seven questions, sixty seconds, and you know whether a chemical repair is on the table for your engine.