The 300TD has been truckin' along relatively well, but it developed an annoying (and filthy) exhaust leak, and the oil consumption was beginning to border on the ridiculous. The car was showing some signs of what I was hoping were worn out valve stem seals - excessive oil consumption being the main one, but I had some secondary issues as well. The wagon has been notorious for eating glow plugs, and the injectors required all too regular fuel treatments to keep them from nailing. My hypothesis was that I was burning so much oil past the guides (and I was indeed burning A LOT of oil) that the excess carbon was junking up the prechambers causing the nailing and the short-lived glow plugs. A stuck ring also wouldn't be out of the realm of possibility, but the car started too easily in cold weather and had too much power for it to be a more serious issue with the rings other than the typical high-mileage cylinder taper to be expected.

Just a disclaimer: if you aren't comfortable working on your motor (performing valve adjustments regularly, quickly, and comfortably at a minimum), if you lack a strong mechanical background, or if you lack a thorough understanding of basic 4-stroke theory, I strongly advise you NOT to use this write-up as a guide to performing this job on your own car. The potential to screw something up is rather large, turning this from a couple hour job into a nightmare of a weekend that could still leave you with a disabled vehicle. Proceed at your own risk!

I started off with a good degreasing of the engine. This was far from thorough or exhaustive, but was good enough to begin disassembly.

Here's where the exhaust leak was.

Off with the air intake housing.

Remove all the nuts holding the manifolds to the head, and the intake manifold can be removed. Don't forget the support brace under the air cleaner housing. At this point, you've got a couple choices on how to proceed. You can either remove the two bolts holding the exhaust manifold to the header pipe and remove the manifold, or disconnect the linkage holding the exhaust to the transmission and leverage the manifold (with exhaust system attached) off the studs while you put on the gasket. The former has a risk of frozen/broken bolts, the latter is physically more demanding. I chose removal.

If you choose removal, loosen the other end of the support brace (circled here) and swing it out of the way to allow removal of the exhaust manifold.

Don't worry, valves, we're getting to you.

Clean the mating surface thoroughly, and install the new gasket on the studs. The old gasket has been superseded by a new part that is much beefier than the old one. Install it clean and dry, no goop of any kind.

Genuine parts. Accept no substitutes! Thanks to Tom Hanson at the Mercedes-Benz Classic Center for getting everything out to me!

New hardware for the exhaust manifold to header pipe connection. Why?

Because there's no way in hell I'd re-use these. Remember that risk of snapping? They'd also round on me while torquing, allowing me to smash my knuckles against the manifold, header pipe, or engine block. My knuckles yield to metal. Yours probably do, too.

Manifolds going back together.

Buttoned back up and preparing to start on the valve stem seals - note that the rocker arms have been removed. The manual doesn't give a torque spec for the manifold nuts. The gasket has to crush to ensure a proper seal. I recommend tightening it in stages until you begin to feel the gasket crush under the torque, and then do the final torque by feel. If that's unfamiliar to you, then I don't recommend doing this job yourself. If you undertorque the gasket, it will leak in short order. If you overtorque it, you risk stripping or pulling one of the studs.

First things first: valve cover removed, align the marks on the cam sprocket and cam tower. This will show you where TDC on the compression stroke of cylinder 1 is.

P.S.: Now's a great time to check the marks on the crank to check your chain stretch before continuing on.

Note the orientation of the exhaust and intake cam lobes for cylinder one to the cam oiler tube. We will use this same position on every other cylinder. Why? Because cylinder 1 is marked, but cylinders 2, 3, 4, and 5 are not. If TDC of the compression stroke of cylinder 1 places the cam lobes in this position, then it follows that the cam lobes for any other cylinder in the motor will be in the same position relative to the oiler tube when it reaches top dead center on it's compression stroke.

Why is TDC so important? Because at TDC, you can remove the adjuster, lock nut, spring perch, and valve spring from the valves, and they will drop less than a quarter of an inch to gently rest on the crown of the piston. Here I've already removed the valve stem seals.

New seals installed, ready to go back together.

Again, a little slower: line up the cam lobes to get the cylinder to TDC.

Remove the lock nut, adjuster, spring perch, and valve spring. I found the the spring was easiest to remove when rotated such that the end of the coil on top of the spring was at 12 o'clock.

Remove the old seal and install the installation sleeve. It will take considerably more effort to remove the seals than you may think. Wear eye protection - many of my old seals shattered on removal. Be sure to remove all the pieces from the top of the head!

Lightly oil the new seal and slide it over the installation sleeve.

Remove the sleeve...

...and press the seal into place. You'll feel it seat on the guide.

Reinstall the spring, perch, and adjuster. You have a choice again here: you can run the adjuster all the way down, or leave it out a good bit. If you run it down, re-installing the rockers is easier, but adjusting the valves to close up the lash takes longer. If you leave if out close to where it was when you removed it, your valve adjustment will be easier, but getting the rockers back in will be a pain.

I ran mine home.

You don't have to do one valve at a time - you can do both, just keep the hardware for each valve separate. Remember, though, you can only do one cylinder at a time! If you try to do more, you will have problems!

The heads of the cylinder head bolts make nice fulcrums for removing the seals. Even after removing all of the seals, I still hadn't found a procedure that I liked - some were faster than others, but produced more chips from the old plasticized seals and required more strength, or resulted in a scraped knuckle or two. Others were safer and produced less mess, but took much longer.

When you're done, carefully get the rockers back in, torque them to spec, and adjust all the valves. You'll probably find that you need the weird third wrench in the adjustment set to keep the valve from rotating.