Shakey Start
 

My '91 560SEL W126 has an annoying starting issue. On turning the key to start, it will fire immediately then die. If I keep the key turned or if I turn it off and start a second time it will catch and run, but the engine and the whole car shakes for about 3-5 seconds before it smooths out and runs normally. It is worst from a cold start and is hardly noticeable on a restart after the engine has been running for a while.

I have replaced all the spark plugs, distributor cap and rotor with OEM spec parts (Bosch) and the leads are fairly new. Other than the initial start the engine runs very smoothly and seems to be producing normal power.
Any ideas?

Any ideas anybody?
Could it be a bad Fuel Injector?

I am having the same issue on my 420sel. My mechanic says it could be the cold start injector.

I just had all 8 of my regular fuel injectors cleaned and tested and they are all working correctly.

Also try this...

my car seemed to fire fine this morning after i forgot to attach these connectors after replacing some things.
If you look at the top of the radiator and follow the big pipe, theres 2 or 3 electrical connectors attached to the metal piece at the end of that coolant pipe - i believe they are temperature sensors. Disconnect that and try to do a cold start. Does it start better?

Could be fuel accumulator.

Thanks guys.
I will check out those items.