What causes water out of the exhaust pipe?

I have a friend whose car is spitting some water out of the exhaust. Someone told him he has a bad head gasket, but I’m skeptical. I’m no mechanic, but I’ve seen plenty of cars do this. Can they all have blown head gaskets??

Thanks, Lee

Gasoline is made up of carbon and hydrogen. When you burn it, it combines with oxygen to form carbon dioxide and water. In the combustion chamber, that water is a gas (steam). If the exhaust system is cold, some of the water will condense to form liquid water, and that’s what you see coming from the exhaust.

This condensation is also why short trips are tough on cars. Moisture in the exhaust system will evaporate once the whole thing is up to operating temperature (~15 minutes) but if it never warms up fully, the system rusts from the inside out. Usually the farthest bits back are the first to go.

Note that this condensation also happens inside the engine, with the same negative result, although on a somewhat more expensive scale. If you’ve ever opened your oil filler and seen a white sludge, it’s most likely caused by short trips or the engine not warming up.

All that said, if you are seeing steam or white smoke coming out of the tailpipe after the car is warmed up, start saving (and hurry) for a new head gasket.


What Desmostylus said, and

C8H18 + 12.5O2 —> 8CO2 + 9H2O

When cold, water condensation on the tailpipe is a pretty good indication of an efficient engine.


When cold, water condensation on the tailpipe is a pretty good indication that the engine is in fact running.

Or, I could be missing your point entirely. What do you mean?

Is it water or is it steam?

Thanks. I told 'em what I learned here and of course we had a big argument. Several people said their cars never do this. My theory is that the shape of some exhaust systems traps water and it just burns off when the piupe gets hot. Am I all wet?

Several people have apparently spent untold hours wandering back to their car’s tailpipes to ensure that they never suffer from condensation drip. I’m gonna go out on a limb and guess that they’ve also never parked on a hill or some incline or started their car when it was particularly cold outside.

Your theory seems valid.

Blown head gasket symptoms.

Other symptoms.

It seems like these days you’re far more likely to get exhaust gasses in your coolant than coolant in your oil. Either way, it tastes expensive!

No, some cars drip and some don’t. In cold weather, it’s not uncommon to see moderate amounts of water vapor coming out of tailpipes, but again the amount varies from car to car.

In terms of what comes out the tailpipe, the archetypal symptom of a blown head gasket is great billowy clouds of white (not bluish or grayish or black) “smoke” (it’s actually water vapor). It’s in significantly greater volume than normal exhaust moisture, and usually has a “hot anitfreeze” odor. And there has to be a corresponding loss of liquid from the cooling system. If the levels in the radiator AND the overflow jar (gotta check 'em both) are not declining, the water coming out the tailpipe is not from a head gasket problem.

Gary, I always thought if the overflow jug was OK the radiator level was OK. Have I just learned somthing new? :eek:

Yes, Jake. The level in the overflow jug is a good indicator if the system has no leaks. But if there is leakage, it’s not a reliable indicator. Some leaks will cause the overflow level to go down when the system sucks coolant out of it to replace what leaked out, but some leaks instead suck air in through the leak, which can leave a low level in the radiator and the engine, while the level in the overflow appears OK. If you want to be sure the system is full, you need to check inside the radiator (cold engine only, of course).

One point of clarification: my above post applies to systems with a reservoir external to the pressure cap (which is usually, but not always, on the radiator). Reservoirs open to atmospheric pressure are properly called overflow jugs.

There are similar-looking reservoirs that are under cooling system pressure, in fact all the ones I can think of have the pressure cap on the reservoir. While it seems that everyone calls them overflow jugs (which they resemble), that is not the correct term for them. Reservoirs that contain system pressure are surge tanks. Since the surge tank is in effect an extension of the radiator, the coolant level in it is a reliable indicator of system level.

My understanding is once the car had been driven, the exhaust cools, then mositure in the air will condensate inside the exhaust leaving small droplets at the top of the exhaust box inside, this will drip off or run down and collect and becasue the box is warmer inside even once engine is cool, moisture in air will still condensate inside causing the white steam in the morning.

If you live in an area with alot of moisture in the air or high humidity , this will always happen and especially if you are parking outside and not in a enclosed garage.

Why dont you just plug the tail piece after you parked , this will prevent moisture from been attracted ino the tail box while its cooling and no more rotting exhaust boxes for a long time, well much longer than if you didnt block it?

Or buy a stainless steel exhaust box to solve it properly.
wherever you get heat , you will get moisture.


This is an ancient thread, but since you came here to post bogus information, I just thought I’d correct you.

Read all the previous posts - they have the right answer. The water comes from exhaust stream, due to combustion products.

Read my first line, to my understanding.!
Im not forcing my thoughts or theory on you or anyone.
And a stainless steel exhaust system wont rot!:slight_smile:

Im going to block off my tail piece after a drive and parked cos i get moisture every morning out exhaust, just gonna try for the hell of it and see if the steam is less in the morning or goes away. What the hell, just try it even if its fruitless and doesnt work.

A potato is perfect for blocking the exhaust pipe.:smiley: