The classic symptom of bad valve stem seals is, if when starting the car, after it has set for a little while (say over night), clouds of smoke appear from the exhaust and then stops after a few seconds. What happens is, while the car is setting, oil leaks down the valve stem past the seal and accumulates in the cylinder chamber. When you start the car, that oil is burned off out the exhaust pipe.

The valve stem seal fits over the valve stem and seats over the top end of the valve guide. (refer to Fig-1) The valve guide is a sleeve that’s pressed into the cylinder head, the valve stem rides in this sleeve. The valve stem seal keeps oil from passing down between the valve guide and valve stem into the combustion chamber. The valve is held closed by a set of 2 valve springs (one inside the other). The springs are compressed down over the valve stem and held in place by steel disk on top of the valve springs call a "valve spring retainer". The retainer is locked to the top of the valve stem by two small halve cone shaped steel wedges called "valve cone halves" or "valve keepers".

In order to get to the valve stem seal, most of what I’ve described has to be removed (16 times, be thankful you don’t have a 32 valve engine). If the cylinder head was removed from the engine and setting on a bench, this job would be a breeze. We could simply compress the spring using a conventional spring compressing tool, remove the various components (keepers, spring retainer, springs and stem seal), then put them back together. But we’re doing this job with the head still on the engine. The problem with this method, is once the valve spring is remove the valve will drop out of the cylinder head. On a bench, no problem, but on the engine, bad news.. The valve drops down into the cylinder and the head has to come off to get it…

In order to keep the valves from dropping, we use an air compressor and pump air into the cylinder we’re working on. With the valves closed and the piston at TDC (Top Dead Center) for that cylinder, the air pressure inside the cylinder will keep the valve up and in place. We can now compress the spring (using a special tool), remove the keepers, spring retainer and springs. The valve stem seal is now exposed and can be removed.

• Air compressor (capable of at least 100psi) *
• Air hose with spark plug fitting at one end and quick disconnect ¼ coupling at the other end (Pep Boys has this hose) *
• Spring compressor (special tool I got mine from www.technictool.com/mercedes.htm $165.00 M123-0361) *
• 27mm socket and 5" extension for turning over engine by hand. *
• Magnetic steel bowl to keep these small parts in as you working. (Sears or Pep Boys)
• Small magnetic grabber to extract valve keeper from the spring retainer (Sears or Pep Boys)
• Small tweezers (squeeze to release) for installing valve keepers (Sears)
• Pump oil can, to lube parts as they’re installed.
• Mercedes Engine Manual

• 2- Valve stem seal kits (1 per side, consists of 8 seal and 4 guide sleeves) p/n 117-050-03-67 $19.00
• 2- Valve cover gaskets and 4 copper VC bolt washers per side (if they haven’t been replace recently)
• 2- 4-port air distribution hoses (1 per side) (if these haven’t already been replaced, will most likely break trying to remove them, the rubber becomes very brittle with heat/age.  You may decide to replace all the air hoses, I did.)

Disclaimer

My car is a 1989 US 560 SEC with a 117.968 engine. The procedures that follow apply to this vehicle and engine, I’m not sure how they may differ across other W126 vehicles and 117 engines.

I found all the tools listed above very useful and I wouldn’t attempt this job without them. In fact, most of the tools (or their equivalent) are required and I don’t know how you’d do it without them. Make sure you have a well organized work area that’s well lit.
This is a fairly involved process and good mechanical knowledge is strongly recommended. You may have to improvise as you go along. The difficulty factor for this job, on a scale from 1 to 10 (1=oil change, 10=complete engine rebuild), in my estimation is a 6. If you feel comfortable with that, go for it..
As you dissemble things pay particular attention to any abnormal wear and obvious damage. In the case of the various components of each cylinder, compare one to the other. They all should look similar, rocker arm, cam lobes, spring, etc.

The follow are Job numbers referenced in the W126 MB Engine manual (or CD).

• 05-230  Removal and Installation of Rocker Arms
• 05-250  Removal and Installation of Valve Springs
• 05-270  Replacing Valve Stem Seal.
• 05-285  Checking, Replacing Valve Guides

When I refer to right side of the engine, I’m referring to the passenger side of a US car, conversely, the left side of the engine is the driver’s side of a US car.

