PeachParts Mercedes-Benz Forum - View Single Post

Hit Man X · #7 01-31-2010, 10:17 PM

It will need a valve job at 240k. Stems seals will not help. I would budget about $1k for head work locally with parts (springs, seals, guides, etc) plus whatever gaskets run. You really run into the "while I am in here syndrome" but I would yank the front cover and do all the lower rails, oil pump chain, etc too. M116/7 bottom ends last and last, but top ends do wear out. From what I recall they have some sort of silicone impregnated sleeves like BMW's M70 V12.

Remember Diesel has no engine vacuum so oil cannot really be sucked past the valve guides... even those could use a valve job at 250k to restore lost power.

I also agree with "Chas H", you need to perform a leak down test also to verify the condition of the top end. I am almost certain this is where the issue lies. My 300SEL fouls a few plugs often as the top end needs a rebuild badly. $1k later and off I go for years to come... that is not even a down payment on a new vehicle.

Also I concur with "pawoSD" on the chain/guides/tensioner issue. Unless the guides are white, replace them. MEYLE makes a set that are aluminum with some sort of rubber coating... cost is a wash over OEM ones. This basically eliminates the chance for upper end failures that occur with jumped chains. New chain obviously brings the cams in time for normal power.

420SEL's are the bargain of W126s to me. Good power/quicker than most of them, adequate fuel economy, simple to fix, less bells/whistles to fail, etc. Plus with yours being an interior face-lift Gen II, it is more desirable than the early cars.

If the body is clean (nice paint, no rust) and the interior is nice, I would rebuild the top end if you plan to keep the car a while. Really just depends on the overall condition of the vehicle if you want to warrant the cash expenditure for the repair.

I would not repower it with a 617, but that is me. I would purchase a 603 vehicle if you want LWB Diesel.