Loss of rest pressure due to bad check valve!

I had something similar that may help you diagnose your car's problem. I
own a '97 E320 (straight six). After about 15K miles I noticed that hot
starting was sometimes delayed; cold starts were fine. If the hot engine
stood for say an hour or three the start was delayed and the engine
started like it was "flooded". On such starts the exhaust reeked of
fuel. I checked the fuel pressure regulator but it was OK. By 36K miles
I found the problem.

The fuel rail's pressure is about 45 psi.; one fuel injector was leaking
fuel into its intake port after shut down. Fuel vapors filled the intake
manifold and so over fueled ("flooded") the engine; excess fuel seeped
into #6 cylinder and wet its spark plug causing some initial misfiring.

The repair was to replace, in this case, #6 fuel injector - 2 hours
labor and about $140 in parts. The engine now starts (hot and cold)
perfectly, as it did at the M-B factory in 1996.

I hope this helps you.