1. Disconnect battery.
2. Remove valve covers and gaskets. The right cover comes off no problem, but to get the left off, you’ve got to disconnect the fuel lines going to the fuel distributor and pressure regulator (17mm). Also disconnect the vacuum line that goes to the EGR valve. Also disconnect the brake booster line where it connects between the rear of the engine and the bulkhead.
3. Remove all spark plugs. (Caution, don’t pull the wires, grab the metal cover and pull that.)
4. Remove the 4-port air rubber distribution hoses from the four injector holders. Take care as these hose (if they haven’t been replaces) are most likely very brittle and hard to remove. You must replace these hoses, so break them not the injector holders. I had to remove them to easily maneuver the spring compressor.
5. Remove the fan and large plastic fan shroud surrounding the fan. Loosen all 4 bolts, but remove only 3 of the bolts (10mm) which fasten the fan and pulley to the water pump. Leave the bottom most bolt in place, simply loosen enough to side the fan over the water pump shaft. (Note: Mind the radiator when removing the fan.) After the fan has been removed, replace two of the adjacent bolts with washers to simulate the thickness of the fan mount. If you don’t secure the water pump pulley this way, it’ll pop off during the process and totally ruin your day.. (I’ll explain late why I kept the water pump pulley in place.)
   The fan shroud is held in place by 2 spring clips at the top of the shroud. The shroud position can be adjusted up and down about an inch or so. So note the current position by counting the number of meshing teeth exposed under the spring clips, so you can return it to the same position on assembly.
   (Note: I removed the fan and shroud so that I could observe the engine as I turn it over by hand. This could be done from the bottom without removing the fan and shroud, but I’m sure you’ll find this method much easier when find the timing marks.)
6. The following steps describe the removal of the rocker arms (Job 05-230) in preparation for removing the valve springs (Job 05-250). We’ll be working on 2 cylinders at a time (one from the right and one from the left) in the order indicated in Table-1.
7. Turn engine over by hand (see note 1) to where the #1 cylinder’s (front most right side cylinder) cam lobes are pointing up and remove the two rocker arms using the valve spring compressor tool (see Fig-3).  Slowly compress springs only enough (see note 3) to remove the rocker arm. The rocker arm simply sets on the hydraulic compensating element and is removed easily when the spring is compressed. Also remove the thrust piece from the top of the spring retainer and store in a separate plastic baggie together with its respective rocker arm. Repeat this step for the companion cylinder.
   NOTE 1: ALWAYS turn engine over in a clockwise direction as you are standing in front of the car looking at the engine, NEVER turn it in the opposite direction..
   NOTE 2: Always return rocker arm and thrust piece to their original positions, I suggest storing them in individually marked plastic baggies.
   NOTE 3: Never compress spring enough so that the valve strikes the top of the piston.

8. Turn engine over by hand (see note 1) to where the #1 cylinder (front most right side cylinder) is TDC. At Top Dead Center (TDC) the piston is at the upper most position in the cylinder. The #6 cylinder will also be in the same position at this time, there for, these are the first two sets of seals we’ll be changing. TDC can be determined by observing the markings on the crankshaft vibration damper, the four markings we are interested in are 0|0, 90, 180 and 270. These are the TDC for each of these cylinder pairs:

   Cylinder Pairing

   	0|0 for cylinders 1 & 6     90 for cylinders 3 & 5     180 for cylinders 4 & 7     270 for cylinders 2 & 8
   	Table-1.

   NOTE 1: ALWAYS turn engine over in a clockwise direction as you are standing in front of the car looking at the engine, NEVER turn it in the opposite direction..

