Closing in on the valve stem seals vs piston rings question.

conor1n

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Hello everybody.

I have my S212 350 CGI with 199k miles and it's burning a lot of oil. Somewhere in the order of 1 litre in less than 1000 miles. At least that was the case before renewing the PCV valve. It still burns a lot.

I have been trying to figure out if the rings or valve seals or gone.

Symptoms are:
  1. Abnormal oil consumption.
  2. After sitting over night, puff of smoke after starting up in the morning.
  3. If I idle or do slow, stop/start driving for a while then pound then floor it in first gear at the bottom of a hill, I get a lot of smoke. One day I filled the entire road! During testing yesterday morning, I even got a misfire code. (Cleared and back to normal).
My current thoughts are that it's the seals. This is because I only get the smoke after the car has sat a while or been idling for a long time and the crankcase pressure sucks in the oil. I believe that if it was piston rings it would be consistently smoky, each time I floor it.

What do people think?

I'm trying to figure out if I can change the stem seals with engine in situ and keeping head. I know the rope method etc, just trying to figure out if I can compress the springs as they are.

Thanks a lot in advance.
 

Stratman

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If the valves themselves are sealing correctly in their seats, a compression test will show if the piston rings are at fault. If a low reading improves with a little oil squirted in through the spark plug hole it's virtually certain to be the rings.
 

Benzowner

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I would also concur it is the piston rings, but not so sure a compression will show it up. If MB use a 3 - 5 ring system, as many manufacturers do, it may well be the compression ring is ok and only the oil seal rings have gone. You would still get compression ok. It could also be badly scored bores which will need a rebore.
 
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conor1n

conor1n

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Thanks lads.

It is a confusing one.

To get into the bores to look is complicated enough. I am wondering if perhaps I could just do the valve seals as I can likely do without taking out engine. And see how consumption goes.

Any works on piston rings or bores is a vastly bigger job as I can do the valve seals on drive at home (I think).

Anybody know if that engine has cylinder liners or just straight up bores as part of the engine block. If I need to bore the block a bit, does one then need to get slightly larger pistons or rings?

Thanks
 

pcthrillrider

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valve stem oil seals they go hard over time when you turn off engine the oil runs down the stems hense blue smoke at start up
 

E55BOF

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The fact that changing the PCV valve improved the oil consumption suggests that the crankcase ventilation system may be the problem. I'd clean it all out very thoroughly; is there a partial blockage - an oil separator or filter - in it that might be the problem? A build-up of oil at low revs and low crankcase pressure, which is forced through when the crankcase pressure rises on hard acceleration, might well give you a sudden cloud of smoke.

If that's not it, read on...

I wouldn't have thought that if the engine was running enough oil would leak past hardened valve stem oil seals and remain unburnt to give a great cloud of smoke on hard acceleration, but I'm also inclined to agree that if it is not consistently smoky, it's more likely to be the oil seals.

If you get a cloud of smoke on hard acceleration sometimes, but not consistently, that would suggest that the bores and compression rings are OK. Have you had a wet and dry compression test done? If the wet compression is significantly higher than the dry, it needs new piston rings at best, and very likely a rebore and new pistons as well. Unfortunately, you can only find out for sure by taking the heads off to see if the bores are sound; even a borescope inspection will only tell you if they're scored or damaged, not whether they're worn evenly and by how much.

If it is possible to do it relatively cheaply and easily I'd replace the valve stem oil seals and see what the result was. If the oil consumption is then acceptable, well and good, but if that didn't do the trick, and the compressions were good, I'd be prepared to replace the oil control rings.
 
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ray_hennig

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Our old W123 280TE used to lay down a cloud of blue when decelerating from 100+ mph. Perfectly clean when accelerating.

Valve seals was the issue as the vacuum sucked the oil into the combustion chambers.

Clean as a whistle after new seals.

RayH
 
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conor1n

conor1n

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The fact that changing the PCV valve improved the oil consumption suggests that the crankcase ventilation system may be the problem. I'd clean it all out very thoroughly; is there a partial blockage - an oil separator or filter - in it that might be the problem? A build-up of oil at low revs and low crankcase pressure, which is forced through when the crankcase pressure rises on hard acceleration, might well give you a sudden cloud of smoke.

If that's not it, read on...

I wouldn't have thought that if the engine was running enough oil would leak past hardened valve stem oil seals and remain unburnt to give a great cloud of smoke on hard acceleration, but I'm also inclined to agree that if it is not consistently smoky, it's more likely to be the oil seals.

If you get a cloud of smoke on hard acceleration sometimes, but not consistently, that would suggest that the bores and compression rings are OK. Have you had a wet and dry compression test done? If the wet compression is significantly higher than the dry, it needs new piston rings at best, and very likely a rebore and new pistons as well. Unfortunately, you can only find out for sure by taking the heads off to see if the bores are sound; even a borescope inspection will only tell you if they're scored or damaged, not whether they're worn evenly and by how much.

If it is possible to do it relatively cheaply and easily I'd replace the valve stem oil seals and see what the result was. If the oil consumption is then acceptable, well and good, but if that didn't do the trick, and the compressions were good, I'd be prepared to replace the oil control rings.

