My 1990 190E 2.6 will try to start everytime you turn the key, however, only about 10% of the time will it stay running. As you turn the key to start position it attempts to start then immediatley dies. After returning the key to the off position and them making a second attempt it will start fine and remain runing no problem.
I did a search and did several small things. New coil, plug wires, plugs ( standard Bosch- not platinum), new rotor and cap. New oil change and have dumped in a bottle of Techron to clean our engine deposits.
Still, rarely stays running first try. Almost never dies second aattempt. Could this have something to do the cold start valve? I need advice...I have run out of option

Maybe?
 

Chris,

My '91 190E did the same thing, except that it would take alot more cranking to start it. After starting it would be fine.

In this case, it was a leaking cold start valve but it took a long while to get to it, diagnosis wise! Changed same things you did but no help. Good luck!