Diagnosing Your Engine Based on the Color of Your Exhaust Smoke

Updated on October 10, 2018
Dan Ferrell profile image

Dan Ferrell writes about do-it-yourself car maintenance and repair. He has certifications in automation and control technology.

Vehicle smoke can mean trouble for your engine and the environment.
Vehicle smoke can mean trouble for your engine and the environment. | Source

Does exhaust smoke mean your car is dying? Not necessarily.

Exhaust smoke happens because of a potential engine mechanical or performance problem.

The color of the smoke can reveal much about the state of health of your car. Sometimes, it means the need for a few adjustments or a new sensor; other times, it means the need for a new set of piston rings or cylinder head gasket.

There are three main reasons why your car might be smoking:

  • A rich fuel mixture. This often causes black smoke.
  • A coolant leak. This often causes white smoke.
  • Oil in the combustion chambers. This often causes blue smoke.

Only the last two problems indicate serious trouble under the hood.

Before you decide on expensive repairs or to send your car to the junk yard, you need to make a preliminary diagnosis. Smoke by itself won't tell you what's exactly wrong with your engine, but this guide to diagnosing smoke will give you an idea about the type of problem you are facing, so you can make the appropriate repair decisions.

Whether you have a gasoline or diesel engine, the following sections help you isolate the potential issues and speed up your diagnosis, helping you save time and money.

Index
1. How to Verify the Color of Your Exhaust Smoke
2. What Does Black Smoke Mean?
3. What Does White Smoke Mean?
4. What Does Blue Smoke Mean?
5. The Risks of Excessive Engine Smoke
Worn out valve train components can lead to blue smoke.
Worn out valve train components can lead to blue smoke. | Source

1. How to Verify the Color of Your Exhaust Smoke

Exhaust smoke usually happens when you either accelerate or decelerate. So it's hard to tell what's going on at the rear of your car while driving.

If you are not sure of the color of the smoke coming out of the tailpipe, you may want to ask an assistant for some help.

  1. Ask your assistant to set the parking brake and set the transmission to Neutral (manual) or Park (automatic).

  2. Have him start the car and let it idle.

  3. Allow the engine to reach operating temperature, if necessary. This will take about 10 to 20 minutes, depending on your model.

  4. Slowly accelerate and decelerate the engine.

  5. Check the color of the smoke coming out of the exhaust pipe and whether it's associated with acceleration or deceleration.

Once you know the color of the smoke, head to the corresponding section below.

A bad fuel injector can "flood" a cylinder and cause black smoke.
A bad fuel injector can "flood" a cylinder and cause black smoke. | Source

2. What Does Black Smoke Mean?

If the smoke coming out the tailpipe is black or gray, you may have one of two potential problems:

  • Fuel in the combustion chamber is only partially burning.
  • One or more cylinders is flooding with fuel.

Causes of Black Smoke You May Need to Check for

  • A clogged filter preventing the engine from breathing properly.

  • Ignition system problems (bad ignition timing).

  • Fuel injection system problems (leaking fuel injector, bad fuel pressure regulator).

  • Emission control system problems (restricted PCV system).

  • Restricted intake duct or manifold.

Sometimes, a fault in the injection system may cause the check engine light to illuminate. If necessary, download the diagnostic trouble codes (DTCs).

On an old vehicle model equipped with a carburetor, black smoke may be caused by a maladjusted carburetor. If you have the repair manual for your model, try adjusting the air-fuel mixture setting first.

