Chain slap makes a sound similar to a hydraulic compensator (tappet) that has failed to pump up - but the noise does not last as long. If you have ever heard it you will know what it is.

ALL 116 and 117 engines are prone to timing chain failures - the chain, rails, and tensioner are the same for both engines. The chain should be checked for slack every 60,000 miles. I believe the manual says anything over 8 degrees off on timing the chain is stretched and needs to be replaced. I would not allow it to go past 5 degrees. The guide rails are plastic and become brittle over time. If they are dark brown then chances are they are original to the engine. Newer ones will appear to be off-white (with some discoloration from wear and oil).

If the car produces a puff of smoke when started (not from cold, but after it has been sitting for a while) chances are it needs valve stem seals. If it smokes all the time, then it more than likely needs a valve job. Mercedes says 1 qt. of oil per 800 miles is acceptable - most anyone here will tell you it is not.

The 126 cars were, in many people's eyes, the last true Mercedes produced in the 20th century (do not start a hate thread or debate over this, just reporting what I have read in books and on the internet). They are very dependable and have enough power for almost anyone. If they have been maintained properly they are wonderful cars. However, preventive maintenance is the key to keeping them on the road AND not breaking the bank. I have had my 420SEL for 4 years and have done quite a bit of work to it - mostly maintenance issues or cosmetic issues and I have been more pleased with this car than any other Mercedes I have owned. It came with a complete set of records so I knew exactly what had and had not been done. I also knew the previous owners and knew the car had not been abused.