AFTER last week's question from a reader about his car's smoking, we have decided to explore the subject further.
A smoking engine is always worrying for a car owner. Last week we looked at the possible causes of blue smoke. But an engine can also produce black smoke or white smoke. What causes that?
Black smoke is an indication of over-fuelling, in other words, too much petrol or diesel being injected into the combustion chambers. The black colour comes from unburned carbon particles in the exhaust gas.
On diesel engines a small amount of black smoke under heavy acceleration is normal.
The injection system is responding to the sudden demand for more power. It allows slightly more fuel to be squirted into the combustion chambers. It's just for a second or so, until the revs can build up. Injectors aren't getting the balance spot-on.
But black smoke pouring from the exhaust pipe in a continuous stream, as one sometimes sees on a truck working hard on an uphill stretch of road, means something is wrong.
It can be a clogged air filter (dangerous on a diesel, because a diesel needs sufficient intake air for cooling the combustion chambers). It could be one or more dodgy injectors which don't deliver the fuel in the correct fine mist spray required for complete combustion. Or, incorrect engine timing which causes general inefficiency in the engine, or an engine that's down on compression due to overall wear.
On a petrol engine with fuel injection over-fuelling should never occur (except for some cars where the computer deliberately gives a rich mixture in brief bursts from time to time to burn the catalytic converter clean).
If you get black smoke from a fuel-injected petrol engine, it is most likely caused by one or more faulty sensors, and the prime suspects are the oxygen sensor and the mass air flow (MAF) sensor. The former sits in the exhaust gas stream. It is the key sensor in the engine's fuel control feedback loop.
The computer uses the signals received from the oxygen sensor to balance the fuel mixture, making it leaner (shorter injection pulses) the sensor reads rich, and richer when the sensor reads lean.
This sensor needs to be hot (at least 250 degrees Celsius) before it operates properly. Some of them have an internal heating element to reduce warm-up time. An oxygen sensor's normal lifespan is 50000 to 80000km. But it may fail prematurely if it becomes coated with carbon or contaminated by silicone from an antifreeze leak or from silicone sealant. It's an expensive component, so be sure it really is faulty before authorising a replacement.
There are very good tests that a workshop with modern equipment can do to tell without any doubt whether an oxygen sensor is malfunctioning.
The MAF sensor is a small component, usually found in the intake air duct between the air cleaner and the inlet manifold, often at the throttle body.
It will have an electrical connection emerging from it by means of which it communicates with the engine computer to tell it what mass of air is entering the engine (per second, say) at any given moment.
This enables the computer to adjust the amount of petrol injected accordingly, to come close to the ideal air/fuel ratio. The fuel mixture is then fine-tuned, as it were, by feedback from the oxygen sensor.
Until the oxygen sensor has heated up sufficiently to become operative (or in the event that the oxygen sensor dies) the computer relies on information from the MAF sensor to balance the fuel mixture.
An engine that smokes for the first few minutes after a cold start might therefore have a faulty or dirty MAF sensor. There have been reports that smoke and fumes in the intake air (such as you get when you are stuck behind a smoking diesel truck) can cause the MAF sensor to become dirty. Sometimes it can be cleaned.
Smoke a telltale sign of trouble
AFTER last week's question from a reader about his car's smoking, we have decided to explore the subject further.
A smoking engine is always worrying for a car owner. Last week we looked at the possible causes of blue smoke. But an engine can also produce black smoke or white smoke. What causes that?
Black smoke is an indication of over-fuelling, in other words, too much petrol or diesel being injected into the combustion chambers. The black colour comes from unburned carbon particles in the exhaust gas.
On diesel engines a small amount of black smoke under heavy acceleration is normal.
The injection system is responding to the sudden demand for more power. It allows slightly more fuel to be squirted into the combustion chambers. It's just for a second or so, until the revs can build up. Injectors aren't getting the balance spot-on.
But black smoke pouring from the exhaust pipe in a continuous stream, as one sometimes sees on a truck working hard on an uphill stretch of road, means something is wrong.
It can be a clogged air filter (dangerous on a diesel, because a diesel needs sufficient intake air for cooling the combustion chambers). It could be one or more dodgy injectors which don't deliver the fuel in the correct fine mist spray required for complete combustion. Or, incorrect engine timing which causes general inefficiency in the engine, or an engine that's down on compression due to overall wear.
On a petrol engine with fuel injection over-fuelling should never occur (except for some cars where the computer deliberately gives a rich mixture in brief bursts from time to time to burn the catalytic converter clean).
If you get black smoke from a fuel-injected petrol engine, it is most likely caused by one or more faulty sensors, and the prime suspects are the oxygen sensor and the mass air flow (MAF) sensor. The former sits in the exhaust gas stream. It is the key sensor in the engine's fuel control feedback loop.
The computer uses the signals received from the oxygen sensor to balance the fuel mixture, making it leaner (shorter injection pulses) the sensor reads rich, and richer when the sensor reads lean.
This sensor needs to be hot (at least 250 degrees Celsius) before it operates properly. Some of them have an internal heating element to reduce warm-up time. An oxygen sensor's normal lifespan is 50000 to 80000km. But it may fail prematurely if it becomes coated with carbon or contaminated by silicone from an antifreeze leak or from silicone sealant. It's an expensive component, so be sure it really is faulty before authorising a replacement.
There are very good tests that a workshop with modern equipment can do to tell without any doubt whether an oxygen sensor is malfunctioning.
The MAF sensor is a small component, usually found in the intake air duct between the air cleaner and the inlet manifold, often at the throttle body.
It will have an electrical connection emerging from it by means of which it communicates with the engine computer to tell it what mass of air is entering the engine (per second, say) at any given moment.
This enables the computer to adjust the amount of petrol injected accordingly, to come close to the ideal air/fuel ratio. The fuel mixture is then fine-tuned, as it were, by feedback from the oxygen sensor.
Until the oxygen sensor has heated up sufficiently to become operative (or in the event that the oxygen sensor dies) the computer relies on information from the MAF sensor to balance the fuel mixture.
An engine that smokes for the first few minutes after a cold start might therefore have a faulty or dirty MAF sensor. There have been reports that smoke and fumes in the intake air (such as you get when you are stuck behind a smoking diesel truck) can cause the MAF sensor to become dirty. Sometimes it can be cleaned.
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