Tuning a diesel engine is not like giving a petrol-powered car a tune-up; in a car driven with standard petrol, a tune-up means changing the spark plugs, but diesel engines don't work with spark plugs. Tuning a diesel engine involves connecting a special piece of equipment, called a tuner, to the vehicle's computer system that controls how the diesel engine functions. This tuner can then make adjustments to those computer commands, "tuning" how the engine and its many components work. Note when you should have this done with your diesel engine and why it can be so beneficial.
When the engine has been modified
If you've added a turbocharger, dual exhaust pipes, or any other custom part to your vehicle, you want to have it tuned. The computer should know to take these changes into account and adjust the level of fuel it pulls from the tank, the amount of oxygen it will now need, and so on. Without a tuning, these parts may not give you the results you expected, as the computer may use old commands to control how the engine functions, even with these upgrades or modifications. You also want to have this tuning done if you've added any significant weight to the vehicle, such as pulling a large trailer behind it, so that the engine provides more torque to compensate for that added weight.
Your fuel economy is suffering
If your engine seems to be using more fuel than usual, it may be time to have it tuned. If there is some type of build-up in the engine or certain parts are worn, the engine may not be getting enough oxygen or is getting too much oxygen to mix properly with fuel and create combustion. The engine may then be consuming more fuel to force combustion. A tuning can adjust the oxygen to fuel ratio so the vehicle uses less fuel while still maintaining combustion.
The engine is struggling
There could be many reasons why your diesel engine struggles to start or sputters and stalls, including worn out glow plugs, poor-quality diesel fuel, or build-up in the engine. However, it may also be that the engine is not getting enough fuel as it operates or is not creating enough torque to pull that heavy weight, as mentioned above. The longer you ignore this problem with the engine, the more wear and tear it suffers, so have this tuning done as soon as you notice the engine is struggling in any way.