Chains themselves are very stout (double row, that is), but tensioners can fail and guides are subject to degradation due to exposure to crankcase gasses and heat cycles. When they break it sets off a "chain reaction" that leads to disaster. It is difficult to tell when to change them just by color. Unfortunately, they are the hardest part to change since removal involves stripping the front of the engine , and once you have done that, you may as well change the chain and tensioner ( and sprockets, too) for good measure.

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80 300SD (129k mi) 82 240D stick (193k mi)77 240D auto - stick to be (153k mi) 85 380SL (145k mi) 89 BMW 535i 82 Diesel Rabbit Pickup (374k mi) 91 Jetta IDI Diesel (155k mi) 81 VW Rabbit Convertible Diesel 70 Triumph Spitfire Mk III (63kmi)66 Triumph TR4a IRS (90k mi)67 Ford F-100 (??)