I am encountering very similar systems to what you have indicated on your 190E. My 1989 300E have very low mileage on it and was not driven a great deal. My mechanic conducted a fuel system pressure test and found that there was a leak in the system. He was able to isolate it to the front of the car versus the rear where the check valve would be (additionally, he indicated that the defective check valve would account for a more rapid loss of pressure in the fuel system). He then proceeded to remove one of the injector and noticed that the spray pattern was irregular. At this point, he suggested cleaning the engine with a special machine which ties into the fuel system. The intention is to clean out the residual build up in the engine and the injectors which may be preventing them from closing properly (which would account for the pressure leakage in the system). This was done several days ago and I have noticed some difffernce, but the car is still difficult to start.

The next step is to replace the injectors.... Will let you know how that goes.

If it were my car, I would first do a double-dose of Techron Fuel Injector Cleaner in less than a full tank of fuel.

The 300e with 240K miles was starting rough, shuddering in warming up and starting to get poor fuel mileage. I replace the cap, rotor etc etc etc and then did the "double-dose" and now runs great.

Hey, maybe this isn't the problem but it sure is an inexpensive way to find out and your injectors are probably dirty anyway.

Haasman