Fuel check valve

[mbzman]

I have a bit of trouble starting my 300CE when the engine is either warm or hot. I have changed out all of the injectors as well as the fuel pressure regulator and accumulator. The one thing I haven't done is the check valve which I've heard being talked about. Where is is located in relation of the rest of the fuel delivery components and is it an item likely to go bad over time or are they fairly reliable?