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Smoke Signals: What the Smoke is Saying About Your Car

Posted by Scott Goldfarb on

Exhaust from your diesel engine should be a light or thin white. If it is anything else, that may signify you have a problem going on somewhere. Luckily, the color of the smoke can be a clue in helping you figure out what is going wrong. Here is a quick guide to understanding what the smoke from your diesel engine is trying to tell you.

White Smoke

As mentioned, a thin, white smoke exiting your vehicle when you start the engine (especially on a cold day) may not be cause for concern. It is likely just water vapor that has collected in the exhaust system.

If, however, the smoke is milky white, the head gasket may be leaking. The coolant is most likely leaking into the combustion chambers, where it is being heated and then blown out as exhaust. If there is coolant leaking into your combustion chambers, you may not have enough coolant in your engine to prevent it from overheating, especially in the summer. Have your engine serviced as soon as possible.

A cracked block or cylinder head could also contribute to heavy white smoke coming from your exhaust. There is no good reason for white smoke to be coming from your vehicle, so have it addressed as soon as you notice it.

Gray Smoke

If you see gray smoke, take a closer look. Gray smoke could simply mean the wrong grade of fuel was put into the vehicle. It can also be caused by a clogged air filter, a simple and inexpensive repair you can probably tackle yourself.

Blue smoke is often mistaken as gray, though, and it could signify more serious problems. Take a closer look if you think the smoke coming from your vehicle is gray.

Blue Smoke

Blue smoke indicates that oil is leaking somewhere in your diesel engine, and it is being burned. The oil can be coming from many different places:

  • Too Much Oil in the Crankcase. If the crankcase is overfilled, the excess will burn and cause the blue smoke.
  • Cylinder Damage or Glaze. If a cylinder is worn down, it can create a gap large enough for oil to pass through. A glazed cylinder prevents the rings from wiping the oil on the way down, and oil will slip through.
  • Incorrect Oil. If the incorrect oil was put in the engine in the first place, it might be too thin, which would allow it to pass the rings or valves to where it shouldn't be.
  • Worn Piston Ring, Valve Guides, or Seals. If the piston ring, valve guides, or seals are worn, they can allow enough space for oil to enter the combustion chamber.
  • Turbocharger Drain Restriction. If the drain is restricted and doesn't allow the oil to flow, it will pass into the intake.
  • Fuel in the Oil. If a fuel injector is stuck open (due to faulty diesel fuel injection parts or plunger fuel injection pump), it will leak fuel into the crankcase. This leak would dilute the oil, which could then pass by and burn.
  • Clog in the Oil Drain Line. A clog in the head of the oil drain line would mean the oil would have nowhere to go. It would find its way out into the exhaust.

Blue smoke is never a good sign. Have your diesel engine checked out by a professional mechanic as soon as possible.

Black Smoke

If your vehicle is giving off black smoke, it is allowing partially burned fuel to pass through the combustion chamber, and there is a problem with the air and fuel mixture.

Let's dive into some of the causes of this imbalance.

1. Air Restriction.

Air restriction is the most likely reason you are seeing black smoke. The air could be restricted in multiple places:

  • The Air Filter. A dirty air filter that needs to be replaced is a common cause of black smoke, and it's an easy fix.
  • The Inlet Restriction. Any obstructions to the air flowing through the engine intake can cause an imbalance in fuel and air (and thus, black smoke). Look to see if the inlet piping is collapsed or if you can find another source of blockage.
  • Incorrect Sensor Reading. Recently engines have been equipped with control units that can measure and adjust air mass to make it the right proportions. If you are seeing black smoke, your sensor could be failing and giving you an incorrect reading.
  • DPF Damage. Check the exhaust and DPF system for any damaged parts that could be contributing to restriction.

2. Bad Fuel.

When you fuel up, pay attention to the quality of fuel you are putting in. Bad fuel can be the culprit for multiple issues.

3. Bad Driving.

If you put the vehicle in a higher gear than it needs to be and then give it more fuel, the engine will inject more diesel, but the diesel to air ratio will be off. The result is black smoke. In this case, nothing is wrong with the engine; the driver just needs to be more educated about how to choose the gear they should be in.

4. Faulty Injectors.

If the fuel injector system is stuck open and giving the engine too much fuel, you could have excess that will be burned as black smoke (if it isn't leaked into the crankcase and causing blue smoke). The injector timing could also need adjusting.

There can be other causes for an imbalance of fuel and air, such as incorrect parts. If you see black smoke, you can do some troubleshooting to rule out easy fixes (such as poor driving or a bad air filter), but if you can't determine and fix the problem, find a professional who can help you, so you don't cause more damage to your vehicle.

The next time you find yourself watching exhaust in your rearview mirror, pay attention to the color to find out what the smoke is trying to tell you.


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