Causes Of White smoke | Black smoke | Blue smoke from Diesel Engines

• White smoke- Incomplete burning of fuel
• Black smoke- Excess burning of fuel
• Blue fuel- Coolant or oil mixing with fuel and then burning

White smoke

White smoke is the unburnt or vapourised fuel, which is produced when there is insufficient heat available In the combustion chamber to burn the fuel entirely.

1- Due to cold starting

Cold starting is a condition that happens when the idle engine is initially started. So due to insufficient engine temperature, the diesel is unable to burn completely (because diesel is ignited by two factors which are high compression and high temperature) which allows unburnt fuel particles to release from the exhaust.

2- Restriction in the supply of air

A  Partially or completely blocked air filter will restrict the supply of air to the combustion chamber due to which there will be insufficient air entry in the cylinder for the piston to compress and generate complete combustion.

This may result in white smoke at the early stage but if operated continuously in the same condition it may result in black smoke, to prevent this condition check air filter, either clean it or replace it.

3- A leak in the cylinder head gasket

If the coolant is leaking into the combustion chamber it will reduce the temperature in the cylinder causing white smoke.

4- Incorrect fuel injection timing

Due to incorrect injection timing either the fuel is injected at an early stage or there is some delay, but in both cases, the fuel will not be able to be compressed completely in the combustion chamber which leads to unburnt fuel particles at the end of the power stroke to exit during exhaust.

5- Poor Compression in the combustion chamber

Poor compressions may lead to leakage during the compression stroke and inevitably less heat would be generated which results in incomplete fuel ignition.

Note-
You can check the evidence of excess burning of fuel by placing your hand in front of the exhaust, but please make sure not too close as the temperature of exhaust gas are very high, if there is excess fuel burning in the combustion chamber you get a strong smell of fuel from your hand may be also small droplets of fuel.

6- Cold running of the engine

This happens when there is some fault in the thermostat and it opens at the starting of the engine which should not be the case.

Note – 
Thermostat is a temperature-operated device that allows the circulation of coolant in the engine, the thermostat valve opens only when the temperature of an engine reaches to a specific range but if it opens before the engine reaches to a specific temperature, it causes cold starting. 

(Once the temperature of the coolant rises to between 180 and 195 F (82 – 91 C), the thermostat starts to open, allowing fluid to flow through the radiator. By the time the coolant reaches 200 to 218 F (93 – 103 C), the thermostat is open all the way.)

Blue Smoke

Blue smoke at the exhaust signifies that there is burning of lubricating oil with charge (a mixture of fuel and air) in the combustion chamber.

Causes of Blue smoke are listed below-

1- Worn or damaged valve stems

It happens mainly due to worn out or damaged valve stems which allow the oil to enter from its sides to the combustion chamber.

Note-
Remember to check the diameter of the valve stem before the assembly that whether it is underspecified limits or not.

Note–
If you disassemble the engine for confirming that whether the oil is mixing in the combustion chamber or not, you will see the deposits of lubricating oil present on the top of the piston crown which will confirm that there is a mixing of oil with fuel. 

2- Damaged or worn-out piston rings

It generally happens due to long usage of cars due to which Piston rings get worn out with time.

Note- 
In some cases, it has been seen that while assembly of the piston rings, they are arranged in-line which will allow easy passage for oil to leak in the combustion chamber.

All the 3 piston rings should be arranged in a way that their opening section should be 120 degrees apart from each other.

3- Worn out cylinder bores

This may occur due to damaged inner bore area as it gets worn out with time due to piston ring sliding movement.

Note-
To check your cylinder bore, you can use a dial-bore gauge in both x and y-axis of the cylinder bore at three-points of height (top, middle, and bottom) this will show whether your bore internal dia is within spec. or not, it also tells you about the ovality and taperness present in your cylinder bore.

Excessive Ovality and Taperness present in the cylinder bore are also the main causes due to which oil enters in the combustion chamber, so consider that point as well. The range of ovality and taper should not range over 9 microns in small engines and 12 microns in large engines.

Black Smoke

Black smoke at the exhaust shows that an excessive amount of fuel (more than desired) is burnt in the combustion chamber which is generally due to restriction in the air intake system.

Causes of Black smoke are listed below-

1- Restriction in the air intake system

Blocked or choked air cleaner element will not allow enough air to enter the combustion chamber and burn the entire fuel.

2- Damaged valve seat and incorrect valve clearance

A damaged valve seat will create a leaking gap between the valve and the combustion chamber, which may allow entry of excess fuel in the port injection engine or allowing air to leave the chamber during compression, which will leave excess fuel behind to burn.

3- Poor compression

The air required for combustion may leak from the cylinder due to damaged gasket,  improper fitment of the gasket, or improper torquing of cylinder head bolts which may create a gap between the engine block and head interface for air to escape.

4- Defective or incorrect injectors

Check the injector to see if the spray is fully atomized and solid fuel is not being injected, Due to improper mixing of air and fuel the fuel does not ignite completely.

5- Low boost pressure

It occurs due to a faulty turbocharger when it is unable to supply enough air for the fuel-injected, this is another form of air starvation.