MK4 Timing belt gurus....how could this have happened?

[mrrhtuner]

Wow, it's like openings pandora's box with this common yet silent issue.

carry on

 

[FlyTDI Guy]

Just speculation but it appears the tensioner requires greater range of travel than it currently allows. Could be since people are now generating much more HP/TQ than stock, greater demands are being placed on this original design. It would be interesting to see if there's any correlation between stock vs modded and this failure (or near failure).

 

[paramedick]

Popping the clutch for a rolling start? I did have one customer that broke a tensioner catastrophically while doing this, resulting in a head replacement.

 

[Lensdude_com]

It does stand to reason the OEM parts will fail when subjected to more torque than they were designed for...
sorta as a rule of thumb... when you approach the limit of the OEM clutch then you have also approached the limit of other components on both ends of the engine.

 

[LNXGUY]

It does stand to reason the OEM parts will fail when subjected to more torque than they were designed for...
sorta as a rule of thumb... when you approach the limit of the OEM clutch then you have also approached the limit of other components on both ends of the engine.

TB system isn't like the rest of your car.. There's no reason to expect a 200hp ALH to eat through it's TB components any faster then a stock car. If this were true, we'd be seeing a lot more disasters on this site... I think most people here are running above stock power levels.

 

[Dimitri16V]

could this be caused by not loosening the nut enough and still tensioning the tensioner ?

 

[jimbote]

TB system isn't like the rest of your car.. There's no reason to expect a 200hp ALH to eat through it's TB components any faster then a stock car. If this were true, we'd be seeing a lot more disasters on this site... I think most people here are running above stock power levels.

this ^^x2

could this be caused by not loosening the nut enough and still tensioning the tensioner ?

possible .... someone needs to do some testing

 

[maxmoo]

could this be caused by not loosening the nut enough and still tensioning the tensioner ?

I could believe that.

 

[Fix_Until_Broke]

Popping the clutch for a rolling start? I did have one customer that broke a tensioner catastrophically while doing this, resulting in a head replacement.

I'll see if I still have my old tensioner laying around in the garage, but back in the day when I was hypermiling, I did thousands of bump starts. I didn't specifically inspect the tensioner, but I don't remember seeing anything like this on it (I think I would have noticed ) and it didn't fail.

 

[greengeeker]

do you think even rolling backward can cause this... unless the cam or pump were somehow stuck when it happened causing major deflection in the tensioner... if you notice it's bent away from the max tension stop indicating it hit the max stop ... how did you line up the marks before you took it apart ?

Honestly, I'm not sure what causes this. If someone were so inclined they could work out through the gearing to figured out how much tension is being applied to the belt when parked on an incline and the engine is being rotated backwards (rolling backwards when in 1st or rolling forwards when in reverse). I can do a quick test with my car by parking in the driveway w/o my ebrake on, notice the position of the tensioner and then have someone push the car a little to see how much more tension is being applied.

By marks do you mean cam lock down etc? I spin the cam over with my spanner until my long reach cam bar can be inserted AND the pump pin is near the slot. Release timing belt tension, pop off cam sprocket, remove belt and tensioner.

 

[greengeeker]

Nick, has anyone else had their hands on the car?... does it still have the ASV?

Nope. Just him doing routine maintenance since I last changed the belt.

Yes, ASV intact and functional. Hmmm. I see where you're going with this. Do you think the cam and IP have enough inertia to force the tang against full tension? Again, another test to be run.

 

[greengeeker]

could this be caused by not loosening the nut enough and still tensioning the tensioner ?

I would think in the case you are suggesting that the other tang (the one that interfaces with the backplate) would be the one providing the reaction force to you torquing on the tensioner.

 

[Abacus]

Any further word on this? I've done 4 or 5 TB's this summer and have only seen one weird anomaly, but it was from a garage that FUBAR'd most everything they touched so it didn't surprise me.

This was the tip-off something was amiss.

And the back: See how the tab is hammering on both sides of the stops? This car had the TB replaced 5-10K prior but did suffer from a walking crank sprocket, so I chalked it up to that, but perhaps something else is afoot here.

I have a bucket of old TB tensioners I can play around with, many of which feel fine and probably are fine, certainly enough to test. But I was curious if anything happened with the testing mentioned above.

 

[Herm TDI]

I have verified and confirmed the cause of the TB tensioner failure.
How is the "Lock Tab" of the tensioner becomes damaged is caused by the mechanic making the tensioner adjustment with the lock nut too tight.

This can be verify by anyone doing a TB replacement.

To confirm this observation you can replicate the exact failure the next time you're doing a TB service.

Before you remove the old timing belt try this procedure (on the old TB).
Loosen the tensioner lock nut so that the tensioner can be adjusted.
Slightly tighten the lock nut and again try to adjust the tensioner.
If you have the tensioner lock nut too snug you'll see the old tensioner lock tab bend as exactly shown in the photos.

This problem is "operator error" and not a product defect.

 

[Herm TDI]

Bingo

could this be caused by not loosening the nut enough and still tensioning the tensioner ?

This is the exact cause of this type of tensioner failure.

 

[Abacus]

Thanks Herm!

 

[Dimitri16V]

thanks for testing it this , Herm

is this only specific to ALH tensioners ?

 

[honda_vtec2]

The only way I see the tab bending this way is if the spring looses tension.
If you took a new tensioner, and measured the tension at full travel (using a torque wrench?) then compared it with one of the damaged tensioners I bet you'd see a significant difference. As a test to prove/disprove Litens' stated cause of failure...
you could take a new tensioner test it dry, then soak it in oil and test it again.
I don't think an oiled clock springs' tension is going to change much if at all.

Came to the ballgame late, but I agree. The internal spring on a brand new tensioner with only 40k snapped on me a while ago. I don't see your tensioner being soaked in oil either, maybe some mist here and there. I hope I never have to experience a pos part like that again. It all comes down to QA and QC.