I have recently discovered a problem starting my ML350. Often (not always), it will crank for 2-3 seconds before firing up. Other times, it will fire up right away. I have replaced the spark plugs and cam shaft position sensor, but it still has not fixed the problem. Any suggestions? Would the fuel filter be the last logical step? Is it possible that there might be air in the fuel line that affects the pressurization of the gas in the line?