For those that prefer the reliability of running a mechanical fuel pressure gauge, you'll want to use this isolator kit. Without the use of an isolator, you're literally running pressurized diesel fuel into the cab of your truck, obviously not a good idea. With the use of an isolator, such as this, you'll run fuel to the bottom of the diaphragm in the isolator, then typically coolant on the top to the back of the gauge. Once the system is bled of any air, it will deliver reliable readings up to 100PSI.