I pulled this off another board -- hope it helps some 190 owners out there if this isn't already common knowledge.

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If your Mercedes 190E is experiencing hard starting, check to make sure the
vacuum hose from the idle advance is connected to the bottom of the
throttle housing. I've seen cars run with the fuel mixture at full rich
with this hose disconnected, usually because the vacuum hose was bypassed
by a green mechanic and confiscated fuel for air to make the car run.

Generally, if the hose comes loose, your car will stall and not restart.
Reconnect the hose and try to start it again. If the car fails to start,
your fuel mixture needs to be reset. It takes expensive machines to get the
exact mix but you should be able to back off on the mixture by depressing
the 3mm Allen screw key down and backing off counter clockwise on the fuel
mixture located to the right side of the fuel distributor and left of the
throttle plate (may be capped). Once your car is started, you should get it
adjusted for emissions by a certified mechanic.

If the cap is still in place, then the mixture wasn't played with and
should be left alone. At this point, the fuel distributor should be checked
for a sticking plunger. Remove the injector lines at the fuel distributor
and remove the fuel pressure regulator line to the fuel distributor (three
retaining or torque screws). Remove and check that the rod or plunger moves
freely up and down. Service if needed and replace the connections.

A cold start valve or fifth injector will not cause a "non-start" condition
in the 190 but it may make the car harder to start. If you have the 2.3
and it's hard to start but runs fine otherwise, you should replace your
cold start valve. If your car starts fine but runs rough and stumbles, a
clogged injector may be your problem.