Many engine problems can be accurately diagnosed by studying the color of the exhaust fumes. Just as with humans where the study of the body’s dejection can give doctors a good idea of the efficiency of the system, the study of the chemical composition of the exhaust fumes will tell you if the engine is correctly burning the fuel it’s fed with or if something is not quite right. Such a task will require the use of an exhaust gas analyzer which is an expensive tool, thus making the operation quite costly for consumers. Before you take out the big bucks and get your car’s fumes analyzed, we suggest you first use your sight and sense of smell to find out what the problem is and how urgent it is to get it fixed.
Read more: Diagnose Types of Car Smells
Table of Contents
Not all smoke is created equal
Different driving conditions and different faulty components will create different smoke colors and density levels. Correctly identifying each of them is crucial to end up with an accurate diagnosis. When visually analyzing your vehicle’s exhaust fumes and deciding whether it’s a normal and temporary condition or your car is actually in need of a revision, always make sure to note down the outside weather and temperature, if the engine was hot or cold as well as the color, the thickness, the smell of the fumes and the overall duration of the problematic condition. All this information will surely help the automotive technician who will be working on your vehicle to find out what is wrong with it a lot quicker.
A white smoke problem is not to be confused with the normal white vapor caused by condensation during cold starts. This type of smoke should disappear quickly under normal driving conditions. On the other hand, thick white smoke that lasts well beyond the engine warm-up phase is a definite cause for concern.
Consequences of burning coolant
Water in the combustion chambers
Even if water isn’t, in essence, bad for your engine, it is still something that should be avoided at all cost. One could argue that water has been used for combustion chambers decarbonization for a long time now and is consequently safe to use…
And they would almost be right.
Yes, injecting water into the combustion chamber has been a solution to prevent premature detonation under hot conditions in the old times, before EFI became a thing. And yes, some people also use water to remove carbon deposits inside combustion chambers by carefully injecting some through a vacuum port.
The idea here is to use the incompressible property of water to knock off carbon deposits from the combustion chambers’ walls. The problem with such a technique is that the carbon pieces falling off can scratch cylinder walls, damage piston rings and possibly bend valve stems, preventing them to open and close correctly.
Unless your engine definitely has a premature detonation problem caused by carbon deposits, the game is simply not worth the prize. The decarbonization of combustion chambers using water is a delicate operation that requires care and precision. The risk of damaging your engine even more instead of fixing it is too great to take a chance.
It is also worth mentioning that as the problem is worsening, more and more coolant will eventually leak, ultimately leading to cold start problems, misfires. rough idles, and a higher risk of hydraulic locks.
Burning coolant is definitely worse for your car than burning water. The combustion of Ethylene Glycol, which is the main component used as antifreeze by automotive coolant manufacturers, will leave deposits inside your exhaust system and catalytic converter which could, in turn, make the check engine light pop up.
Ethylene Glycol also produces toxic fumes when heated. In addition to polluting the atmosphere, these fumes could be very dangerous to your health too, especially if your exhaust system is leaking a bit.
It’s worth mention that burning coolant will also cause the coolant level to slowly drop and will tend to cause engine overheating when it reaches a certain level.
The most frequent reason why an engine starts burning coolant is definitely a broken or cracked head gasket. Old and dried out head gaskets tend to crack in between the oil or coolant passages and the cylinders. If a crack appears between any of the passages and the cylinders, the car will start producing thick blue smoke.
Higher-end performance cars running with a leaking water-cooled turbocharger could also produce white smoke without having a faulty head gasket problem. No matter if the leakage occurs on the cold or the hot side of the turbo, the leaking coolant will either be sucked into the air intake and the cylinders or it will be pushed into the exhaust system. Both situations will produce white smoke under acceleration.
How to fix it
1. Check your fluid levels
The first thing to do to find out if your car is burning coolant is to check the coolant level. And, that’s also correct for any other kind of smoke. If fluid is leaking somewhere, the first thing you should notice is a significant drop in the fluid level on the dipstick.
Make sure the coolant is filled to the appropriate level and trace a line with a Sharpie on the expansion tank. Drive your car for a couple of days and re-check the level. If it’s lower than your mark, the coolant is definitely leaking somewhere. Before you jump to conclusions and wrongfully start thinking that your engine head is cracked, inspect under your car for signs of leaks.
