File photo: Reuters

A smoking exhaust is always a cause for concern, and I recently was asked to look at a diesel-engined car from which the smoke could be blue-white, or black.

The car had recorded about 150 000 kilometres and the engine started easily and ran well.

The owner said he had renewed the air and fuel filters, but, as this made no difference, wondered if he should think about renewing the injectors.

Black smoke emitted from the exhaust appears when some of the fuel injected is not burned during combustion, perhaps due to over-fuelling, or injector problems, which allow fuel droplets that are too large into the combustion chamber.

White smoke, which is often steam, can be caused by a coolant leak, resulting in water in the combustion chamber. Water in the fuel is another possibility. If the smoke clears quickly, it is probably condensation or even a very slight coolant leak.

Blue smoke is the result of oil being burnt during combustion, perhaps because of worn cylinder bore/rings. Oil leaking past valve stem seals is another possible cause. The smoke will be most obvious after the car has been left standing for some time, while a bore-sealing problem will produce smoke at higher engine speeds and especially under deceleration.

Something amiss with the crankcase breather system may produce blue or whitish smoke. On cars with turbochargers, the seals should be inspected too.

As the owner was determined to do the job himself, I advised to first check the crankcase breather system. Then, rather than fitting new (and expensive) injectors, to have the car checked by a diesel specialist who should be able to pinpoint the problem and cure it far more cheaply by using a professional fuel system cleaning agent.

On another subject, I must once again stress the importance of using the correct coolant when dealing with modern engines. Gone are the days when we could take liberties and rely on simply topping-up with water and pouring in some anti-freeze with no thought of checking specific gravity with a hydrometer. Guessing on a modern engine is simply not good enough.

Why is this? It is because alloy heads with steel gaskets and iron blocks need the correct concentration of the specified coolant for your car's engine. Failure to do this may result in the various metals to react to each other and encourage corrosion, with possibly disastrous results.

Some manufacturers recommend using a pre-mixed coolant, but many of us still use the traditional water and coolant mixture with no adverse effects. All the same, some recommend using only distilled water, claiming that tap water is full of chemicals that bode ill for the cooling system.

Regularly check the coolant level, and if it drops too regularly, find out why. I saw an engine recently on which, as the coolant level fell, the engine oil level increased. It was obvious that coolant was entering the oil system through a head fault.

In this case, the car was old and would have been expensive to put right, so the owner took it to a breaker. Sometimes it pays to face facts and cut your losses. -Star Motoring