My car is overheating

Stop driving now

If the gauge is climbing toward the red, or you see steam, or you catch that sweet smell, pull over as soon as you safely can and shut the engine off. Do not keep driving to get home or to the shop. An engine that overheats for a few minutes can warp a head or blow a gasket. An engine that overheats for twenty more miles because you wanted to make it home can turn a $150 fix into a $3,000 one. Once it's stopped, leave the hood down for a few minutes before you open it. Do not open the radiator cap or the coolant reservoir cap while it's hot. Pressurized coolant at 200-plus degrees will burn you badly.

What usually causes this

Most overheating comes down to one of a handful of things. Low coolant is the most common, usually from a leak somewhere in the system: a hose, the radiator itself, the water pump, or a small crack in a plastic reservoir. A failed thermostat is next, one that's stuck closed and not letting coolant circulate. After that, a bad radiator fan or fan clutch, especially if the overheating happens at idle or in slow traffic but seems better on the highway. A failing water pump can also cause this, sometimes with a whining noise or coolant dripping underneath the front of the engine. Less common but more serious: a blown head gasket, which lets combustion gases into the cooling system or coolant into the cylinders.

How to tell them apart

A sweet smell, usually with steam, points to a coolant leak somewhere, often small and out of sight until it isn't. White smoke from the tailpipe, especially with a rough idle, points toward a head gasket problem, which is the one you really don't want to drive on at all. Overheating that only happens sitting still or crawling in traffic, but not on the freeway, points to the fan. Coolant that looks milky or has a film in it, or oil that looks milky on the dipstick, is another head gasket sign. If you can pop the hood safely once it's cooled and see coolant pooled or dripping, that tells you where to look. If you don't see anything obvious, don't guess, that's a job for a pressure test.

What the repair involves and roughly what it costs

A low coolant top-off with a small hose or clamp repair might run under $200. A thermostat replacement is usually a few hundred dollars in parts and labor. A radiator or water pump job runs higher, often somewhere in the $400 to $900 range depending on the vehicle, since a water pump often means draining and refilling the whole cooling system. A head gasket job is a much bigger repair, often into four figures, because it means pulling the head off the engine. We can't tell you which of these it is over the phone. What we can do is pressure test the system, check the fan operation, and look at the coolant and oil condition to narrow it down before anything gets taken apart. If your car is sitting on the shoulder right now, get it towed rather than driven in, even if it seems to have cooled off enough to start again.