Ulfi, sorry 'bout my mechanic statement... I was referring to my experience in US.
The oily gunk and deposits on the fuel nozzel you replaced adds to the probability that the combustion chamber in Cyl #5 is fouled with excess carbon deposits which changes the combustion process within the chamber, leading to one very well known source of "pinging"... uneven explosion during combustion. It doesn't prove it, but is additional evidence of a detonation source problem along with the fact that removing the spark eliminates the sound.
Given that the prior owner told you the pinging sound was "always" there, the question then becomes what condition exists or existed to create the excess carbon deposits in the 1st place. A fouled plug or poor spark at the spark-plug gap would aid this process over time..... and once the carbon deposits were established, the likelyhood of an improved spark or plug condition wouldn't materially undo the deposits. A quick check of the spark plug deposits would tell you whether the currently used plugs are also getting fouled prematurely or at all in Cyl. #5. If so then it's may be due to a poor seal on the oil ring (either worn, cracked, or sealed to the piston ring gap).
If, on the other hand, the spark plug points are not being fouled on your current plugs and driving, then there would be no reason to suspect a worn or broken oil ring having caused the initial combustion chamber assumed excess carbon deposits, so it could be simply that at one time the prior owner had a poor plug or contact to it for some period of time while the carbon built up in the chamber... and since then he "always heard the sound".
If the distributor cap hasn't been replaced, you can check the contact for roter to Cyl. #5 contact on the distributor cap for signs of arcing.... an indication of an inefficient transfer of electrical conduction from rotor to the Cyl #5 distributor contact.... anothe poster suggested this previously.... waqas.
The point is that something's causing the Cyl #5 chamber to foul or having at one time been caused to foul and assumedly then having create excess carbon build-up within the chamber... on piston surface, valve edges, etc. ... leading to the uneven detonation in that cylinder without being created in the others.
By the way, the higher than normal CO2 emissions would be expected from a poor detonation condition in one cylinder (which is producing ~17% of the total emission if all cylinder's combustion were evenly distributed and equal... so if Cyl. #5 is less efficient in combustion process, then it's going to produce greater levels of C02 than the others, leading to higher than normal C02 emissions ... certainly not the only reason for higher CO2 emissions by a long shot, but still in the direction of expectations with poor / uneven detonation process in Cyl #5.
In other words, all the symptoms described thus far are consistant with a detonation problem, uneven detonation more specifically, in Cyl. #5. If the combustion chamber is found indeed to have excess carbon build-up on valves &/or piston head, then the issue that will have to be dealt with ultimately is the source and cause.
Ultimately the source will be due to either just poor spark (see distributor cap issue above) which caused the excessive carbon build up in the 1st place... having a good spark won't fix it, so the excess carbon has to be removed in any event, or given the fouling condition noted on the fuel nozzel, a possible oil ring seating, wear, or crack in that ring of that cylinder. If the latter is found ultimately to be the source, then all other cylinder's are equally suspect in having similar wear, or aging to lead to cracking or sticking to the piston ring gap it's supposed to slide & rotate in (though not sufficient to have created enough fouling and carbon build-up to cause "pinging" yet).
By the way, another source for excess oil in the combustion process are the valve guides... excess wear.... which would definitely lead to excess carbon build-up on the valve edges. If that's the case then the head needs to be reconditioned... new valve guides, valves, and valve seats ground.
Coincidentally, perhaps, when I bought my 250SL in '84, Cyl #5's spark plug was fouling so much that I had to clean it or replace the plug every thousand miles or so... after driving the car for about 5-6k miles it got so bad that I had to clean the plug every 500 miles or so... a process I quit doing by not driving the car for the next 15 years or so... just parked it one day after work in it's spot in the garage and forgot to drive it again for 15 years. When I finally had the engine rebuilt, the oil ring in cylinder #5 was cracked... the mechanics (a pure MB shop specializing in the SL's and other 60's era MB's) told me that Cylinder #5 for some reason is the one most prone to problems with ring wear... they thought it was due to Cyl #5 heat dissipation or Cylinder warpage being geater than in the others. I don't know if their statement has merit in fact or whether they were just trying to get me off their back with any old explanation that sounded good... but ...this is on the 7 main bearing 2.5 liter engine. It's certainly possible for the engine heat dissipation to be uneven from one cylinder to another and for one in particular to have issues, but I can't say this is a fact for the 2.5 or 2.8 liter 7 main bearing engine or not. Food for thought though.
