Fuel System Internal Leaks
WARNING: This page does not describe the selected car, but rather 6 other vehicles, including the 2002 Mercedes-Benz ML320, 2002 Mercedes-Benz E320, 2002 Mercedes-Benz CLK320, 2001 Mercedes-Benz ML320, and 2001 Mercedes-Benz E320. However, it is still accessible from the selected car via links, so may be relevant.
Connect a fuel pressure gauge to test port. Start engine and idle for approximately one minute. Turn engine off and observe fuel pressure gauge. Fuel pressure should be more than 44 psi (3 bar). After 30 minutes, fuel pressure should be more than 36 psi (2.5 bar). If fuel pressure quickly drops off, possible cause is faulty check valve at fuel pump. Repair or replace as necessary. If fuel pressure slowly drops off to less than specification, possible cause is leaking fuel injector(s) or faulty fuel pressure regulator. Repair or replace as necessary.