I have a 90 420sel that has about 115k on the clock. I have had the timing chain/rails/tensioners replaced about 4k ago. I have noticed in the last little bit that while sitting in a parking place with the car idling it will occasionally knock or peck a single time. What could this be? It will do it a single time and then it will be probably a minute or two before it does it again. Since I have had the timing job done recently, I don't think it could be anything related to that. Can anyone tell me what it might be and is it dangerous as far as a dissaster happening if not fixed?

It sounds a lot like the reset that happens when either the idle system or the lambda system gives up and resets. It will run at the end of range for a while and then jump to the middle and start over. Something like that. I would verify closed loop control and idle control.

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Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician