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I performed the 2mm test for cam timing/chain stretch today and came up with a value of 19 degrees BTDC. The spec value for my car, a 83 240d is 11 degrees ATDC. I followed the maintenance manual instructions to the letter and got the same result four times. What could account for this large discrepancy?
I timed the injector pump to 24 degrees BTDC and the car ran really poorly. I used the bubble method which, unless completely useless, should have got me in the ballpark. The cam marks, when lined up, showed 7 degrees ATDC.
yup!
 

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How many miles on the engine?

Such a stretch is not uncommon for high-mileage engines. I would replace the timing chain.
 

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Not likely a tensioner issue as it's on the slack side of the chain trail. Like Govert said, 8 degree of stretch is not uncommon and for sure it's time for a new chain. That's too much stretch for a correction using an offset key.
 

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I also think a new chain would be best - but for the record chap 5-215 specifies a 1.3mm offset key to adjust "about" 10 degrees. That's the biggest available though.
 
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