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Why diesel engines smoke and what that means for servicing them or possibly failing your next WOF.

To let you "observe" when your diesel engine is saying "I need attention" I've brought together some basic views from those in the industry. The colour of engine exhaust fumes can often be a good indicator of engine condition, power output and economy. Even an engine in excellent condition will give off some visible fume due to the nature of diesel engines. However if it is excessive, you may be able to "read" what is ailing it by determining its exact colour.

Diesel engine exhaust colours commonly seen are black, grey, translucent purple/blue and white or white/blue, these are commonly recognized as resulting from the following engine complaints.

Too much excess diesel exhaust fumes and you could find yourself with a failed WOF

Black smoke

Or unburnt fuel in the exhaust system is commonly observed when powering up a hill. As the throttle position increases more fuel is injected into the cylinder faster than more air is inducted. This over rich mixture produces black smoke, which is acceptable on maximum throttle, but not otherwise. An over-fuelling injector pump will have the same result. Economy and engine life will be reduced. Black smoke can be the result of worn injector nozzles.

If the spray pattern is imperfect the fuel does not atomise and the tiny droplets of fuel are too big to be completely burnt in the parcel of air which surrounds them. The air-fuel mixture then continues to burn down the exhaust pipe, power and fuel economy being lost. In a normally aspirated diesel only the fuel burnt in the cylinder before the exhaust valve opens will contribute power to the engine. Another cause of black smoke can be airflow restrictions such as a dirty air cleaner or insufficient airflow into the air cleaner.

A side benefit of a snorkel fitment is the increased fuel economy, up to 15% depending on the model, the economy being derived from increased airflow"

Cloudy white smoke

Indicates incorrect timing or low engine compression. If the timing is retarded or advanced, ie the fuel is injected before the piston and valves are in the correct position, then this could be the injector pump incorrectly aligned or timed to the crankshaft so it delivers the fuel to the injectors either early or late. Or the injectors may have low opening pressure.

With low opening pressure the fuel enters the cylinder before the maximum pump pressure is realised resulting in poor atomisation of the fuel, and it occurs so early that the air is below maximum temperature.

With injection timing: Optimum timing = maximum air temperature = most efficient burn of fuel = maximum power

Blue or grey smoke

Can indicate a governor diaphragm problem. If the diaphragm is perforated it fails to control the rack and governor.

With continued high rpm the rack moves to a high fuel supply position. When the throttle is quickly released the rack reacts slowly (due to the perforation) and the pump continues to supply fuel at the high rpm rate but the air is reduced quickly as the throttle was released. The blue/grey smoke is unburnt excess fuel.

Translucent purple-blue smoke

When starting first thing in the morning is unburnt fuel. This unburnt fuel may be the result of the glow plugs not preheating the fuel sufficiently or a timing issue.

Light black smoke

On starting the engine with no movement of the throttle. If with each repeated start-up, say 5 start and shut off's in quick succession, there is a puff of light black smoke it suggests the injector nozzle or pintle is worn and fuel is dribbling onto the piston top after the engine has stopped firing. This produces a hot spot on the piston top and will certainly reduce engine life.

The radial clearance between the injector pintle and orifice is very small, between 0.02 to 0.05 mm. A deposit of only 5 microns thick can restrict fuel flow by up to 25%. Fuel gums, waxes and varnishes can easily build up on the critical surfaces of the injection system analogous to that of deposits of 'gunk' on the inside of human arteries. And we all know what that means!

Water is the catalyst of injection problems. With water will come fungal growths, acids and corrosion. Water accumulates over time from condensation in the tank and can also enter the system with fuel. Use of a non-hydrocarbon based fuel system cleanser (Fuel Set) will mix with the water to carry it through to the injectors and finally be burnt. This maintains a healthy system but will not rebuild worn parts.

Grant reckons most common repairs for an excessively smoking diesel engine are either failure to Service the diesel injectors after 100,000km or water in the diesel fuel or filter.

All the above problems lead to reduced engine performance, engine life and fuel economy but please remember that diesel engines are extremely reliable and efficient when maintained correctly.

If you are suspicious that your engine may have a problem then have the symptoms assessed by one of our diesel mechanics experienced with injection system problems