Should I fix it?

Can you drive with a blown head gasket?

Usually, yes, for a while. The honest answer from a technician who has seen more than 1,100 of them is: as little as you possibly can, because every mile costs you options.

What happens every mile you drive

The pistons push 100 to 200 psi or more against the head gasket on every stroke. Once there is a breach, that pressure erodes it a little more each time the engine runs. The small mist of coolant out the tail pipe becomes a cloud, the quart per 100 miles becomes more, and the misfire gets worse. Eventually the engine will not start.

That last step matters more than people realize. A chemical repair needs the engine to start and run for at least 15 minutes. The day it stops starting, the least expensive repair on the table is gone, and you are back to a $2,000 to $7,000 replacement or the salvage yard.

When you should not drive it at all

• It is overheating. Coolant leaving as steam drops the level, and low coolant overheats any engine. Heat is what warps heads and cracks blocks. If the gauge climbs, stop.
• You cannot keep coolant in it. Dave's yardstick: under a quart per 100 miles is not a major breach. Well past that, the breach is large, and every mile makes it larger.
• It will not turn over. It may be hydrolocked: coolant has filled a cylinder and the piston cannot move. Do not keep cranking. Pull the spark plugs and turn the key so the coolant can come out, then get it looked at.

If you have to drive it

1. Check the coolant level before every trip and top it up. Watch how much you add per 100 miles; under a quart is not a major breach.
2. Keep trips short and watch the temperature gauge. Any climb toward hot, pull over and shut it off.
3. Do not add a pour in sealer that mixes with antifreeze and hope. In Robert Hooper's words, no product that is compatible with antifreeze is capable of repairing your vehicle, and thousands of cars are lost every year that way.
4. Get the diagnosis confirmed. A parts store will lend you a block tester and the fluid is about $10: How to do a block test.

The mistake that turns one problem into two

Most head gaskets blow because a cooling system part failed first: a stuck thermostat, a weak water pump, a clogged radiator, a fan that does not come on. If that part is still failing, every mile you drive is overheating the engine again. Find the cause before you fix the gasket, or you will be fixing the gasket twice.

How long does the repair take?

Once an engine qualifies, the repair itself is measured in hours, not days. Dave's own line on a Toyota: "we can repair a Toyota in about an hour to an hour and a half." The cure finishes over the next three days as you drive normally and bring the engine up to operating temperature at least four times. Details: Can a blown head gasket be fixed without replacing it?