Leaky valve seals
 

Recently purchased 1991 420sel with 170k. My local MB mechanic confirms that the reason I get a big puff of light blue smoke at startup and continous smoking is because the valve stem seals are worn. Maybe one, maybe all. I can do this myself and would like to do it without removing the heads but just taking the valve covers off. In addition to the seals and springs what else do you recommend that I replace while I'm in there. I'm a novice but mechanically inclined with tools. I suspect I could do lifters and I've heard talk about rails and tensioners. This is not a daily driver, so I have time. I just want to replace the easy things under the covers while that part of the car is open.
BTW the car runs good, it just smokes.

There are many opinions here. Those with $ will recommend a 'might as well replace it while you're in there' approach, usually without regard to testing the component, just an 'on principle' thing since the engine may have lots of miles or is just 'old', or when you have to pay someone else to tear it down for reason A, so might as well take advantage of the labour $ spent and put in the parts that require the same time to get at them.
I don't think replacing the valve springs is worth the $, as I never heard of them failing or wearing out on the M116. On the other hand, if you have ticking at one or more cam lobes, this would be a good time to isolate the culprit(s) and replace the HLC(s) and maybe the rocker(s) and/or shims(s). You'll need to educate y'self on the procedures, all well documented on this forum.
Removing the valve covers is relatively minor work, but you should inspect the upper chain guides and tensioner/guide rail while you have the opportunity, and do a test to determine timing chain condition. See the DIY section for details.
With the right spring compressor, the valve stem seals are not especially hard to do, just a tedious job, and potentially frustrating if you're not very careful when removing the 'keepers'. While doing this, check the condition of the valve guides too. One worn guide and you're into head removal, I believe.

Many, many thanks to all that have contributed to this forum. I have read hundreds of threads here and saved thousands of dollars. When I opened the valve covers, I found almost 1/2 the left upper rail just lying on the chain which after measuring it had 10 degrees of stretch. Boy did I dodge a bullet by not driving it. So with your help I installed timing chain guide rails on both sides (upper and lower), new tensioner and rolled in new timing chain. After putting every thing back together and re timing it, it purrs like a kitten. (not bad for a first timer) :)

Problem is, it still smokes. so back to the leaky valve seals. What spring compressor do I need to buy and how do you remove the rocker arms. This is a 420sel so do I need to remove the cams, or how is this done. Can't seem to find the right thread for this engine.

Thanks

170k is getting up there in miles (if it's true miles, VDO odometers always break), if the car's a keeper... I'd just pull the heads.

I wasted my time doing stem seals on my M103 and the guides are well worn past the point of stem seals helping.

Yes, I'm sure it's not what you want to hear... but it's what I'd do since stem seals are big labor on the V8s (and probably not the only issue considering the chain was stretched TEN DEGREES and you had broken guides... that points to me serious lack of upkeep over the years prior to your purchase).

I agree with donbryce, you get the "while I'm in here" deal... your miles/age means most everything rubber needs to be replaced. Breather tubes, injector seals, fuel distributor boot, intake manifold seals, etc

Check ebay for reman heads, you may get a steal on a set.

I have all the tools to pull the cams and heads if you want to borrow them.



My 2¢