What to do (and when?) on '91 420SEL
 

Car is in great shape, still gets 17 mpg in city on midgrade gas. Uses very little synthetic oil between changes. Plugs are not fouled. No blue smoke. Very small oil leak evidence at rear of driver's side head, not enough to drip but enough to see a dark spot. Tranny runs fine & shifts are smooth & great.

Getting somewhat close to 180K major interval: PO had timing chain, tensioner & guides all done @103K in 2000. Not hearing any real rattle at startup, runs very smooth and especially strong at upper end (as when flooring it occasionally at 35 or so and running it up to 100 mph). Maybe a very slight idle "tick" when really hot outside (for Portland, that would be a rare 90+ degree day) and car completely warmed up, a/c on, etc.

Not enough to bother me but just enough for me to notice.

QUESTION: should I just (A) have the timing chain guides, tensioner, etc. work done at the 180K interval or go all out and (B) have the all the top end work done, i.e. heads rebuilt?

What is a decent "ballpark" fair e$timate of the costs of (A) vs. (B)? On the one hand I don't want to service the chain and guides only to have to rebuild the heads a year later, nor do I believe it makes any sense to just "RUN IT UNTIL IT RATTLES" and then be faced with a blown engine &/or a bunch of broken parts that are not broken now.

I guess there is also option (C) which would be to wait until 200K or so...

Cheers!

There are some maintenance tips to be found in the buyers guide for the 560SEC that I co-wrote back in '98, and this specifically is mentioned in there:

www.mbcoupes.com

It's not a bad idea to have the timing chain, tensioner and guide rails done, at 100,000 miles, as preventative maintenance, and maybe the cam oiler tubes and injectors if they need it.

The guid rails are white plastic. If you take off a valve cover, and look and you see they are root beer brown, probably they are past their "sell by" date, and should be replaced, they get hairline cracks, and will crumble soon, and you know the rest. On tyhe 560 they let go more often on the left side.

You can gamble and wait, but without records, on your car, proving this has been done, yours is probably way past due, and if you hear timing chain rattle on start up, you could be seconds away from a major catastrophe in that scenario.

I have heard the 420 is less critical than the 560 with regard to this, but in any case I wouldn't wait on this one.

JimB. Thanks for your reply, but please note that the "..PO had timing chain, guides & tensioner DONE @103K in 2000".

I'm wondering if I'm being paranoid in thinking that I should get the entire top end redone at 180K vs. spending a lot of $ I don't have to prematurely...

Mostly looking for the ballpark $ figures for

(A) timing chain, guides & tensioner service at 180K $?
vs.
(B) topend rebuild of heads, etc. $?
vs.
(C) just waiting until 200K (RISK factor vs. $?)

Thank you, again, for your most prompt reply.

Cheers!

p.s. I did read your mbcoupes.com & all relevant search threads on "timing chain" (over 250 postings!), "head gaskets", "head rebuild", etc.

Replace the rails. They are the cause of all failures. We go 5 years or 100k whichever comes first. I wouldn't do anything else except maybe to pull the rocker arms and see if they are in need of replacing. They will cause the ticking sound if worn.