See full version: Why Does the Outboard Coolant Stream Sometimes Steam
The steam you notice is a normal occurrence on most outboard motors. The exhaust gases exiting the engine block may reach the 1,200-1,500-degrees F range when under heavy load. They are directed down an exhaust chamber inside the midsection and out the propeller hub. Some of the cooling water from the engine is directed to quiet and cool the exhaust gases as well as surrounding the exhaust chamber, further silencing the motor. more
At higher speeds the exhaust chamber becomes very hot internally and the cooling water injected into it turns to steam and is expelled through the propeller hub while the boat is underway. When you slow down by pulling the throttle back, the pressure and the volume of exhaust is greatly reduced, yet high temperatures remain inside the chamber and the steam then exits through the idle relief port. more
The whitish gray vapor that you see coming out of the back of the motor is perfectly normal. As the motor idles, the exhaust gas temperatures subside and the visible steam practically disappears. In cooler humid weather, you may notice it more, just like out of the exhaust pipe of your vehicle on a cold day. [links]
A BoatTEST member recently asked about the stream of water coming out from the rear of this engine. “Why does the water streaming out of outboard steam when I slowdown and come to a stop, but when I start again, it goes away?” [links]
Petrol when burnt produces co2 and h2o. here
"more" or "less" visible depends on many factors I suppose but from the above you can work out which may apply.
here
Petrol when burnt produces co2 and h2o. more
After 10 minutes it will literally drip out of the exhaust pipes.
When i clicked on this thread i was thinking of my old S Type R
If white steam is accompanied by a sweet or pungent smell it means the engine is burning coolant. This can happen by one for four ways the most popular failures is a blown head gasket which can be tested for. Additional causes include a cracked cylinder head or engine block or the intake manifold gasket failure if it's used to distribute coolant into the cylinder head (engine designs vary). Many times white smoke can begin after the engine has been overheated. here
Combustion chamber coolant leaks can be tricky because the repairs can overlap and it's difficult to know what the problem is before disassembly in most cases. Example: A repair shop has determined coolant is entering the engine combustion chamber as they disassemble the engine they discover the intake manifold gasket failed. It's up to the honesty of the repair shop to alert the customer the repair will be less than quoted. here
There are different levels of white smoke/steam which could be a normal occurrence. A small amount of water is produced as a normal bi product of the combustion process and until the exhaust system warms up the water evaporates and can be seen as steam. When an engine heats up under normal driving conditions the exhaust system will get hot as well. After the engine is shut off and allowed to cool the air inside the exhaust system becomes cooler as well which draws in moisture/water. This is normal and is why exhaust systems have small holes at the rear of the muffler to allow water to drip out. When the engine is again started the exhaust system will heat up evaporating the water causing steam. In cold weather a moderate amount of white smoke/steam the first 10 to 15 minutes of run time is not unusual. The image above shows an abnormal amount of steam which indicates a problem. more
Or the opposite can happen, example: A repair shop has estimated a blown head gasket replacement for $1200.00 (US). After disassembly they inform you the head gasket is okay and need to send the cylinder head out for pressure testing. The tests come back negative leaving along the engine block as the failure which means it's time for a new engine.
The worst case scenario is that your entire engine block has a crack in it. If this is indeed the culprit, be prepared for an expensive and time-consuming replacement. You’d most likely require professional service in this case.
When you encounter white smoke from exhaust, it’s important to note that this means different sources of problems in gas- and diesel-powered engines.
There are three possible methods for repairing a cracked engine block, that is using a cold-metal patch over the crack, cold-metal stitching it shut, or re-welding the crack. These require the work of a professional. Regardless of which route you opt for, it is certainly not cheap. The labor for an engine block repair could be anywhere from 12 to 35 hours, depending on the make and model of your vehicle, as for some models it’s much harder to get to the engine block and disassemble it. This can set you off between $2,500-$4,000 for a temporary fix. [links]
Thin white smoke on startup then disappears is usually a result of condensation build-up within the exhaust system. It tends to occur in the winter or cold mornings. The smoke should be thin, not too visible and only be coming out in small amounts upon startup, and will burn out fairly quickly after your engine warms up, particularly after about 30 seconds to a minute. It is not a sign of trouble, but if it occurs in conjunction with other occurrences of reduced engine performance or difficult starting, it may indicate a more serious issue within your engine.