I have a family owned 1988 560 SL 38,000 miles. About 2/3 of the time when starting from cold it has a very loud knocking sound that lasts for about 2 seconds. Starts perfectly when warm. It always starts with no problem. Idle is good but goes up and down 150 rpm. The other thing it is doing is the engine will cut out under very hard acceleration. Fuel filter is new.

 

The knocking sound is most likely the timing chain hitting the front cover due to a chain tensioner that no longer holds pressure when the engine is off. You had better check this as well as the plastic chain guide rails. The chain, the tensioner, and all of the plastic guides need to be replaced ASAP if not already done. The issue is age, not miles. The idle issue is likely due to a sticky idle air valve or a vacuum leak. The cut out could be ignition related as high cylinder pressure can affect the spark with a weak system. The timing chain system and camshaft oil tube fittings (also plastic) are must-do items on these cars.

 

Thank you so much for this great reply. It is clearly time to go through the car and bring it up to date. Low miles but lots of years! I will follow up and post pics if what I find. Going up on the stands right now!