Mercedes: 500 SL 1992, fuel additive, cold start

Question
My 1992 500 SL is hard to start when cold. When it does start it idles rough until it warms up

Answer
Jerry,
I too have this problem with my 420 sel. There is a 9th injector on the V8 called the cold start injector. It inriches the fuel to improve the start ability. I have concluded that because these injector are not use often enough they are subject to getting clogged or sticky. The new fuels are really bad about creating varnish in the system over time.
I would first try adding some tetron (sp?) fuel additive.
The vehicle has some many electrical inputs to the starting cicuit it is had to trouble shoot.
I am not sure that a dealer has the ability to determine the issue unless a code ispresent.
If you have not changed plug lately, that too can cause hard start and rough idle problems.
Also if you do a lot of start and stop and short distance driving without allowing the catalytic converter to warm up completely you the vehicle can develop bad engine habits.
Good Luck
Let us know how it turns out.
Rob