Mercedes-Benz 300SL Smoke from engine or exhaust Inspection at your home or office.

You can learn a lot about the health and well being of your car from the color of its exhaust. If it’s operating in tip-top shape, tailpipe emissions should be nearly undetectable. If exhaust smoke is visible, you likely have a real problem, and one that will require a skilled auto technician to remedy.

First, an important note on engine smoke:

Smoke rarely comes from the engine bay, and if it does, the issue is typically critical already. Smoke is not a good indicator that some malady is brewing within the engine; the color of your car’s exhaust is a better diagnostic tool. If there is a problem within the engine that produces smoke, that smoke will exit the engine through the exhaust.

A leaking valve cover gasket might produce smoke caused by oil dripping on a hot manifold. If the leak is large enough, you will likely see the oil dripping on your driveway or garage floor.

A fire under the hood would produce smoke, but flames would follow that, resulting in a very dangerous situation. If this ever happens, quickly exit and move far away from the car.

Again, smoke from the exhaust is nearly always the type of smoke detected in a diagnostic situation, not engine smoke. Read on for common issues that can cause exhaust smoke.

Common causes of exhaust smoke, according to its color:

While the presence of exhaust smoke can indicate a serious car malady, there is a silver lining: the color of that tailpipe plume can give an indication of what might be wrong, which helps to generate a quick and efficient diagnosis.

If the exhaust smoke is black:

Black exhaust doesn’t always indicate impending doom. If your car emits a little black smoke at start-up, but it clears up as the engine warms to operating temperature, don’t worry – that’s normal for some cars. If it continues after the car warms up, there’s cause for concern. If that’s the case, here are the likely culprits:

• Clogged or dirty air filter: An essential part of a vehicle’s fuel system, an air filter stops airborne contaminants from getting sucked into car’s engine, where they can clog up the works. The remedy is simple: replace the filter.

If the problem persists, it’s an indication that your vehicle’s air-to-fuel ratio is askew – specifically, it is burning too rich or more fuel than air. Two of the most common reasons for this are a malfunctioning fuel pressure regulator or leaky or clogged fuel injectors.

• Bad fuel pressure regulator: A vehicle’s fuel system is designed to work within a specific range of pressure, which is controlled by a fuel pressure regulator. If the regulator is on the fritz, the pressure will fluctuate outside of the system’s specified range. If it falls too low, there may not be enough fuel reaching the engine to start it. Too much pressure may result in over-fueling, which can make your engine run rough and deliver poor fuel mileage, among other things. The remedy is to replace the fuel pressure regulator.


• Leaky or clogged fuel injectors: Fuel injectors are small electro-mechanical devices used to spray a predetermined amount of atomized gasoline (a mist of gasoline) into a vehicle’s intake manifold, which is directly in front of the intake valve. The gas is drawn into the combustion chamber and mixed with a predetermined amount of oxygen to optimize the efficiency of the burn or combustion event. The remedy is to repair the fuel injectors.

If the exhaust smoke is white:

Does the exhaust smoke quickly dissipate after leaving the tailpipe? If so, it is probably the result of condensation building up within your car’s exhaust system. This is a common sight when cars, even modern ones, are first started in the morning. If it continues well after start-up, then you may have serious trouble. White exhaust smoke often happens when an engine burns coolant that has leaked into the combustion chamber. The following are the most common culprits:

• Blown head gasket: This is not so great news, but your engine’s structural integrity has most likely not been compromised, so it’s not ruinous. An engine essentially consists of an engine block and a cylinder head. The block is where the cylinders and coolant passageways are located, and the head is where the valves and rocker arms reside. The head gasket is placed between the two to seal the connection. If it is damaged or compromised, coolant can find its way into the combustion chamber. This is a costly, but fixable, issue.


• Damaged cylinder head: This fix is more of a Greek tragedy. Most of the explosive action happens in the cylinder head, where the combustion chamber is located. A cracked or extremely warped cylinder head will cause the engine to lose compression and misfire. Why? There’s too much heat. Overheating puts undue stress on all of an engine’s metal components, especially the cylinder head, which is at the center of the heat production in a car. Excessive heat, normally caused by a cooling system failure, can cause the head gasket to fail, which in turn can cause the cylinder head to crack as components warp and pressure builds. This is not an easy fix. Heads have very precisely milled surfaces to provide a smooth and flush fit with all the connecting parts. You’re better off replacing the head altogether. Depending on the age and value of your car, you might consider buying a new vehicle.


• Cracked engine block: Call a crash cart: your car engine is dead. As we said above, the block houses the cylinders and their components inside a cooled and lubricated crankcase. It’s cast from one piece of metal – typically iron or aluminum – to be extremely strong and sturdy. It also supports the rest of the engine’s components. Cracked blocks are virtually impossible to repair. An engine swap is the only reliable remedy, but that can be extremely expensive and thus sometimes not worth the investment. Depending on the age and value of your car, you may want to consider cutting your losses and buying a new car.

If the exhaust smoke is blue or gray:

Thick blue or gray smoke is an indication of the vehicle burning oil, which means that oil is somehow leaking into your engine’s combustion chamber. Burning oil like this can cause a wide range of issues from reduced fuel economy to less-than-stellar acceleration. There may also be an increase in hydrocarbon emissions, as well as damage to the catalytic converter.

• Malfunctioning valve stem seal: Valve stem seals regulate the amount of oil applied to the valve stem interface, which is then used to lubricate the valve guide and ultimately the combustion chamber. The remedy is to replace the seal (an often costlier fix that involves a partial rebuild) or rebuild or replace the engine. Valve seals normally fail due to a clearance problem between the valve and the valve guide in the cylinder head. When they become loose, the valve is able to “rock” side to side in the bore and thus hogs out the seal. A cylinder head rebuild or replacement is often the recommended fix.


• Failed piston rings: Failed piston rings (or loose piston-to-bore clearance) is more rare of an issue than malfunctioning valve guide seals, but it still happens from time to time.

The telltale sign of a valve guide seal problem or a piston (or piston ring) issue is when the smoke happens. If you are sitting at a stop for 30 to 60 seconds and as soon as you begin to throttle, the car lets out a puff of bluish grey smoke (and then clears up), that’s a dead ringer for a valve guide issue. If you see smoke only under heavy acceleration, that is indicative of a piston or piston ring problem.