A gradually descending coolant level means that the coolant is leaking indeed, but it doesn’t automatically mean that it’s leaking into the combustion chambers. Coolant can leak from multiple locations like the hose clamps, the thermostat housing, the radiator, etc. Simply make sure you aren’t misled towards a false diagnosis by an unrelated leak somewhere.
2. Visually inspect the coolant
Whenever a crack is created between the coolant passages and the cylinders, fluids and contaminants resulting from the fuel combustion will be able to travel both ways. When the engine is running, the higher pressure on the cylinders’ side will lead contaminants to accumulate in the coolant and will gradually make it turn darker. If your coolant is dark, slimy, smells like burnt fuel or contaminated with small black particles, your head gasket may well be leaking.
Positive pressure coming in from the cylinders can also create air bubbles in the coolant system. If after removing your radiator cap, you can see small bubbles bursting in the coolant, further testing will be needed.
3. Perform a leak down test
The next step is to perform a leak down test. To confirm that the head gasket is not air-tight anymore, you’ll need to remove all the spark plugs and connect the leak down tester to the spark plug threaded holes. Inspect the spark plugs and look out for white deposits as a sign of coolant combustion. Position the cylinder you are testing to “top dead center” and inject compressed air inside it. Do the same for all cylinders. If the head gasket is leaking or if there’s a crack somewhere, air bubbles will start to come out of the radiator or the expansion tank.
If bubbles appear, you now know for a fact that there is a leak somewhere. But… you aren’t out of the woods yet. You still need to find where the crack is so you know if you only need to replace the head gasket or the engine head as a whole.
4. Inspect the components
There’s no quick and easy way to inspect a head gasket. The engine head will need to be removed as well as everything connected to it. Removing and disassembling an engine head is no easy task and should be reserved for full-fledged auto mechanics only.
Once the head is removed, the technician will use a dedicated crack detection spray to help him detect even the smallest cracks. If nothing is found, a pressure test could be performed on the engine head to make sure it’s not warped or damaged. Any faulty component found will obviously need to be replaced.
Smoke with a slight blueish hue is definitely the easiest smoke color to identify, mainly because of the oil smell usually tagging along with it. Blue smoke is pretty much always related to an oil consumption condition.
Most engines consume oil reasonably in normal conditions. There is no need to worry if your car’s oil level drops a little in between oil changes. Car manufacturers tolerate something around 1L to 1.5L of oil consumed for each service interval to be normal.
If your car just started to produce blue smoke though, especially under hard accelerations, the problem may become a concern in the near future.
Consequences of burning oil
Engine oil is crucial for your engine to stay in tip-top shape. Oil is allowing everything to move freely while reducing the wear on the various components. Without oil, it only takes a few minutes for an engine to seize.
If oil can enter the combustion chambers because the engine block or the head gasket is cracked, fuel combustion residues can also contaminate the oil in return. Fuel is the enemy of oil and fluids because of its detergent property. Starting in 1996, automotive fuel minimum standards were set and detergent started to be added to the mix to help with carbon deposits and to keep injectors as clean as possible. The problem is that when fuel infiltrates the lubrication system, it will literally render your engine oil useless. The oil will degrade and will lose its viscosity, leading to bad lubrication and excessive premature engine wear.
Any leak between the engine block and the engine head will also lower the compression ratio. Bad engine compression will cause misfires, lack of power, stuttering and a multitude of recorded DTCs.
Burning oil also leaves petroleum deposits everywhere it touches. Oil fumes will accumulate on the spark plugs, preventing them from creating good sparks. Without sparks, no combustion. And without combustion, well.. the engine just dies.
Burnt oil fumes will also clog the catalytic converter causing various problems such as a lack of power and a total inefficiency of the anti-pollution system.
Not to mention the fact that driving a car constantly followed by a huge cloud of blue smoke will often get you in trouble with the cops and you risk having your car towed for a general inspection.
Leaking cylinder head gasket
Exactly as with the white smoke problem, a cracked head gasket, engine block or engine head can also let oil infiltrate the combustion chambers. Most of the smoke will be produced under acceleration and right after a cold start. The smoke will usually stay the same until the engine has reached its normal operating temperature. Once the engine is hot and metal expansion is at its maximum, cracks tend to reduce in size, and oil consumption will be kept to a minimum until the next cold start.
