Hello friends! I am the owner of a Mercedes C180K W203! For almost a year I have problems with starting.I have a long crank and finally the car starts.Today I have made a pressure test and with the ignition on the pressure is about 47PSI but right away it drops to 0 PSI.When I turn the key and starts cranking the pressure rises slowly and when the pressure reach about 45 PSI,the engine starts.I shut off the engine and pressure drops to 0 . I put a link with the test:
Any advice is important to me.Thank you!

Looks like your fuel system is not holding residual pressure, may be the fuel pump dying or very clogged filter as you said...assuming you have no leaks..

The fuel pump is new, so I am sure that it is not the problem. If the fuel pressure regulator on the rail goes bad, can he drop the fuel back in the tank?

Bad regulator. I believe on that system it's part of the fuel filter. The check valve that holds pressure when the pump is off is failed.

I know for sure that I have the fuel pressure regulator on the fuel rail. I will try to replace it and hope the start problem will go as away. Thank you!

Today I had replaced the pressure regulator on the fuel rail but the problem was not solved.I will try to change fuel filter,I will buy MANN WK 720 fuel filter with integrated pressure regulator.Hope that will solve the issue.

Pump was replaced? Perhaps it's faulty.
New? from where? China?
Always go back to the last you did.

Fuel pump is new! I replaced it 3 months ago.

What year is this car. Does it have an external fuel filter or an in-tank unit?

did you even read the post above this one?

The car was made in 2003. The fuel filter is external. It is located under the car, close to the rear left wheel.

You know for sure that the regulator is on the rail! Except you are wrong. It's in the filter. There are only 2 ways your problem can occur.. bad pump or bad filter.. or a 3rd option. Bad workmanship when u replaced the pump.
Good luck

Precisely my point.

Glyn! You old tart! Nice of you to drop into the Mercedes W203 ghetto....
ha. Even my lady was asking about you....no joke!
Is there a 'lady's man' reputation I don't know about spreading all the way from the Dark Continent to my humble abode?
(As it was called by author Henry M. Stanley for it's mysterious and largely unknown features in the 1800's.
Figured I'd better qualify the term for all the uneducated heathens out there
who might mistakenly take it for something derogatory. )

C230 coup, I am also at the bottom of the dark continent. No worries we all good here, except for a lack of water.

The problem was solved with the replacement of fuel filter.With MANN WK 720 filter all is ok!Thank you!

Ah. Well, definitely something that should be done in tandem with a FP if the filter hasn't been replaced recently. Every 60K miles according to MB. Probably why the old pump broke.