Recently I had a problem with the timing belt on my 850. The belt slipped, put the motor out of time and bent most of the valves. I had about 30 to 50 lbs. compression on each cyclinder. After I pulled the engine I examined the timing belt, camshaft and crankshaft sprockets and they all appeared to be in good condition. Next I examined the belt tensioner and noticed that when it was being compressed that it leaked a small amount of oil. I am guessing that the tensioner might be the culprit here. Has anyone else had a similar failure? Does anyone know of a retrofit that could prevent a future (very costly) failure?

From what we were once told at school, the tensioner failures are more common with manual trans cars that are parked in gear on a hill and without the e-brake used. That combination puts added tension on the t-belt and the tensioner can compress slightly while parked.

Technically the tensioner is suppose to be checked with a belt tension tool before replacing the T-belt (replaced if the tension is less than spec) but few techs know that or do it. The tool is a $500 special tool that was mainly used for the old B234F engines.

I've seen a few of the the tensioners leaking oil and have replaced them with the timing belt change. There is a white plastic spacer that is suppose to be inserted after the pin is pulled, (it looks like a Pack-Man)that small about $1 piece of plastic will keep the belt from jumping teeth should the tensioner fail.