Pistons, rings, and cylinders
Pistons and cylinders ovalization will create a small gap between both and will let oil go through to reach the combustion chambers. Be aware that this one is not a common occurrence though. Out of specs pistons are mostly found on seized engines or after an engine has been driven without proper lubrication for some time. Ovalization is normally a consequence of an excessive oil consumption problem and not a cause.
Worn or scratched piston rings will also let a thicker than expected layer of oil on the cylinders’ walls. It will be the case especially if the “oil control ring” is at fault. As its name suggests, its job is to control the amount of oil left behind when the pistons go down during the intake stroke. As the pistons come back up for the compression stroke, the excess of oil will then be pushed back up and will be burnt during the combustion process.
Engine oil viscosity
Using an incorrect engine oil grade can bring a lot of problems including excessive oil consumption, especially if the oil grade selected is lower than what is normally recommended by the manufacturer.
As opposed to popular belief, oil thickness isn’t the same thing as oil viscosity. Thinner oil doesn’t mean a lower viscosity level and different oil grades can have the same specs and level of protection. The important thing is to use the correct oil grade for your vehicle at all time.
In a case where lower oil grade was used, the thinner oil could infiltrate and leak into the engine. If a higher oil grade was used instead, the thicker oil could make the oil pressure rise significantly and ultimately lead to oil seals popping out or starting to leak inside the combustion chambers.
Bent valve stems and worn valve guides can also allow oil to slowly drip inside the cylinders. Damaged valves are still a rare condition often a result of another engine problem like a broken timing belt on an interference engine or if a foreign object comes into contact with one of the valves while it’s operating at full speed.
Some turbochargers bearings are lubricated by the engine oil and will often leak into the intake system as they age. If one is lucky enough, the turbo will leak to the outside of the unit and will only leave small oil spots on the ground. The unlucky ones are those with an inside leak. The positive pressure created on the cold side of the turbo will definitely not help to keep the oil consumption under control. In both cases, the turbo will most probably need to be replaced.
How to fix it
While blue smoke is definitely easier to detect than white smoke, it’s not as easy to diagnose. Most possible faulty components are located inside the engine and simply can’t be visually inspected like that. This is exactly the kind of situation where your mechanic’s work experience will truly be worth every penny invested.
A leak down test could be performed to find cracks and leaks before starting to disassemble everything. The next step would be to remove the spark plugs to look for oil and potentially leaking seals. If everything looks alright, removal of the engine head to visually inspect the valves and block surface will be needed.
Most internal components can’t be repaired or rebuilt and consequently, the replacement of the faulty parts, as well as some other single-use gaskets and seals, will be required.
Internal combustion engines require a precise air/fuel ratio to work efficiently and as eco-friendly as possible. Excessive air volume leads to a lean mixture, whereas the opposite produces a rich mixture. A perfect air/fuel ratio is called a stoichiometric ratio and equals to 14.7g of air for 1g of fuel. Even a slight modification to the mixture can have a dramatic effect on a vehicle’s performance and reliability.
Black smoke is almost always synonymous of a “too-rich” ratio. Black smoke is, in fact, soot residues caused by partial combustion of the fuel. Fuel will burn slower and less efficiently inside the exhaust line than it would have burned inside the combustion chambers, creating black smoke instead of transparent fumes like it should.
Consequences of a bad air/fuel ratio
A car running on anything else than a 14.7:1 ratio will certainly produce, at the very least, a small amount of smoke out of the exhaust pipe. Race cars often use a different air/fuel ratio configuration as a leaner setting often leads to better performance and smoke coming out of your rear end is not really a concern on a race track. On a daily driven vehicle, however, performance is not the main objective and compromises are to be made to achieve better fuel efficiency and ride comfort.
An incorrect air/fuel ratio can also compromise compression which will, in turn, make it harder for the engine to efficiently burn the fuel. Running a too-rich mixture for a long time will also end up filling the catalytic converter with gas fumes. Once filled, the temperature of the catalytic converter will start to rise to a point where the honeycomb structure will literally melt and clog the pipe. The obstruction of the exhaust flow will result in a loss of power and could even make the engine stall.
The Mass Air Flow sensor is the component calculating the amount of air that enters the air intake. The Powertrain Control module then calculate the quantity of fuel it needs to send to the injectors and for how long it has to open them to achieve the desired air/fuel ratio. If the MAF isn’t measuring accurately, as it often happens when the tip of sensor gets dirty, the PCM can’t calculate the correct amount of fuel to send since it doesn’t have a good air volume and oxygen concentration reading to use as a reference.
