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1988 420SEL
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Hello, my 1988 420SEL (126k miles) shakes sometimes right after a start and only after a start... this is standing still of course. After maybe couple minutes of driving It idles quite well. Already replaced distributor cap and rotor and ignition wires.

Please help, thank you.
 

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9,909 Posts
Sounds like weak initial fuel pressure which could be injectors , pump , fuel distributor , fuel accumulator , fuel filter etc
 

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350SDL, '17 GLS450, "Grandpa's Roadster" Project Car
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3,647 Posts
I would first check the injector spray patterns. A warm engine is much more tolerant of poor atomization than a cold one.
 

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a lot of fine running cars , with high mileage and injectors are not spraying that great when you pressure test them .what does not work that great is when they stick open , do you know what happen then ????. and how you diagnose that on the car , waiting for response .
 

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1991 560sec. 1991 560sel 1969 280SL
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1,571 Posts
Now is the time for you to shine and dazzle us with your superior knowledge of motor vehicle technology of years gone by. Enjoy the forum, your experience is apparent and it’s great you are sharing it with us. No need to be so thin skinned, life is too short for that.
 

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602 Posts
Now is the time for you to shine and dazzle us with your superior knowledge of motor vehicle technology of years gone by. Enjoy the forum, your experience is apparent and it’s great you are sharing it with us. No need to be so thin skinned, life is too short for that.
i dont know how you fix something like that on the phone .first thing i would have done is a quick ignition scope test [ still have a scope ] .i had years ago 2 cars with that issue , cars ran perfect hot .when you first start car ran on 7 cyl [ both 420 /126 ] for some minutes then was fine .did notice then while cranking cold it sounded like one cyl was not quite there .so , warm up car , did a compression test , near perfect 170 , next morning did same , all near 170 except one at 110 . did put that cyl at tdc , valves closed , put some air pressure , and air in this case coming out of tail pipe .so leaky exhaust valve , replaced the rocker arm pad with next size under [ 3 sizes available ] , problem fixed .the cause was a exhaust seat / valve that was wearing , so the valve climb up , and hydraulic lifter run out of room . now why cold only .the camshaft is on aluminum towers , when cold they shrink , so the camshaft move down just enough to let the valve close all away .hot it rise .the quick way to tell is with valve cover off , if you look at a cam , only the the part that actuate the valve is shiny , the round part has a color to it , on a tight valve it is shiny all around .now , that will be $ 20 .happy hunting .
 
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