560 SEL - Hard to Start

[jgl1]

1991 560 SEL, 150 K miles - over the past two months the engine has become increasingly difficult to start, most noticeably when at operating temperature... engine can be running, shutoff and then a restart attempted... but the accelerator pedal has to be depressed fully and the engine cranked for several seconds before it will start. Once running, the vehicle's performance was a bit sluggish, with a noticeable stumble when moderate acceleration was required. Occasionally, when hot, or frequently, when cold, the engine would start immediately without any need for depressing the accelerator pedal. Inspection of spark plugs reveals a rich mixture. Plugs, distributor cap, rotor and air filter are new. Cam and crank timing is correct, engine vacuum lines and connectors are intact and free of leaks. Check engine light is not illuminated and on-board diagnosis with pulse counter reveals no fault codes. Oxygen sensor appears to be the factory original.

Replaced the fuel pressure regulator, both fuel pump check valves and fuel filter. Performance is significantly improved but difficulty in starting still remains. Installed an EHA of known quality, with no noticeable improvement.

Can a faulty fuel accumulator result in a difficult hot restart attempt? Any suggestions as to the source of this starting difficulty?