How a Colour Test Can Help You Determine What's Wrong with Your Diesel Engine

Posted on: 23 June 2017

The modern-day diesel engine in your car is a complicated device, and it can sometimes be quite difficult to diagnose any problem that you may come across. While a special computer is normally plugged into the vehicle to pick up some clues, you as the driver can often predict what may be wrong by assessing the vehicle's performance and looking for some visual and audible pointers. What can you tell your mechanic to help them pinpoint the issue?

Where to Look First?

The first place to look is not under the bonnet, but at the back of the vehicle.


Have a look at the colour of the smoke that is coming out of the exhaust pipe. If it is black, then this means that the fuel to air ratio is out of balance. Too much diesel is in the mixture or the engine is being starved of some oxygen. In this case, a dirty air cleaner could be to blame. It's also possible that the injector pump needs attention, or the injectors need to be cleaned. The mechanic may also have a good look at the turbocharger or intercooler, while the exhaust gas recycler could be malfunctioning, causing a timing issue with the engine valves.


White smoke from the exhaust typically means that the fuel is not being burned properly within the engine. This will signify that the motor is running with low compression or that water has infiltrated the mixture. The fuel pump might be misfiring, as it is receiving an insufficient volume of diesel.

It's not unheard of for people to make a mistake when fuelling and add petrol instead. This would certainly result in white smoke, but do not breathe this in as it is toxic.


Blue smoke suggests that the engine is burning much more oil than it should. Has it been overfilled by any chance? If not, perhaps the stem seals at the bottom of the valves need to be changed. Another place to look is the injector pump, which could be allowing oil to seep into the diesel fuel. If you're really unlucky, the issue will be within the engine itself, where worn piston rings could be allowing oil to escape.

Taking Action

It's amazing how this simple colour test could help you narrow down the problem. Take the vehicle in to your mechanic for them to verify your findings.

For more information, contact local professionals like those found at Cummins South Pacific.