I have ventured over from the diesel section....

My brother has an '89 420sel with 240K miles on it. His mechanic (not me)said that two cylinders were leaking oil past the rings and fouling out the plugs AND that a rebuild was in order. I come from the diesel world so a MB engine with 240k on does not sound like it needs rebuilt. The car runs very well but smokes ever so slightly from a stop light in the summer.

Should I dump some marvel mystery oil through the plug hole and let it soak? We think the car has sat for a long time... so I think the rings are stuck.
If the rings are junk should we try to put new rings in?

He hate's the idea of junking the otherwise nice car because of some fouling spark plugs.

Thanks

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08 R320 CDI current

Past
95 E420
87 300D Turbo 5spd
90 300TE
83 300SD
85 300TD
92 400E
85 190D

More likely than rings is that it needs valve stem seals. The oil is probably leaking in from above down the valve rather than past the rings. My 300E needed this....it was fouling all the plugs with only 133k on the engine, after new stem seals it uses 0 oil between 4,000 mile changes. Amazing difference.

Also, if that 420 has not had the timing chain and guides replaced, it needs it BAD!

And why junk the whole car because the engine needs some normal repairs/maintenance? That is the typical wasteful mentality of the USA......

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-diesel is not just a fuel, its a way of life-
'15 GLK250 Bluetec 118k - mine - (OC-123,800)
'17 Metris(VITO!) - 37k - wifes (OC-41k)
'09 Sprinter 3500 Winnebago View - 62k (OC - 67k)
'13 ML350 Bluetec - 95k - dad's (OC-98k)
'01 SL500 - 103k(km) - dad's (OC-110,000km)
'16 E400 4matic Sedan - 148k - Brothers (OC-155k)

are the guides semi easy/straight forward to replace? I am not real interested in working on his car but I guess I will

__________________
08 R320 CDI current

Past
95 E420
87 300D Turbo 5spd
90 300TE
83 300SD
85 300TD
92 400E
85 190D

I forgot to mention..

His mechanic tested these two cylinders and said they had very low compression. Now does that indicate worn rings? Can you get low compression from worn valve seal thingy's? I know nuttin about gasser MB's. When I look under the hood it looks like I am on board an alien space craft.. It takes some alien fuel called premium unleaded too.

We are not looking to "junk" the whole car for normal maintenance issues. Just for poops and giggles we asked a local shop how much to rebuild the engine and they said $4500. For that amount of money you can get another 420sel in very good shape.

So are we looking at piston ring issues or bad valve guide seals? Any ideas??

I told my brother that I would buy his car and put the om617 out of my SD in it. The 420 has beautiful brown leather

__________________
08 R320 CDI current

Past
95 E420
87 300D Turbo 5spd
90 300TE
83 300SD
85 300TD
92 400E
85 190D

Low compression is bad news.
The mechanic could/should do a leakdown or wet test just to comfirm the cause of low compression.
A salvage engine may be the most cost effective repair.

My brother says that his car seems as though it looses power at times. Like it's fast/powerful one day then slower the next. MB's are pretty rare in the yards around here

__________________
08 R320 CDI current

Past
95 E420
87 300D Turbo 5spd
90 300TE
83 300SD
85 300TD
92 400E
85 190D

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.

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I'm not a doctor, but I'll have a look.

'85 300SD 245k
'87 300SDL 251k
'90 300SEL 326k

Six others from BMW, GM, and Ford.

Liberty will not descend to a people; a people must raise themselves to liberty.[/IMG]