267,000 miles, been in the family since new, but puffs like a tea kettle out of the valve cover with the cap off. Been doing this for at least the last ten years. Should I be concerned? I just keep driving it. Uses about a quart of oil every 500 miles pretty much for the last ten years. How long will this engine last?

Probably another 10 yrs at least.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

Just keep the oil changed, adjust the valves every 15k and drive it. The only fix is to tear it down and do rings at the least, or possibly oversize pistons. Wait till the oil consumption gets to be too much.

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Chief

1991 350SD

During the warmer months try straight 40 diesel oil. The oil consumption will drop somewhat. The higher viscosity or thickness of the oil may extend the engines remaining life even further by providing a better barrier between worn parts.

From My notes:
Marvel Mystery oil results sticky rings
Original thread also has + response
Compression Test...how low is low..... - PeachParts Mercedes ShopForum

Well it loosened the carbon as the compression has increased in the two low cylinders! # 3 was 320 is now 380 and #5 was 160 and is now 340! Drove it about 20 miles hard and fast, there was some carbon coming out the exhaust! Will be taking iton two longer trips this next week: Monday I'll be going over to the coast (SLO) and the SoCa gtg on next Saturday. The trip over the Grapevine should help!
Email response about if it reduced blow-by
The blow by was reduced and also with the nice drive to San Luis Obispo my rpms are lower than they were before! Before 65@3,250 now 2,850, before 70@3,500 now 3,150? and boy does it have power... but the glow plug light is now out (starts ok but the weather is warm. The relay works, so I'll be checking the plugs) and the fuel gauge is dancing (ground?). Oh, well gives me something to do...

Some have said that switching to Synthetic Oil and after driving 200-300 miles also will unstick Piston Rings if the happen to be stuck by Carbon.

Depending on the condition of you Valve Stem Seals; poor ones can contribute to the Oil consumption issue.

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84 300D, 82 Volvo 244Gl Diesel