Black Smoke From a Diesel Engine

Black smoke coming out the exhaust of a diesel engine, as with a gasoline engine, may indicate fuel not burning properly, or a rich air-fuel mixture, but it can also be a symptom of other potential problems. Some of the most common include:

  • Leaking fuel nozzles (if engine is running smoothly)
  • Bad automatic timing advance (if engine is running rough)
  • Injection timing problems
  • Air leak into the fuel system (if engine is running rough)
  • Restricted air inlet system (clogged air filter)
  • Bad injection pump
  • Engine overheating
  • Using the incorrect grade of fuel
  • Problems in the emission control system
  • Low compression

A blown head gasket may allow coolant into the cylinders and cause white smoke.
A blown head gasket may allow coolant into the cylinders and cause white smoke. | Source

3. What Does White Smoke Mean?

Although ideally no smoke would come out of your tailpipe, white smoke from the car exhaust on a cold day is not something to be concerned about.

However, if the smoke persists after the engine has reached operating temperature, there's something serious going on.

White Smoke in a Gasoline Engine

In a gasoline engine, this is symptomatic of serious engine problems. Most likely coolant is reaching the combustion chamber and burning along with the air and fuel mixture.

Coolant can reach the combustion chamber because of:

  • Blown out head gasket
  • Cracked engine block
  • Cracked cylinder head

White Smoke in a Diesel Engine

White smoke coming out of the exhaust system of a diesel engine is usually not as serious as with a gasoline engine.

On some diesel models, outside cold temperatures can cause smoke to come out the tail pipe if the engine is left idling for an extended period. This is not a problem.

Nevertheless, white smoke can be symptomatic of serious problems that need attention. The most common reasons include:

  • Late injection timing
  • Overheating engine
  • Bad glow plugs
  • Bad fuel pump timing
  • Faulty injector spray pattern
  • Air leaks in the fuel system
  • Thermostat stuck open
  • Low engine compression

And, just like in a gasoline engine, white smoke in a diesel engine can also mean coolant leaking into the combustion chambers:

  • leaking head gasket
  • cracked engine block
  • cracked cylinder head

Worn or broken piston rings can allow oil into the combustion chamber and cause blue smoke.
Worn or broken piston rings can allow oil into the combustion chamber and cause blue smoke. | Source

4. What Does Blue Smoke Mean?

Blue smoke coming out the exhaust system, from gasoline or diesel engines, usually means engine oil is burning in the combustion chamber.

Oil Can Reach the Combustion Chamber in Several Ways

  • Worn cylinder wall
  • Bad valve stem seals
  • Worn out piston rings
  • Worn valve guides

Also, in a diesel engine:

  • Blue smoke may come from too much oil going into the crankcase.
  • White-blue smoke is due to incomplete combustion or a faulty injection system.

Other symptoms you may notice when oil is leaking into the combustion chambers include:

  • gradual drop in oil level
  • rough idle
  • foul spark plugs
  • misfire
  • loss of power

What if I Only See Blue Smoke When the Engine is Cold?

If the blue smoke appears, only upon starting a cold engine, or after the car have been parked for an hour or more, this usually this indicates worn valve stem seals or valve guides.

After you turn off the engine, oil begins to seep through the valve seals or guides, slowly accumulating in the combustion chamber. Then, when you start the engine, the oil burns along with the air-fuel mixture and you see it as blue smoke through the tailpipe.

This normally happens on high-mileage engines.

If you want to have an idea what blue smoke looks like, watch the following video.

5. The Risks of Excessive Engine Smoke

On a diesel engine, small amounts of smoke coming out of the tailpipe are normal during engine start up, cold engine operation, or rapid acceleration.

Excessive smoke, though, from either a gasoline or a diesel engine is cause for concern.

But you can use the color of the smoke coming out the tailpipe to help you diagnose potential engine mechanical or performance problems.

When your car begins smoking, try to diagnose the problem and make the necessary repairs as soon as possible to prevent further damage. Excessive smoke coming from the exhaust of your car for long periods of time is not only bad for your engine but for the environment as well.

If you know what the problem is, you may be able to fix it with an adjustment or a relatively inexpensive repair.

This guide helps you speed up your diagnostic without spending too much time and money to get your car back on the road as soon as possible.

Questions & Answers

    © 2018 Dan Ferrell

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