Why It’s Important to Prime Your Diesel Fuel System After a Filter Change

After swapping out your fuel filter, it might be tempting to just turn the key and go. But when you’re dealing with a diesel engine—especially one equipped with a Bosch CP4 high-pressure pump—that’s a shortcut that can cost you big.

Here’s why priming the fuel system after a filter change is something you should never skip:

1. Prevents Dry Starts and Pump Damage

When you replace the fuel filter, the housing and lines are full of air. If that air reaches the CP4 pump, it can run dry for a few seconds before fuel reaches it. The problem? Diesel fuel also acts as a lubricant and coolant for the pump. Without it, internal parts grind together, causing metal wear or even complete pump failure.

2. Keeps Fuel Flow Consistent

Priming fills the new filter and lines with fuel before the system pressurizes. That means smoother starts, steady fuel pressure, and less strain on the high-pressure side. It’s a simple way to make sure everything stays happy and reliable.

3. Protects Injectors and the Entire Fuel System

Air pockets and debris can wreak havoc on your injectors and high-pressure rail. Properly priming after a filter change purges trapped air, helping prevent misfires, rough idles, and long cranks.

4. Long-Term Reliability

Taking a few minutes to prime isn’t just good maintenance—it’s insurance. It keeps your fuel system clean, lubricated, and ready for years of reliable use.

Extra Protection with an RCP CP4 Bypass Kit

Even when you do everything right, the CP4 pump has a reputation for self-destructing if something goes wrong internally. That’s why we make the RCP CP4 Metering Valve Bypass Kit—it isolates contaminated fuel from the high-pressure side, filters it, and keeps clean fuel flowing to protect your engine.