Obviously, if the air intake is either obstructed or leaking, the Mass Air Flow sensor won’t be able to do its job. Look out for small cracks in the main tubing and leaks at the air intake gasket. On newer car models, intakes and plenums are often made out of plastic and tend to dry up and crack as time goes by. Spraying the intake and every vacuum hoses with brake cleaner while the engine is running will help you quickly locate defective and leaking tubing and hoses.
Thanks to the use of detergent additives and the recent direct injection technology developments, fuel is now cleaner than ever and excessive combustion residues build up on injectors is pretty much a thing of the past. Injectors operation, on the other hand, is still taken care of by small electric motors, sensors and electronic control modules and these are still subject to breakage and electrical faults. If you doubt that one of your injectors may not be opening or closing correctly, most good OBD2 scan tools can test and activate them independently. Switching them on and off while touching them with your fingers to see if you can feel them click will easily let you know if they are working or not.
Finding out if they are correctly calibrated and if they open and close at the correct speed, frequency and duration is totally another game, though. If this is where you are at in your troubleshooting procedure and you are not so sure what to do next, you might be better bringing your car in for a quick checkup by an auto mechanic specialized in fuel injection systems. Fuel mapping is not be taken lightly and usually requires professional tools anyway.
The fuel pressure regulator can also become weak and start sending too much or not enough fuel pressure to the injectors. Use a fuel pressure gauge to make sure everything is between the thresholds set by the manufacturer or thick black smoke is guaranteed.
Just like air intakes, any leaking hoses and malfunctioning sensors can lead the PCM to think there’s more air entering the engine than there really is. The PCM will react by sending more fuel to match the higher air volume resulting in a too rich mixture creating more black smoke. A faulty boost pressure sensor will do the exact same thing and PCM will once again be unable to control it via the wastegate.
An o2 sensor’s job is to monitor and analyze the fumes coming out of the engine and send feedback to the PCM. They are the ones measuring the air/fuel ratio and the catalytic converters efficiency level. No need to explain how crucial they are in order to maintain a stoichiometric air/fuel ratio at all time.
As soon as the PCM detects that an o2 sensor’s reading is out of a certain threshold, it will cancel it out and will work based on a default mapping to ensure the car doesn’t stall. A car driving around with a bad o2 sensor will typically be less fuel efficient, less eco-friendly and could sometimes produce black exhaust fumes.
The exhaust gas recirculation valve, as it names implies, is used to return some of the inefficiently burned fuel found in the exhaust gas to the intake in order to reduce emissions and cool down the engine. The same soot creating black smoke behind your car is also found there and will gradually accumulate and obstruct the EGR valve. The resulting dirt will often prevent the valve from closing correctly and the recirculated gas will then be returned to the engine at random, changing the oxygen ratio of the air/fuel mixture, producing even more black smoke.
How to fix it
Mass air flow sensors, as well as most other sensors, can’t be played with and the only solution is to replace the problematic ones. Using a scan tool is probably the best technique to test and make sure sensors need to be replaced. A quick look at the OBD2 data will help you know if it’s working or not. Make sure the data is between thresholds and that it changes with the rpm or it will need to be replaced. Recent car models give mechanics access to tons of various sensor data and values. Testing sensors by hand with a multimeter is not as accurate and definitely a lot more time-consuming.
Turbochargers can often be rebuilt instead of replaced. Machine shops specialized in forced induction systems usually have most big brands parts in stock and the total cost of a rebuild including the labor time will be a lot less expensive than replacing the whole snail.
Oxygen sensors problems are probably the most common occurrence of all the problems listed in this post. Sadly, even though O2 sensors are quite easy to replace and appear to break on a somewhat regular basis, they are still kinda expensive. Even worse, your car can’t really go without one and the increased fuel cost of driving on the default fuel mapping setting will probably end up being more expensive in the long run anyway.
In the end, the analysis of the color of the smoke coming out of your car’s exhaust is not an exact science and it won’t necessarily tell you the exact cause of the problem. It will only give you a better indication of what system may be concerned, where to look out first and if it needs to be fixed right away or if it can wait until the next paycheck comes.
Always keep in mind that whatever the density or color of the smoke that comes out of your vehicle, if the condition remains persistent for a couple of days, we strongly suggest you seek the advice of a specialist without any delay.