Does white smoke from exhaust mean a blown head gasket?

Does white smoke from exhaust mean a blown head gasket?

by Rick Muscoplat

In auto forums, posters often ask if white smoke from the exhaust is a sign of a bad head gasket. It can be, but it depends on when you see the smoke and how much of it you see. Here’s how to dig in a little deeper.

White Smoke From The Exhaust at startup is normal

As your engine burns gasoline, the combustion process creates unburned hydrocarbons (HC), water vapor, carbon dioxide (CO2), carbon monoxide (CO), oxygen (O2), and Nitric oxide (NO), Nitrogen dioxide (NO2), Nitrous oxide (N2),  and Nitrogen peroxide (NO5). As the hot exhaust exits the tailpipe, the hot water vapor condenses in the cooler outside air, appearing as white smoke. The colder the outside air, the more noticeable the white smoke.

More white smoke from the exhaust when the engine is cold

A cold engine requires a richer air/fuel mixture to overcome the “quenching” effect of the cold metal in the combustion chamber. Since combustion in a cold engine is not as efficient, the exhaust contains a higher concentration of unburned fuel and oxygen, resulting in a more noticeable emission of white smoke. Then, as the engine heats up and the vehicle computer reduces the amount of fuel, which reduces the amount of water vapor produced.

The catalytic converter’s effect on white smoke from the exhaust

The catalytic converter’s job is to convert 2CO + O2 into 2CO2 and to convert 2NO into N2 + O2 and convert 2NO2 into N2 + 2O2. It does this by storing excess oxygen and using that oxygen for the conversion process. For more information on how a catalytic converter works, see this post.

The catalytic reaction process requires heat, so the catalytic converter isn’t really working right away at cold startup. The hot exhaust gasses begin to heat up the catalytic converter until it reaches around 500°F. At that point, called “light off,” the conversion process begins, generating even more heat. When fully operational, a properly running catalytic converter is about 1,200°F so it emits less H2O. In other words, you’ll see far less white smoke coming from the tailpipe as the engine and the catalytic converter heat up.

What about blue smoke?

Leaking valve stem seals, worn valve stem guides, and worn piston rings allow more oil to enter the combustion chamber. When there’s excessive oil in the combustion chamber, the unburned oil creates blue smoke. In the old days, before catalytic converters, it was common to see blue smoke coming from the exhaust from a worn engine. However, once a catalytic converter reaches its operating temperature of around 1,200°F, that excess oil (HC) gets burned off. So you may see a brief period of blue smoke on cold startup, but that blue smoke will stop once the catalytic converter reaches operating temperature.

What else causes white smoke from the exhaust

Head gasket failure causes white smoke from the exhaust

If an engine head gasket fails, and the failure point is between a coolant passage and the combustion chamber, the coolant can get sucked into the combustion chamber on the intake stroke and burned during the power stroke. Coolant in the combustion chamber creates excessive amounts of H2O and polyethylene glycol in the exhaust stream; so much, that the catalytic converter can’t treat it all. So you see quite a bit of white smoke from the exhaust, even if the engine is up to operating temperature.

Turbocharger failure can cause white smoke from the exhaust

Turbochargers can run at speeds up to 300,000 RPM, so they require a constant flow of engine coolant to keep them cool. Engine coolant temperature is around 200°F, while turbochargers can run around $1,750°F, so engine coolant can really drop the temperature at the turbocharger’s bearing. However, if the turbocharger’s seals fail, it can inject engine coolant directly into the intake manifold, causing a massive amount of coolant to enter the combustion chamber, resulting in white smoke from the exhaust.

©, 2021 Rick Muscoplat