Sorry, I think I may have been a little unclear before. Since putting more miles on after the PCV valve change, I can confirm consumption is still quite high. At this point I have changed everything in the attached picture which is effectively all the PCV system (exterior anyway).

I am inclined to agree on the course of action. I am getting comfortable with the prospect of doing the valve seals on the driveway and taking it from there. If no change, then I will need professional help.

May I please ask about the process of boring. I get the concept. But, when you go at the bores do they get significantly wider that you need bigger piston / rings? Or is it usually within tolerance that the normal components work? How do I find out if the M272 engine has cylinder liners or not? If it has liners, I assume it's just a case of replacing?

Our old W123 280TE used to lay down a cloud of blue when decelerating from 100+ mph. Perfectly clean when accelerating.

Valve seals was the issue as the vacuum sucked the oil into the combustion chambers.

Clean as a whistle after new seals.

RayH

Interesting.. I don't get anything when decelerating.

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E55BOF

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Sorry, I think I may have been a little unclear before. Since putting more miles on after the PCV valve change, I can confirm consumption is still quite high. At this point I have changed everything in the attached picture which is effectively all the PCV system (exterior anyway).

I am inclined to agree on the course of action. I am getting comfortable with the prospect of doing the valve seals on the driveway and taking it from there. If no change, then I will need professional help.

May I please ask about the process of boring. I get the concept. But, when you go at the bores do they get significantly wider that you need bigger piston / rings? Or is it usually within tolerance that the normal components work? How do I find out if the M272 engine has cylinder liners or not? If it has liners, I assume it's just a case of replacing?



Interesting.. I don't get anything when decelerating.

If the bores are so worn that they need reboring, rather than just honing (if that - think of it as a light rubbing with fine grade sandpaper, so very little material is actually removed), to restore them to good condition, then it's very likely you would need new pistons. If not, you might well get away with just new rings. You can only find out the degree of wear with the heads off, though.

I've not heard of the 'rope trick' (apart from the Indian Rope Trick, but I doubt that would be useful... :D ), but I believe the pros use an adapter and air line (no, not like Virgin Airways, silly...) to pressurise each cylinder in turn to hold the valves closed while the lifters and springs are removed to get at the valve stem seals.
 
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conor1n

conor1n

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If the bores are so worn that they need reboring, rather than just honing (if that - think of it as a light rubbing with fine grade sandpaper, so very little material is actually removed), to restore them to good condition, then it's very likely you would need new pistons. If not, you might well get away with just new rings. You can only find out the degree of wear with the heads off, though.

I've not heard of the 'rope trick' (apart from the Indian Rope Trick, but I doubt that would be useful... :D ), but I believe the pros use an adapter and air line (no, not like Virgin Airways, silly...) to pressurise each cylinder in turn to hold the valves closed while the lifters and springs are removed to get at the valve stem seals.

Ok, I see. Honing remove very very fine wear/scores then. With reboring do the regular pistons provide enough clearance?

The rope trick involves inserting some rope into the fully open cylinder and then turning the crank to get the piston to compress the rope and push it up into the valve area. This prevents the valve from slipping into the cylinder when the keepers are removed. The theory stands pretty well anyway.
 
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conor1n

conor1n

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A rebore will need new oversize pistons.

Fair, makes sense.. Do MB OEM offer pistons in staged sizes or would you be going totally aftermarket on this. Would the rebore be of enough significance to offer extra BHP?
 

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Anybody know if that engine has cylinder liners or just straight up bores as part of the engine block. If I need to bore the block a bit, does one then need to get slightly larger pistons or rings?
Is it an M272 or M276 engine?

If the former, I believe it has coated cylinder liners, if the latter then early versions also had coated cylinder liners but from early 2013 it has Mercedes-Benz's 'Nanoslide' coating sprayed directly onto the aluminium block's cylinder walls. Not sure if the liners are cast in or removable, but either way coated liners can't be re-bored. If it's the later 'Nanoslide' coated bores then that can't be re-bored either.

If it turns out to not be the valve stem seals, I would suggest that a donor engine will be the most cost-effective solution.
 

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I wouldn't attempt to do valve stem oil seals on a multi valve head without removing it, even with a decent valve spring compressor tool. The amount of collets, snap rings and springs you will need to remove to either get at them with a proper seal tool, let alone a home made one, and then press the seal carrier back in place properly leaves you open to many possible error points, between 12 and 24 obviously. Without the correct tools its certainly not a kerbside motors job. You will likely need to remove the cams as well, so a timing set will be needed. Plus all the VVT testing and adjusting gear. You are disturbing a 200k mile timing chain as well.... if the tensioners/sliders are worn you will have trouble re-timing it.

You may have worn valve stems, which unless you can remove and check the dimensions with a micrometer, will just mean the seals failing again prematurely, probably within a few hundred miles. The tolerances are tight, so cleanliness is very important. Even so, it might be wise to replace upper valve train components whilst you are in there due to the mileage (buckets, lifters etc).
 

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