Always perform a compression test on each cylinder before digging into an engine with only a high oil consumption problem. Many shops just recommend doing a head job because thats how they'll make money when in fact 190Es are known to get a big improvement from just replacing the valve stem seals (check out MBs TSBs on 190Es high oil consumption). I worked on a friends 2.6l 190E car with same oil burning, spark plug fowling problem as yours. I replaced all his valve stem seals with the head in the car. It was a 1/2 day job and now, more than 8,000 miles later the oil burn and plug fowling has gone a way. The carbon in the combustion port will eventually burn away. The valve stem seals are very2 cheap.

Don't be surprised if the old valve stem seals look perfect. All of the ones i replaced were intact and only a bit stiffer from heat/age. But in the end it made all the difference with new ones. If your shop screwed up the installation then they probably didn't bother using the insertion tool (actually just a dinky plastic sleeve) that come with the new valve stem seal kit. The insertion tool prevent the valve stem from damaging the seals as they slide on. For the piddly cost you should have them put in a new set of seals and make sure they lubricate the insertion tool with motor oil.

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1972 450SL
1982 300D Turbo