How do I read / interpret the timing marks on my 89 420SEL, and are there any special precautions or steps to take to make sure I'm on the specified 12 degrees BTDC ?
On a side note - it pinks like the devil - not sure if it's dodgy / bad gas from certain stations or the timing or something else. Any ideas ?
Also, #4 (R/H side closest to bulkhead) spark plug is covered in crud - the space between the insulator and plug body is almost full of build-up and it's a darkish colour. The remaining seven plugs all look far more acceptable - not much in the way of deposits and all a good colour. Just put in eight new plugs and it runs better now, but I'd like to know if I'm facing bigger problems ?

But here goes.Are you talking dark brown or black crud gumming up you'r spark plug?If so I'm guessing you'r car burns some oil also.If these are both the case,you've got oil fouling of you'r spark plug.This could be caused by you'r piston rings sticking and not sealing properly.Or it could be valve stem wear,and or a bad valve stem seal.You need to run a compression test on that cylinder to determine exactly what's wrong there.

like you have a lot of carbon build up. i would give it a dose of injection cleaner and run it hard in third gear for ten miles or so to get the carbon good and hot and hope to blow it out.

ricks comments about oil consumption are good. the one cyl sounds as if it may have those.

if your car is new to you it may not have been getting regular oil changes or maybe not good oil. perhaps a good synthetic might clean out your rings if they are stuck. broken rings in a benz seem unlikely.

non of this will help if valve guides and seals are shot.

tom w

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[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.

bit more on the history of the car - bought in July 2003 at 123700 miles, in generally very good and well cared for condition. Everything is - or looks - original and well maintained. Appears to have been used as a short trips vehicle (123700 / 14 = 8800 miles/year) so may have suffered much 'cold' running. It does indeed burn a fair amount of oil (much more than the 235000 mile 380SE we previously owned) and does around 18 to the gallon. Service history is unknown, though there are indications it was looked after by a local reputable MB dealer.
I'm just concerned that only one of the cylinders has this heavy build up of dark brown deposits - the remaining seven are almost perfectly clean. That plus the pinking has me wondering ...............

The carbon buildup the one cylinder has,is enough all by itself to cause you'r engine to knock.The carbon has probably built up enough that it has both raised the compression too high in that cylinder.As well as the carbon itself causing hot spots in the cylinder causing preignition.Tom recomends injector cleaner to try cleaning out the deposits.I've have my best luck cleaning up both fuel systems and carbon build up using Sea Foam.Either way,it's a good place to start.