why won't my 1986 mercedes 420 sel start when engine is cold?

possible your fuel pump check valve or cold start injector are bad

As others have stated spray some penetrating oil on the backside of the spark plug and spray countless it in there. If I even have been you i'd desire to warmth up the engine first to have extra effective smart consequence with the oil. The penetrating oil will cool down the spark plug, the penetrating oil will warmth up and overall performance a miles better smart risk to artwork its way down the threads for the spark plug. you may consistently place a leather-based jacket or something on the heads so which you will not burn your self. using the splendid suggestions of course with a ratchet that has an extraordinarily long handle, whether you will have the skill to desire to stay with a cheater pipe. What you desire is controlled rigidity, with the cheater pipe you have the leaverage with minimum rigidity utilized. attempt to tighten the spark plug using approximately 30 lbs of rigidity on the top of the cheater pipe. Then attempt to loosen the plug with the comparable quantity of rigidity. Repeat the attitude 3-5 circumstances. If it hasn't broken loose, carry on with extra effective penetrating oil if mandatory and allow it sit down down for a pair of hours and then repeat the attitude. under no circumstances would desire to you utilize direct rigidity (effect rigidity) like an effect wrench or a hammer on the ratchet. This has a severe potential of ruining the threads on the heads.

just do a simple tune up, spark plugs, wires, fuel filter, and the timing may be a little off, to increase the idle loosen up the distributor, and turn in until you idle gets higher,

voltage to cold start valve dont show 12 volts Overvoltage relay gives the voltage to the cold start valve needs relay

If it has a "cold start" fuel injector, make sure it's working.

Try replacing the spark plugs.

The automatic choke plate needs to be adjusted so that it works(which it doesn't at this moment)

You may find it is fitted with an inhibitor.