9. With all the other rocker arms still in place, secure engine from being able to turn over (see special note-a). Screw the special air hose into the sparkplug hole of the cylinder we’re working on. Then connect your air compressor hose to that sparkplug hose (see note-1).
   NOTE-1: Your compressor must be capable of producing at least 100psi, which should be enough pressure to hold the valve up and allow us to compress the spring enough to release the valve keepers.
10. Get yourself to ready, have all the necessary tools at easy reach (see special note-c). Slowly start compressing the spring as you did when removing the rocker arms, the valve itself should not move this time. The air pressure should hold it up. If the valve does compress with the spring, the air will release from the cylinder with a pop. Simply let up on the spring compressor, position yourself so the pressure of the spring compressor is directly over the center of the valve retainer and try again. If the spring compressor is slightly cocked, it’ll bind against the valve and the valve will compress too. If the valve still wants to compress, try increasing the air pressure 10psi. (I started off with very low psi and worked it up to 100psi which worked well. see special note-b)
11. As the spring retainer and spring compress down and the valve stem is held up in place, the tiny valve keepers (valve cone halves) will loosen and separate. As they do, use you magnetic grabber to pluck one half out and place it in the magnetic bowl. Then go back for the second half and place it in the bowl. As I mentioned before, try to maintain pressure on the spring compress directly over the spring retainer. Otherwise the retainer will bind against the keeper and not release the keeper enough to be removed.
    Note: You arms may get tired holding the spring compressed while trying to get that second keeper out. With the single keeper still in place, you can release pressure to realign yourself.
12. With both keepers out, slowly release pressure on the spring compressor. Remove the spring compressor and remove the retainer and springs. Because I’m a very cautious guy, I immediately took a piece of wire and tied it to the tip of the valve stem. Just in case the engine decided to turn over, the wire would keep the valve from dropping into the cylinder. (Am I paranoid or what?).
13. The prize, the valve stem seal, is at the bottom of the valve stem. It sets in a groove in the valve guide and can be a bugger to pop off. With age the rubber hardens and doesn’t want to easily slip out of that groove. The book says to pry the old seal off with a screwdriver or pair of pliers. I found it necessary to sometimes use both. Get in there with a rag and clean the oil off the seal and guide so you can see where the seal meet the guide. Work you screwdriver between the seal and guide and try popping the seal up and off. Some just pop off and some you’ll have to work your way around the seal, prying it up at little at a time.
    On some cylinders there’s not much room to get a long screwdriver on the base of the seal and you’ll have to try to pull it off the pliers or a shorter screwdriver. This is where you may have to improvise.
    NOTE: With all the pulling and prying, be careful not to damage the valve guide or valve stem. Don’t worry about the old seal.
14. Take a rag and clean off the stem and top of the valve guide. Don’t press down on the valve stem, but try and move the stem from side to side there may be a very miniscule amount of play (see note-1). If there’s excessive play, the valve and/or guide is worn and that’s a big job. Also, check if the valve guide is loose by trying to twist it. They are pressed into the head and shouldn’t turn. (I found one of mine could be turned, so I’m figuring on pulling the heads somewhere down the line.)
    NOTE-1: The MB manual (Job 05-285) calls for a special gauge to be inserted into the valve guide while the valve is removed. Based on how far that gauge can be inserted, indicates whether the guide should be replaced.  This job is outside the scope of this write up.
15. The valve springs set on top of the rotocap, which acts like a little lazy-susan for the spring set. Check to see if that looks damaged. I just made sure it turned freely.
16. By now you’re probably thinking "take it to the shop, let the mechanic do it", but after the first couple and you get the hang of it, it’ll get less confusing.
17. Now we’re ready to install the new seal. Squirt some motor oil on the stem and inside the new seal. The seal kit should come with 4 plastic guides that look like short straw with a rounded end. This plastic guide protects the seal lip from any rough edges on the stem. Slip the guide over the valve stem and slip the new seal (spring end up) over and down the plastic guide.
    The book calls for a mandrel to set the seal in place. I didn’t have one, so I gently press the seal over the valve guide, being careful not to distort the seal lip and spring. I could not seat the seal flush with the valve guide with my fingers, so I very gently worked my way around the seal tapping the metal edge of the seal down with a pair of screwdrivers, using one as a punch and the another screwdriver as a hammer.
    This seemed to work fine, but a mandrel would have made a better job of it. Perhaps a short of pipe piece (1" or so) the diameter of the metal portion of the seal that could be slipped over the valve stem and tapped down with a weighted open-end wrench?
    NOTE: Be very careful not to damage the rubber part of the seal.
18. Remove the plastic seal guide. Inspect the spring, retainer and keepers for any abnormal wear or obvious damage. Slip the spring set over valve stem (make sure the same end of the spring goes down, the book says the tight coil goes down, I found it hard to tell one end from the other) and set the spring retainer on top of the springs.
    Use a pair of press-to-release tweezers to hold one of the valve keeper halves in a position that allows you to simply drop the keeper into the retainer cup when the spring is properly compressed (see Fig-4). This is a critical step since, once again, you’ve only got one hand free and you want to install the more difficult of the two keepers first.
    Note: Make sure your tweezers are not magnetized because you won’t be able to simply drop the keeper in place, it’ll want to stay stuck to the tweezers.
19. Carefully setup your spring compressor, compress the spring being very careful to NOT catch the end of the valve stem and press that down too. (you'll loose pressure and valve drops… BAD THING). When you’ve got the retainer compressed enough to allow the keeper to simply drop in, drop it in and slowly release pressure on the spring compressor. The keeper should naturally lock into its correct position and look just like neighboring spring. Ready the second keeper, compress the spring again and drop the second keeper into its place.
20. One done 15 to go…
21. Return to step 11 and do the second valve of the current cylinder you’re working on.
22. Return to step 9 and do the second cylinder of the pair you’re working on.
23. Disconnect the compressor air hose and remove the sparkplug hose.
24. Now reinstall the four thrust pieces and rocker arm to their original positions by rotating engine until the appropriate cam lobe is pointing up. Squirt oil over in the retainer cup. Install thrust washer (flat side down). Compress spring enough to slide rocker arm in place, wiggle the rocker arm around a little to make sure it’s seated properly on the hydraulic compensating element and aligned with the slot of the thrust piece on top of the spring retainer. Then slowly release pressure on the spring compressor.
25. Proceed to the next cylinder pair by returning to step 7.  Be sure to release the engine, to allow it to be turned over for the following steps. 
26. That's it..  After you've completed all four cylinder pairs, inspect the valve train area for any tools or debris.  See that all springs keepers are properly install and nothing looks out of line.  Then replace the valve cover (with new gaskets if required) and torque the VC bolts to 11nm.  Remove the 27mm socket wrench from the crank pulley bolt.  (You don't want to forget this, it would make for a bad day if you tried starting the engine with that wrench still in place.)
    You should have removed what ever device you rigged up to hold the engine from turning after step 25.
27. Install (replace if required) the 4 port auxiliary air distribution hoses to both sides.  (you may want to replace all these air hoses, if needed, at this time)
28. Reconnect the brake booster line, fuel lines and vacuum line to the EGR valve.
29. Install  fan shroud and blade.  Again, mind the radiator and the shroud clearance around the fan blade tips.

