Black smoke or white smoke from exhaust: What’s wrong with your car?

Smoking is injurious to health. And smoke from a car’s exhaust is a symptom of an engine that is in poor health. A petrol car billowing white smoke – what could be the issue? A diesel car belching out black smoke – what is the problem? We look at some common causes and suggested remedies for smoky exhausts for both petrol cars and diesel cars.

India is ready to move to Euro V emission norms in metros and Euro IV norms elsewhere, which would mean even lower emission standards from vehicles. Pollution Under Control (PUC) certificates are now mandatory for all vehicles. A well-maintained car should not have any visible smoke 90% of the time. But if you do have a problem with a smoky exhaust, here’s what the likely problems could be.

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Smoke from a petrol car

Petrol cars usually have next to no visible smoke from the tailpipe. So if a petrol car is emitting smoke, then there is a problem with the car.

Black Smoke

If a petrol car is emitting black smoke, it is a sure sign of the engine running too rich. That means excessive fuel is being burnt, with too little air, causing soot to be blown out of the exhaust. The common causes for this happening are if the air filter is clogged or dirty and you try to accelerate. This is also associated with a loss of pick up or stalling of the engine. Another cause for black smoke in older vehicles (that had manual timing adjustments) is poor ignition timing. Another cause for some black smoke in a petrol car especially in the morning is if the electronic choke gets stuck – a petrol car usually runs rich for the first few minutes when cold, till the engine warms up. (In older cars with carburetors you had to manually use the choke, and failing to release it fully led to black smoke and soot build up inside the exhaust).

White Smoke

Some wispy white smoke may not actually be smoke with a petrol car. It could be water vapour on a cold start, which is actually a sign of a healthy engine. But if you get strong exhaust smell and see white smoke in large clouds it could be a problem. White smoke is usually associated with coolant entering the cylinders and getting burnt. This happens if you have a blown gasket in the engine (the thin part that separates the water chamber from the cylinders between the engine block and the head – the top part of the engine) or even a cracked engine block.

Bluish Smoke

If you see bluish, white smoke coming out of the engine with an oil burning smell, it is precisely that – oil is getting burned when it shouldn’t. When you have Bluish smoke you will also have associated symptoms of higher oil consumption, lower power and more engine noise. This is usually caused by worn out piston rings or an engine that is about to seize. An engine burning oil needs to be attended to immediately.

Also read: Engine decarbonization – when do you need it?

Smoke from a diesel car

Diesel cars emit some amount of visible smoke from the exhaust occasionally and it is not a sign of alarm. Small puffs of black smoke on hard acceleration is normal and some white smoke in extremely cold weather too is normal. But if there’s excessive black or white smoke, there is an issue with the engine. Diesel engines usually have more issues with smoke compared to petrol engines.

Black Smoke

If a diesel engine is billowing black smoke, which is mostly the case, it could be one of a number of issues. Black smoke could either be a too rich fuel mixture or excessive soot. A dirty air filter will cause the engine to give out black smoke and loss of pick up. At high altitudes, the lower oxygen levels in the air again make the engine run rich and give out black smoke – this is not really a problem, but a car with a healthy turbocharger and clean filters won’t have too much black smoke.

A cracked intercooler hose or turbo hose can also lead to a drop in boost pressure, leading to black smoke coming out of the engine. This is usually accompanied with a hissing sound and loss of pick up. Dirty or clogged fuel injectors can also cause black smoke from a diesel engine, due to an improper spray pattern by the injector or delayed injection. Adulterated fuel is one of the main causes. Cleaning the injectors or adding an additive from time to time will prevent this.

Diesel vehicles also have an exhaust gas recirculation circuit, where some of the exhaust gas is pushed back into the intake to reduce the combustion temperature and subsequently emission of nitrous oxides. If the EGR valve is faulty, it could lead to black smoke again.

Driving a diesel vehicle at below optimum RPM in bumper to bumper traffic for a long time, leads to a build up of soot in the exhaust. Over time this accumulates in the catalytic converter and muffler. So if you suddenly switch to high RPM driving, you will get black smoke for a while (and then it clears up, also known as an Italian tune-up).

White Smoke

White smoke from a diesel car is symptomatic of improper fuel combustion. In very cold weather when a diesel car has not warmed up, some white smoke is normal. But if this condition continues to persist and there’s a loss in power, it indicates that the car is losing compression and hence belching white smoke.

Another cause of white smoke in a diesel car is when diesel fuel is adulterated with kerosene especially. This has a characteristic smell too, besides the constant emission of white smoke. Choked valve seats can also cause some amount of white smoke. A good additive can help clean up the valves and eliminate this problem.

Water or coolant entering the fuel system or combustion chamber is also characterised by white smoke. A blown head gasket (as described above) or water in the fuel can lead to this issue.

Needless to say, whatever the colour of smoke, your vehicle needs attention. Hope this helps you diagnose the causes more easily. Post your comments on CarToq if you need further assistance.

Also see: Diesel engine maintenance tips

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