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Smoky Diesel Engines | Smoking Diesels

Why Diesels Smoke - Diesel car engines get smoky for many reasons but identifying the smoke colours, often white or black, helps you diagnose the cause of the problem to hopefully cure and stop a smoking diesel engine.

If you can see smoke-filled clouds emitting from your vehicle exhaust then you are already half way to reading the signals and diagnosing which parts are failing inside your smoky diesel engine.

If your engine is running but kicking out excessive plumes of black, white, or blue smog, there is a lot you can learn from the colour and characteristics of foggy emissions blowing from your car or truck.

Recognising the most common symptoms of 'blow-by' and 'oil usage' can help you determine why you have a smoky diesel engine and which components need most of your attention. It is a visible sign that your automobile is not running well, the level of pollutant emissions may not be functioning properly, and you may need some advice or assistance from a qualified mechanic.

Most diesel engines (if not all) emit an insignificant amount of black smoke when the vehicle is accelerating. This is normal because it takes some time for the engine system to develop enough revolutions (RPM) and power to burn the fuel. This is how a diesel engine works and they are not the typical signs of a smoky diesel engine. But if there is a noticeable excess of coloured exhaust smoke bellowing out behind your car during acceleration, then this introductory guide to smoking and smokey diesels will help you in your quest to put things right.

Smoke from Diesel Engines

What makes a car smoke out the exhaust? A modern diesel car in good condition and regularly tuned and maintained, should produce almost no visible smoke from the exhaust - under most operating conditions.

Diesel engine smoke problems materialize for varying reasons but most drivers accept there may be evidence of short puffs of gray smoky haze or vapour when the engine is accelerating hard or under heavy load.

Heavy duty diesels and most trucks emit some smoke which is most often a result of bad driving technique (lugging), poor injector maintenance, excessive fuel delivery rates, or a combination of all three.

Smoke emissions are warning signs that the vehicle is wasting fuel, or burning it inefficiently, and some level of engine damage is probably occurring or deteriorating.

Perhaps the most prudent solution is to employ routine servicing which helps to eliminate many problems that cause smoke emissions. In the long term it is most likely to save you time and expense.

Nevertheless, as most diesel car owners know, mechanical problems can and do occur even after regular vehicle servicing and with relatively new or top of the range models.

Diesel Engine Smoke Diagnosis

As you troubleshoot the reasons for your smoky diesel engine, always make an initial inspection for a clogged air filter. A common reason for vehicle smog is insufficient air being drawn or sucked into the system.

The cause is often something as simple as dirt or grime clogging the filtration system. A cloggy air filter does not allow the diesel to burn properly and this would be one of the first things that we recommend when checking for causes of smoky engines.

Checking the Filter

Even the most untrained mechanics will find it easy to check the air filter for dirt. Remove the filter from its casing and rev up the engine. Revving the engine without the filter will not do any harm for a short period of time.

If the engine still smokes, then the diagnosis is that something else is causing the problem. You might be a little disappointed at this stage because renewing the air filter is normally the easiest part to replace and one of the cheapest repair solutions.

Failing this, perhaps further checks should be performed by a competent diesel fitter because the next area to check is the injectors. Having a dodgy injector suggests a low burn quality of the diesel and often produces black smoke. It is possible to fix new injectors, but we advise getting professional help to stop black smoke from a diesel engine.

Poor engine timing also causes black smoke because any general inefficiency of the system results in unburned fuel. That means you are likely to see smoke-filled blackened plumes blowing out of the exhaust.

Diesel Engine Smoke Colours

Black or Grey Smoke

What is black smoke from a diesel engine? When things go wrong, diesel cars may produce unburnt black soot emitted from their exhaust. It is actually an imbalance in the air to fuel ratio where diesel particulate matter consisting of carbon compounds have not burned properly. The common reason for this incomplete combustion failure is low local temperatures creating situations where fuel is not fully atomized.