1. On Securing Engine from being able to turn over.
   This is a CRITICAL step. When we attach the 100psi air hose to the cylinder we’re working on, piston may want to go down from the pressure. If this happens when the valve spring has been removed, the valve will most likely drop into the combustion chamber.. A VERY BAD THING. That would require removing the head and under no circumstances turning the engine over while the valve is down there, because valve would be compressed on the up stoke of that piston..
   To keep this from happening MB has a special tool that appears to go place of the starter and locks the flywheel in place. I did not have this tool, so rigged up a couple of vise grips, a piece of steel right angle stock and a 7/8 open end wrench to hold the engine in place. (See Fig-2) I clamped the right angle stock to the suspension leveler reservoir bracket, then put the open end wrench on the power steer pump pulley nut and clamped that to the other end of the right angle stock.
   Then I made sure I couldn’t turn the engine by hand and attached the air. I waited a minute or two and went about pulling the old seals out.
   This was the reason I left the water pump pulley secure with the proper belt tension. Also leaving the other 6 cylinder’s rocker arm in place helps add to the drag on the engine turning over.
2. Leak Down Test
   This procedure of pumping air into the cylinder while the valves are close is essentially the same procedure as a "Leak Down Test". This test allows you to determine where you may be loosing compression, if this happens to be a problem. With the Leak Down Test you’d use a gauge to measure how fast a cylinder is loosing pressure and compare that to other cylinders. Also listen from where the air is escaping, (intake, exhaust, crankcase or cooling system). I didn’t use a gauge, I simply compared the sounds of the escaping air from one cylinder to the next and determined where the air was leaking. All my cylinders sound the same, a very slow, low hissing sound coming from the crankcase. Since I’m not burning a lot of oil and I have no performance problems (knock on wood), I assumed this was normal..
3. Tool to have ready
   When you’ve started removing the valve spring retainer, have everything ready to go (like a surgeon) . You’re going to have one hand busy with the spring compressor, you want to make sure everything else is within easy reach of the other hand. I found it very helpful to have one of those heavy duty fender covers, the one with the pleats on top, so you can place tools within easy access. Also, as I mentioned above, have a magnetic bowl accessible to put small parts in as they’re removed. I also found that helpful in keeping the spring set together and upright as put them in the bowl. You have to put the springs back in the same direction as they were removed. And you really don’t want to be on your hands a knees looking for that tiny valve keeper should that fall on the floor.

Figures

Valve Components

Fig-1

Securing Engine

Fig-2

Valve Spring Compressor

Fig-3

Handling Valve Keeper

Fig-4