What causes white smoke from the exhaust?
A question that first comes to mind when you see thick white fumes coming out of the exhaust pipe.
The reasons are various, some are more serious while others aren’t a problem at all.
Problems with white smoke appear both on gasoline (petrol) and diesel engines so whatever kind of car you drive this article may be useful.
Also, after reading this you’ll hopefully get a good idea of what’s causing the problem. The reasons start from the more harmless and end with the more severe ones.
1. WATER VAPOR
Water vapor is the most common reason for what causes white smoke from the exhaust.
As you probably know, when the engine is working it produces hot exhaust fumes.
While they come out of the exhaust pipe, they heat up the rear muffler. But when they stop, the muffler cools down and a water condensate starts to form inside of it.
When you turn on the engine again, the water inside heats up and white vapor starts emerging from the muffler.
It might be just a thin, barely visible smoke or it might be a real smokescreen.
It all depends on how much condensate has formed inside and how long it takes to evaporate.
What’s characteristical of this is that the vapor, in most cases doesn’t have any noticeable odor, like when the engine burns oil for instance.
It also may be followed by drops of water coming from the muffler, especially when you rev up the engine.
Also, if water is the reason, the white smoke should disappear in a matter of minutes.
This reason is more present in colder and damper climates where the condensate is more likely to form.
2. RICH AIR-FUEL MIXTURE
When there’s more fuel than air in the air-fuel mixture, white smoke may appear.
Unlike other reasons, you’ll easily recognize this one. Besides the thick smoke, it will have gasoline (petrol) smell to it.
Reasons for a rich engine mixture are various and we’ll mention some of the main ones lower in the article. In short, there can be problems with the fuel injection system, fuel intake, air intake, sensor problems, and so on.
If the problem is minor and temporary, the odor will disappear in a couple of minutes at most after starting the car. It usually takes until the engine warms up a bit.
But if the odor is present all the time and the exhaust smoke doesn’t clear, you should check out the problem as soon as possible.
3. FAULTY FUEL INJECTORS
Over time, fuel injectors and seals become worn out mostly due to the fact that fuel is an aggressive substance and because of the high injection pressures. Material fatigue simply takes its toll.
When any of this is present, the fuel injector may deliver too much fuel into the combustion chamber disturbing the air-fuel mixture ratio.
This then causes white smoke to come from the exhaust. Besides the smoke, it will have a gasoline scent to it.
Some other side effects are fuel leaks in the engine bay and increased fuel consumption.
4. FAULTY FUEL INJECTION PUMP (FOR DIESEL ENGINES)
With diesel engines, the correct timing and fuel pressure are critical for the engine.
If these are not harmonized, then the engine gets more fuel than it should, causing white or gray smoke to appear.
Besides the smoke, when the pump is the problem, it may be accompanied by the engine working unevenly and the fuel consumption may be increased.
5. BAD ECU UNIT
The Electronic Control Unit is a very durable car part making this reason one of the rarest.
But if there were problems with the electrics and electronics, it might get damaged.
Due to this, the circuits inside may get damaged and the ECU will stop monitoring and managing all of the systems properly.
It may issue a faulty command to the fuel injection system for instance causing a richer mixture. This then causes white smoke to come out of the exhaust.
6. BAD SENSORS
When talking about what causes white smoke from the exhaust, the sensors that monitor the air and fuel intake system may also cause problems.
Sensors send input to the ECU and if they’re faulty, they send the wrong data to the ECU which then issues the wrong command to the rest of the system.
In this case, it could be the MAF sensor, lambda sensor, or else.
7. DPF FILTER REGENERATION
If you drive a car with a modern diesel engine then there’s a good chance you have a Diesel Particulate Filter.
In short, it’s in charge of thoroughly cleaning the exhaust fumes from a diesel engine.
In order to do that more efficiently, within specific intervals an automatic or induced regeneration process has to be done.
The regeneration, in short, is a process of cleaning the soot that gathers over time inside the filter. If you want a more thorough explanation about this process, check out this great article from Wikipedia.
When the regeneration process starts, white smoke appears from the exhaust. It can also be mixed with a gray or black color.
Sometimes the smoke is thicker, sometimes not, all depending on the car type and how much soot there is inside the DPF filter.
Some drivers don’t know about this so when the white smoke appears it may surprise or worry them.
Best look up the driver’s manual if you drive a diesel with a DPF filter and see how it works on your car so you don’t mix this reason up with something else.
8. DAMAGED HEAD GASKET
Head gaskets are in charge of sealing the cylinder head and block together. Besides this, it prevents coolant and engine oil from getting into the combustion chamber.
If it’s damaged, coolant starts getting into the combustion chamber and starts burning together with the air-fuel mixture.
The end result is a thick white smoke coming from the exhaust. When the damage is minor, you’ll only see a thin, barely visible trace of white smoke.
As the problem gets worse, the smoke becomes thicker.
Other common symptoms are: the smoke will become more obvious as the engine heats up and there will be a low level of coolant all the time.
It’s a pretty serious fault that demands taking off the cylinder head in order to replace the gasket. Not that bad, but it costs a hefty sum of money and takes some time.
9. DAMAGED CYLINDER HEAD
If the gasket is OK, but the symptoms are the same, then you should check the cylinder head.
Especially do this if you’ve had previous overheating problems for a longer period of time.
When the cylinder head is cracked or damaged, coolant gets into the combustion chamber, burns out and this causes white smoke to come from the exhaust.
This is a serious and costly problem. It involves taking off the cylinder head, which is to some extent a partial engine overhaul.
In most cases, once the cylinder head is removed, it has to be repaired, milled, etc. It’s preferable to change the head gasket also.
Other symptoms are a noticeable decline in the engine power output that you’ll notice while driving and a low coolant level all the time.
10. CRACKED ENGINE BLOCK
The final and most severe reason for what causes white smoke from the exhaust is a cracked engine block.
Within the engine block, oil and coolant have their separate canals through which they flow. When the engine block cracks, they may mix with each other or get inside the combustion chamber.
Reasons for this are various: overheating, physical damage, poor quality of casting, etc. More frequently, the problem is water that’s leftover in the coolant system during winter.
If the crack is minor, white smoke will appear when the engine starts to gain working temperature. But if it’s worse, the smoke will appear just after you’ve started the engine.
You’ll also notice a frequent low level of coolant.
Cracked engine blocks can be repaired or completely replaced. This usually means a partial or complete engine overhaul.
It all depends on how severe the crack is and where it’s located. The repair, if done properly, can last for some time.
But to be honest, regarding this problem, nothing beats replacing the whole engine block. It’s the most expensive but also the most long-lasting solution.
Another option that is worth considering if this happens is to buy a new engine.
One of the most important factors when answering the question of what causes white smoke from the exhaust is a quality diagnostic of the problem.
If you see the smoke disappear in a couple of minutes and you don’t notice any other problem with the car, then you shouldn’t worry about the smoke.
But if it’s accompanied by a loss of engine power, overheating, smoke with a gasoline-like smell, low coolant level all the time, or else, check out what’s wrong as soon as possible.
In some cases, you might prevent bigger damage from happening in the near future.
All you have to do is observe for a couple of minutes and make a few simple checks.
Written by: Sibin Spasojevic
Former car technician, life-long car and DIY enthusiast, author for Despairrepair.com