Another cause of black smoke from the exhaust could be when your engine is burning too much fuel. Check the vehicle's intake components, starting with the air filter, fuel injectors, sensors, and the fuel-pressure regulator. Although not so common, it is worth mentioning that a clogged fuel line may also produce similar symptoms.

The least amount of damage occurs if you replace worn or damaged components once you have identified the problem. A good tip for diesel owners is to keep on top of engine tune issues. This includes regular servicing of air, fuel, oil filters, and setting up valve adjustments.

Black diesel engine smoke when starting on cold start, or while accelerating hard, is not uncommon with older cars. But buying fuel from disreputable outlets is almost certain to cause you problems at some stage. Even though dirty fuel creates murky components, cleaning your injectors is relatively straight forward using an effective and reliable fuel system cleaner.

Sooty black or grey smoke is not a good sign if your vehicle is a late model car (or truck) with a 'clean diesel' engine. Modern diesels are fitted with electronic controls and high pressure common rail injectors to reduce smoking problems.

Blue Smoke

Smoky Diesel Engine ProblemsBlue smoke is a true indication that oil is being burnt. Puffs of blue smoke, often at the back of the engine, suggests it is burning deposits of oil and happens most likely because it is entering the combustion chamber. This can happen for several reasons including;

Seeing blue coloured smoke during a cold start (after start-up) usually reflects a problem with reduced oil control. Common reasons for this include fouling deposits around the piston rings or cylinder glaze (Carbon has been deposited in the machined cylinder crosshatching).

In simple car terms, blue smoke suggests you need to restore physical cleanliness to all the engine's components and replace any worn mechanical parts. If a major overhaul is required it involves taking the engine apart. Before you attempt such a drastic repair you could try freeing up the cylinder rings with a can of Wynn's. Wynn Oil Treatment helps to stop excessive oil consumption, restore engine compression, and increase oil pressure.

White Smoke

What does it mean when your car emits white smoke? Typically, this would be a sign of an internal leak and the engine is burning coolant or transmission fluid. The most common reasons for diesels producing smoky white coloured clouds from coolant leaks are a leaky head gasket or perhaps even a cracked cylinder head (block).

If the problem is transmission fluid, most likely the engine is sucking transmission fluid through one of the vacuum hoses which connects to the transmission. It may also occur when raw diesel comes through the exhaust system and is completely intact and unburned. Other considerations include;

A diesel mechanic knows how a diesel engine works and he can test the engine compressions. If you have worn cylinders, then there may not be enough compression for complete burning of the diesel fuel. It may also be as a result of worn valves or valve guides in which case you are likely to see smoke puffing out of the engine crankcase breather pipe.

Note: Steam caused by a leaky head gasket or other water leak into the combustion system can disguise itself as smoke. Steam will dissipate rapidly once it hits lower temperatures than ambient air, whereas smoke, all things being equal, will persistently blow and hang around in the air.

Diesel Exhaust Smoke Testing

The free-acceleration test (vehicle stationary) is commonly used to test for exhaust smoke. But because it does not simulate the typical conditions which exist when the vehicle is under load, you may find the results inadequate. It can also be difficult for the driver to observe any evidence of exhaust smoke from inside the cab. Therefore, observations should carried out from behind and while the vehicle is being driven on the road and under varying conditions of load and speed.

The vehicle should be traveling at normal operating temperature during the test from a cold start (unless a faulty choke is suspected). It should be road-tested at regular intervals and after mechanical servicing.

Our team of engineers hope this guide helps to sort out your smoky - or smoking - diesel engine.

Disclaimer: Technical data about smoky diesel engines is for general informational purposes only. The advice and opinions given are not intended to be a full diagnostic guide and should not be used as such. We use reasonable efforts to ensure that information herein is accurate and up to date but we make no warranties or representations as to the accuracy of the content and assume no liability or responsibility for any errors or omissions. We also recommend that carrying out any diagnostic work, servicing, car repairs, or other work to smoking diesel engines should be conducted by a trained and competent mechanic who is knowledgeable in diesel engine workings.