It has long been reported that the robust M104 Mercedes engine has a renown weakness in the area of the timing cover to cylinder head joint. Because of a design weakness, it is common if not obligatory for these engines to leak at this point. The often publicised method of repair is to remove the front timing cover and replace the ‘C’ gasket that seals upper and lower halves of the timing cover.
I have read many ‘how-to’ posts and owners write-ups on this problem, and in all but a very few cases the seal made by the ‘C’ gasket only fails in this one weak spot. (left hand combination joint, above the alternator support bracket) In many cases once disassembled the gasket has proved to be sound and in good condition, apart from in this key area where there is practically no material for the seal to press against causing a troublesome unpressurised weeping leak.
The leak itself is caused by the internal casing design that is leaned at an angle, this allows localised pooling of oil fling from the timing chain and subsequent leaking from the poor seal area between head, block and upper timing cover. Bearing this in mind, knowing that the leak is not a ‘pressurised’ leak from an oil-way or gallery, it is only oil escape to atmosphere by a poor external seal. Although not serious and frequently the amount of oil lost in everyday use is slight, it still makes a mess and is annoying at the very least.
A couple of US guys had tried with some success the sealing of this area externally and I decided this was what I was going to try for myself as there is really nothing to lose – it would either work or it wouldn’t ! Worst case would be the timing cover seal would need replacement to fully stem the leak, so nothing really lost in trying this method first.
I set about gaining access to the troublesome area, on the R129 Mercedes SL 320 access is made possible by removing the air box and plastic cam chain housing decoration panel only. Once these are removed, clean down the area with Gunk or Jizer de-greaser. Use a selection of bent pieces of wire and rag to clean the area thoroughly, raking out any oily crud that lies in the seam between the upper and lower covers. When you are satisfied the area is spotless and clean (the cleaner and more time you spend de-greasing here, the more chance of the repair working) Finish off by either a tooth-brush over with methylated spirits or carb cleaner.
Use a gas blow lamp to apply localised heat to ‘dry off’ and warm the area for a coupe of minutes, take care when doing this that you don’t damage any other components, although the area is quite clear of any combustibles, cables and plastics, just remember to fire-watch! Choose a good quality RTV silicone sealant, high temperature and rapid curing. Using the supplied nozzle on the RTV tube, cut to its smallest hole, inject the sealer sealer into the area cleaned. As the silicone builds, squidge it into the gaps with your thumb working it into the seam and corner area of the join. Apply more sealant and push it into the corners with a small wooden stick (Coffee stirrer) and really work the area well. Now apply the final squirt and smooth the area off, neatening as much as you can without disturbing the main blob of sealant, if you are particular how it looks.
While the plastic front cover was off I noticed a leak from the lower edge of the variable valve timing clutch on the end of the timing cover. I removed the three cap head screws that fix it to the cover and remade the gasket, clearly this original gasket had hardened and was allowing oil seepage down the front face of the timing cover. Cleaning the finished front area as much as possible to allow monitoring of any further oil leaking over time, was in order before leaving the RTV silicone sealant to fully harden for the rest of the day. Re-build the front cover and air box to complete the job.
I have now monitored the leakage for a day or two now and thus far it has not returned. I was unsure if most of my leaking was down to the VVT clutch gasket or the more complex seal point on the timing cover. However and whatever has been rectified in this operation has currently stemmed the oil flow from the front of the engine, it will be interesting to monitor the situation over the next few months and see if it returns, but as of now I am pleased with the outcome, for what was basically very little effort.