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940 Car skipped time (940 Turbo)

O

ortho stice

Active member
Joined
Feb 9, 2015
Location
Braddock, PA
I had a thread on a no-start condition last month that must have gotten black-holed in the server collapse...

Anyway, kudos to whoever it was that suggested I check timing. I rolled my eyes and thought "yeah right the belt has 15k miles on it" but, somehow, he was right! Skipped five teeth on the camshaft somehow. I loosened the belt, set the timing again, and the car is back to running great.

So my question is; what could have caused the belt to slip? I don't want it to happen again obviously. Tensioner nut was on nice and tight and the spring felt appropriately strong. The belt is a Contitech from like two years ago. For context, the car shut off going over a large bump in my driveway at like 1100 rpm.

Any ideas appreciated.
 
I don't use the spring to tension the belt. I smack it tight with a hammer and a screwdriver and then tighten the nut.
 
Belts will stretch over time. New belts stretch the most. It's a good idea to revisit the tension every 10-15k miles on your timing belt.
 
I had a brand new belt jump off the cam from failing to re-tension it after installing. Drove fine for a couple days then on my way home one day it just popped off and tried to push through the cover
 
Interesting! Thanks for the info; I feel more at ease about driving it around now. I guess that explains why there’s a rubber plug right over the tensioner nut in the timing cover…
 
I always load up the tension between the cam and crank before taking up any slack with the tensioner, which simulates the tension the belt will have under operation. I wouldn't be surprised if a new belt stretched a little.
 
On some cars the belt gets loose for some reason(tensioner not set right?) Just picked up a 740 last month and I was checking belt. It was super loose. Change it anyways for peace of mind.
 
Mr. V said:
No reason not to, and it's vert easuy; just loosen the tensioner nut then retighten it to specified torque.

That should take up any slack that managed to work its way in there.
Click to expand...

I think that is the problem with the initial installation and re-tensioning in most cases. People line up the marks and then lock the tensioner. What needs to happen is to turn the crank so that it is pulling the cam, then, loosen the lock nut and re-torque it. That way the belt is being tensioned on the leading side and the tensioner spring adds the correct amount of tension to the trailing side.
 
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