How do you know its time for a new timing chain? I have a 1976 240D, starts and runs great. I have most of the records but no evidence of chain being replaced. 250000 miles. The car does make a slapping noise until its warmed up

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'05 Ford Escape 100,000 miles
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'02 Audi TT 100,000 miles
'00 Ford Excursion V-10 121000 miles
'92 Mercury Capri 100000 miles
'02 BMW 325XI 60000 miles
'92 230CE 160000
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'93 Rinker Captiva 209
5 Kids
2 Dogs
1 Wife