88 300E 270K Km

One of my lovely 300Es has developed a new headache. It started a month ago when I noticed a miss and the number 6 plug kept fouling. Wet, black carbon buildup when cleaned off, the motor would return to that nice purr we all love to hear. It has gotten progressively worse but/and it is still intermittent. I've swapped distributor cap and wires, new plugs etc and at this point I'm thinking ring, lifter or valve guide. Very on and off, I get some serious blue smoke which might occur for 3 or four minutes and desist.

Clean the plug again and until this week I'd get a couple of weeks smooth sailing. Now I'm cleaning every other day. There is no valve noise of any kind and I've not been able to do a compression test yet. I ran some Lucas additive when this first started about 2 months back and it cleared up briefly. If it is a lifter would there not be noise associated with? I'd hate to find out this was a broken ring but doubt it would be since the problem comes and goes. Are any of those additives any good?

This is the 300E I was having problems with the OVP and FPR which really isn't completely resolved but I've cleaned and checked all the sensors and idle sensors and the idle isn't as bad as it was. Only thing left to check is the O2 sensor in the next day or two.

This miss is unrelated however IMO, it's only that one cylinder, so I'm wondering what suggestions anyone might have. I plan to sell this car asap since I've got the other running well but I'd like to at least have a better sounding engine than it currently is to get a half decent price.


tia