It's ALWAYS the sealant, and yes when MB was trying to figure out what sealant to use, we got comebacks on lip seal jobs ALL the time. Well, OK, eventually the lip seal will get hard and maybe leak, but will last as long as a valve cover gasket for example, 5 + years. 5900 is what MB recommends for this job, those of you with access to WIS can research this, there is an SI that was put out in regards to it, find it in the same group you find work instructions for replacing the front cover, the info may also appear in the work instructions by now, or there is a link to it in the work instructions. The main place it's used is on the sealing face between the front cover and the head, no reason to "pile" it on there, about a 1/8" bead is enough, certainly less than 1/4" bead. The only other spot is about 1/4" ball of it at the very ends of the groove on top of the timing cover, in the ends of the small channel the lip seal fits in to. As I said it's a great benefit to have the installation tool that slips in to the camshaft, really helps line everything up when slipping the cover back on. There are TWO different sized tools, I believe the bore in the front of the camshaft changed sizes some point early in production. If you borrow the tool and it won't go in to the camshaft or is way too loose, you need the OTHER one. Just a light film of motor oil (synth or dino
) will help the cover glide back in to position, you do need a slight amount of downward push to get it to go back in to place, if you can't get it perfectly flat in back, don't worry the bolts will finish the job, but by ALL MEANS check the gap between the front cover and timing cover (the lower, main timing case cover) to make damm sure the lip seal didn't "roll" or come out of place! Then go ahead and torque the 4 bolts and you want, as a DIY'er, for this stuff to cure as long as practical, overnight would be a good thing. Don't start the engine during this time, you don't want oil to get on this stuff, that's why I mentioned a THIN film of oil on the new lip seal, just enough to make it slick. Be sure to clean all the old junk off the front cover, including where the lip seal contacts it, and especially where it contacts the head, and of couse the tough part is getting the old junk off the front of the head, drivers side is a bit harder than the passengers side, due to the tensioner shock, etc being in the way. I use a tiny screwdriver to pick away at the old junk around the 2 steel dowels, also clean off the tops of these dowels and clear the dowel holes they fit in to. Clean the old junk out of the groove the lip seal fits in to. If you look at the ends of this groove, you'll see the small gap between the head and engine block, it'll be kind of obvious that this is what the small ball of 5900 is supposed to seal up. Still, don't overdo it, this isn't a case of more=better. But realize what you are trying to do.
Gilly
PS Many times you will see the old junk is a creamy white, sometimes turns yellowish to light brown, that was recommended for a few years, see that quite a bit, sometimes a greenish stuff was used, lasted a short while. You may also see the typical blue or red RTV's used, these don't last long either, usually by shops or DIY'ers that never really researched what should be used.