Using long needle-nose pliers or a special tool, unscrew the lid holding the fuel filter in place. | 
Lift out the old fuel filter. |
When the old filter is out, flipping this yellow tag will allow fuel removal and, more importantly, any water to exit through a drain on the right front of the vehicle into your waiting drain pan. | 
Prime the new filter with clean diesel fuel or a diesel-fuel conditioner. |
The seal for the fuel filter may be a square-cut seal that installs either side up, or it may have a triangular cross section that installs flat-side down and pointed-side up. | 
Install the new fuel filter. Cleanliness is key due to small orifices throughout the fuel-injection system. |
Use Great Caution-Extremely High Pressure!
These diesels sometimes need tender loving care. They are no problem to keep running if you know what you're doing, but proper training in diesel diagnosis and service is absolutely essential. It could be worth your life. No matter how good of a wrench you are, that isn't a gasoline engine under the hood of your diesel truck, and if it needs service, take it to a specialist. Why? You have to use great caution when working around diesel-fuel pressures. Today's common-rail-type diesels run up to 29,000 psi. You can no longer use the old technique of loosening an injector line to check for a dead cylinder. Doing so could end your life! At that extreme pressure, the fuel could easily cut through you!
A look at the Diesel engine
First, let's take a look at what makes a diesel a diesel. You might be surprised to find that none other than Rudolf Diesel, a German engineer who developed the idea for the engine in the late 1800s and patented it in 1892, invented the diesel engine. Diesel sought to build an engine with greater efficiency than the crude gasoline engines of that time, which were notoriously inefficient.
The diesel engine, unlike the gasoline engine we all know so well, is a compression-ignition engine. In this type of engine, the diesel fuel is ignited by being injected into the cylinder, which contains extremely hot pressurized and compressed air.
This "diesel cycle" can use many different fuels, and Diesel's first working engine, demonstrated at the 1900 World's Fair, ran on peanut oil.
Today's diesel engine uses Diesel's concept of the fuel being ignited by introducing it into a very hot combustion chamber filled with compressed air at extremely high cylinder pressures. The diesel fuel is delivered to the combustion chamber by pressure-activated injectors in a highly atomized state.
When a gas is compressed, its temperature rises. Air is drawn into the cylinder of a diesel engine and compressed by the rising piston, and the air temperature is 700-9,000 degrees Celsius or 1,300-16,500 degrees Fahrenheit. The fuel is injected into the combustion chamber at extremely high pressure through an atomizing nozzle and mixes with the hot, high-pressure air. The resulting mixture ignites and burns very rapidly, and this combustion causes the gas in the chamber to heat up rapidly, increasing its pressure and forcing the piston down.