Smoke out of an exhaust system in a 4 stroke engine is a bad sign nobody wants to see. In general, what it means is that said engine is burning something that it should not and we can identify what that is by the color of the smoke. Once this is figured out, it becomes easier to determine where the problem is at. Check this list for causes and fixes for when the smoke is white!
Be careful to not confuse the white vapor from mornings with white smoke. If it is vapor, it should stop after a short amount of time, just until the water is out of there. If it is smoke, it will stay there until you fix it or burn the whole fluid which can be catastrophic.
1. Blown head gasket
The head gasket is the component responsible for sealing any small gaps between the cylinder head and the cylinder block. When this gasket gets too old or the engine overheats for example, it can fail opening a passage for coolant to leak through and this will cause the engine to burn coolant. The result of this is white smoke coming out of the exhaust.
Remove the cylinder head and replace the head gasket with a new one. Do not go cheap on this as the amount of work to get to the gasket is big (draining fluids, removing valve cover, removing valves and camshaft etc). Look for a good quality gasket so you do not have to repeat this process anytime soon.
2. Cracked/warped cylinder head
If the cylinder head cracks or warps because of excessive heat or any other reason, this will also allow coolant to leak through. The result will be the same as before and coolant will end up burning inside the engine leading to white smoke coming off the exhaust.
In another scenario, coolant can also get mixed with oil and reach the combustion chamber by passing through the piston rings (this will happen because the viscosity will change and the rings will not be able to keep the fluid to get inside the combustion chamber).
The easiest way to find out if this is what is happening is to check the color of the oil on the stick or the color of the fluid in the radiator (both done with the engine cold for safety reasons): if the fluid looks like chocolate milk then they are getting mixed. This mix can also happen in scenario #1 described previously where the gasket failed.
In some cases it is possible to mill down the cylinder head making sure the surface is flat again and will seal with the engine block well enough. However, most commonly the damage can not be reversed and a new cylinder head will be necessary.
You can find an aftermarket one, buy OEM versions or even from junkyards if you have one near you that you can trust. Again, do not try to save money on this as it is not worth it; go for good quality components that will last.
3. Cracked engine block
When an engine block cracks, depending on where the crack is located, it can connect with the coolant galleries that run around the engine to exchange heat.
If the crack starts from the outside, coolant will leak to the outside and burn when the engine is hot, creating the white smoke in the engine area (stop the engine immediately as its temperature will only rise due to the less coolant you have in the system).
On the other hand, if the crack starts on the inside, the coolant can get inside the combustion chamber, burn and produce the white smoke. Also, just like mentioned before, coolant can get mixed with the oil and come through the piston rings.
If you are on a budget, some of these cracks can be filled and last long enough if the job is well done. Another option would be to buy a new cylinder block and replace the cracked one.
This is a lot of work as it requires you to disassemble the original engine and move the components to the new block. But it is a good chance to rebuild the whole engine by replacing gaskets, seals, chains and whatever else does not look like will last too much longer, so this is the way to go if you were already planning on rebuilding it.
Last but not least, you can buy a whole new engine and just keep the one with the cracked block as a parts donor for the good ones that it still holds.
4. Coolant/water pump cover gasket/o-ring damage
The coolant pump gasket can also fail which would allow for coolant to leak to the outside of the engine. This will cause the white smoke to come off the side of the engine and you should turn the engine off as running it without enough coolant can cause it to overheat which can cause worse damage such as the cylinder head warping from #2..
Drain the coolant, remove the pump cover and replace the gasket/o-ring. Torque the bolts as recommended by the manufacturer and replace the coolant with the proper specification and quantity.
5. Failing radiator cap
It is a rare case, but it can happen that the radiator cap will fail. This will allow for the coolant to leak from the top of the radiator which not only is dangerous due to the temperature that it can potentially be at, but it will also create white smoke if it touches any hot surfaces and burns.
Replace the radiator cap with a new one. Attention to only remove the one in place with a cold engine and I know what you are thinking: the answer is no, of course you should not use the engine without the cap on the radiator.
6. Damaged radiator hoses
The radiator is connected to the engine by hoses that take the low temperature coolant from the radiator to the engine and return the high temperature coolant to the radiator. These hoses can dry out with time – due to sunlight exposure for instance -, get damaged/cracked and even the clamps that hold them in place can fail, all leading to coolant leak.
Depending on the location of the failing point, coolant can fall onto the engine and burn due to the temperature of it or you could lose coolant, overheating the engine and we already know what comes next (most likely #1 and #2 of this list).
Check your coolant level; if it is low, this means it could be leaking somewhere. Check all the hoses and connections looking for any damage that can be causing it. Once you have found the reason, replace the component with the proper parts; keep in mind that because it is a high temperature system, you should only use hoses and clamps designed for the task.
7. Unburnt fuel
White smoke can also be a sign of unburnt fuel as the engine compresses it. This will generally be associated with other problems such as bad spark timing – not burning the full mass of fuel – or carburetor jetting issues – allowing for too much gas to be sucked in.
Cleaning the carburetor and replacing parts with a repair kit of seals and jets is always a good start; this should fix the issue if related to jetting. If the problem is related to bad spark timing, you would be hearing misfires and feeling loss of power coming from the engine. Checking the spark plugs and the cables condition as well as the CDI should reveal where the problem is.
8. Lean fuel mixture
Lean fuel mixture happens when we have too much air for the amount of fuel admitted. Same as before, this could be a problem with the carburetor jetting not allowing too much gas to get mixed or the boots that connect it to both the air box and the engine could be leaking. The excess of air would cause the engine to condensate the water in it and push vapor out of the exhaust.
While working on the carburetor/injector body, checking the boots that connect the airbox and engine to it could reveal a crack through which more air is getting sucked in and causing the mixture to become lean. These cracks can happen because of the plastics getting too dry and replacement for them can be easily found on the aftermarket as well as a carburetor repair kit.
Sometimes, it is hard to tell if the smoke out of the engine is really white or blue. If you can’t find any issues with your coolant system, it might be worth checking if the engine is not burning any oil. This would cause blue smoke to come off the exhaust, but depending on how much oil is burning, it gets easy to mistake it by white smoke.
White smoke in 4 stroke engines is something that can range from really bad news to just a simple component replacement if you notice it fast enough. It is important to pay attention to the engine’s operating temperature and also the coolant level; any signs that one of these two is off, you can start to look further. Hopefully, the problem will be on the simple side of the range.