Is it Common For Chain Tensioners to Unscrew Themselves?
 

Car: 2008 E350 with later M272 Engine.

For the past three weeks I noticed an increasingly severe oil leak in the front-right corner of the engine. Upon closer inspection, and after a costly "repair" by a shop that misdiagnosed it as the oil pressure sensor, I saw that he leak was coming from behind the alternator. The only thing there is the timing chain tensioner.

After reading how-to's and watching instructional videos, I decided to do the repair myself. What shocked me was that the tensioner was so loose that I could unscrew it by hand!

Is this a common fault??

I've never seen that happen before and I've worked on a lot of them. I'm guessing it wasn't torqued properly during a prior repair?

Sounds as though you may have prevented a catastrophic engine failure if the tensioner fell out.

Can't say with any certainty.

The car was low mileage when I bought it (owner drove less than 5K miles/year) and the tensioner I removed appeared original, with the 17mm head vs the 19mm for current ones.

When doing my research for the cause of the leak, I saw two mentions of the exact problem. One person simply retorqued his, which I was tempted to do as well, however, I was worried my tensioner was failing internally and did not want to repeat the job so I went ahead and installed the new one.

The old one I could compress about 1/3 in hand, with some visible (and audible) oil seepage at the base, so it appears to have indeed been failing.

That thought gives me nightmares. Even with the alternator in its way to prevent it from falling out entirely, it could conceivably unscrew enough to cause serious damage -- assuming the massive oil leak that would occur first did not get my attention. :)

That thought also made me apprehensive about fixing it myself. There is a procedure that involves holding the exhaust camshaft stationary while turning the crankshaft about 5 degrees counter clockwise to put pressure on the chain guide, and thus the tensioner thrust pin, to unlock it. I was haunted by the possibility of that not happening, starting the car, and the engine being destroyed. :eek:

I am pretty certain I heard a click from the tensioner unlocking, just before hearing the click from the torque wrench that told me I had turned it far enough.

Overall an easy enough repair. The hardest part was reattaching the first bolt of the alternator while holding it in one hand and aligning and screwing in the bolt with the other, in a tight space.