Just thought I'd share some of my experience with my recently acquired '74, 450SL, equipped with D-Jetronic F/I system. When I purchased the car, the seller/owner claimed to have no knowledge of the timing chain and track guides ever having been replaced; an honest, if unwelcome, bit if news. He also added, he would have no qualms of driving the car long distances, with only a tune up! To which I said to myself, "right!" The car has 210K miles on it and was running very rough and rich at idle.
After reading everything I could get my hands on for replacing the timing chain and rails, I ordered the parts and planned the event. I am in the thick of it, as I write. This is a daunting task, to say the least, and I am a profesionally trained mechanic/machinist/ engineer with many years of working on engines of various types including, aircraft, and with access to a very complete, well equipped shop, though not specific to MB. It is a difficult task, not because of the technology involved, but because of poor design on the part of the manufacurer (YES, MB!), in the placing and mounting methods for the engine driven accessories, in addtition to poorly written manuals and thirty plus years of crud build-up in the surrounding work area. Placement of these accessories prevented proper cleaning of the surrounding work areas, prior to initiating the dismantling operation. Deciphering MB's budget-driven tech writing is only surpased by those of developing Asian manufacturer's, on how to assemble your child's swing set or a BBQ grill -- If you now what I mean! And, the Haynes manual -- well, what good can be said?
My findings, so far: don't waste your time making a slide hammer to attempt to remove the guide rail retaining pins. It, quite simply, does not work. Unless, the pins are fitting loosely in the head and the weight of the sliding mass is over five pounds; in which case the threaded 6mm bolt will fail and ruin your day by snapping off while embedded into the guide pin. Other methods will need to be employed if the pins are snug into the cylinder head(s), as mine were found to be. I will be glad to answer specific questions about this off-line, at Coresa@aol.com
. No, I'm not selling anything!
A broken guide rail on the left cylinder bank, tells part of the story. The engine made no mechanical noises whatsoever and, aside from the uneven running, I was/am impressed with its quiet operation. This failure could have resulted in catastrophic engine failure, withing a few miles of driving. For once, I listened to my inner voice and did not procrastinate. It is a credit to the design of the engine that the chain and rails lasted 210 K miles! There is no evidence of this engine ever having been serviced internally! A solid record of timely oil changes and related filters and what not, are a testament to how important these preventive measures are: there is no evidence of build-up of any kind, and no perceptible wear on the camshafts. Really, a thing of beauty! I am also sure this prolonged the life of the chain and its related components. The cam sprockets show no wear whatsoever.
During my evaluation process of what would cause the engine to run so rough at idle, I used the common test equipement: timing light, dwell meter, DVOM, vacuum gage and a shop-built, cylinder leakdown tester. The vacuum gauge told most of the story: steady, low vacuum readings, at idle, under no load, at low throttle settings. The timing signal was jumping all over the place and suggested worn timing components and faulty distributor internals. The late valve timing being attributed to a severely worn timing chain and the broken slide rail (loss of chain tension and chain slap). Other factors included, a non-functioning ignition distributor, (advance mechanism frozen), and poor electrical connections.
The object of my post is not to critisize anyone or anything, but as an objective finding (as I see it, HA!) and to warn all of those who may be due for a timing chain replacement and are waiting to hear strange noises coming from the engine, broadcasting a warning of an impending need for this service. DON'T WAIT! This could cost you some serious bucks, and ruin your day, I assure you!
I hope this is of some benefit to someone.