What causes cambelt jump a tooth?

[mk2bal]

my cambelt kept going two teeth out, everytime someone lookd at it they noticed the belt was out and reset it, car would run sweet initially then go back to no power, pig to start etc. after a few times of  it happening and been told cambelts dont jump two teeth then go back on where they are, we checked the pulley and like matey said earlier the keyway had gone on crank. put the car in 4th gear and push it back and forth, there shouldnt be any/much play on the pulley before the belt moves, mine moved quite a bit then started to turn the engine/belt. youll see if there is play there...

[AudiA8Quattro]

correctly installed cam belts don't jump teeth : FACT

it was ether fitted wrong or fitted a tooth out

Not fact! I could make a list as long as your arm that could cause a cambelt to jump!  thorough investigation is the key here to avoid serious damage
Thom

Start making the list then. I'm quite interested to hear this.

[Neo Badness]

Cheers for the input fellas, I'm no mechanic but if it's the keyway on the crank that means it's proper boogered isn't it? Or am I thinking woodruf key or whatever it is called? Trying to gain as much knowledge for my negotiations with said garage.

Thom89 could you give me a top5 reasons for cambelt to jump a tooth or teeth?
 I've always left cambelts to the pro's as I considered bent valves are a hurdle to achieving true performance... Not saying there are bent valves as I haven't looked. It's odd though as mate who is an engineer looked it over and said he'd checked timing marks and they were correct? It was running dog rough then though.

subtly imply it was their shoddy workmanship 

Good thinking Jay, I'm going to see how they resolve this before I throw the toys out of the pram. My old dear is getting hearty sick of me nicking her car to go to work in though... Guy who owns the garage is a the mate of a mate so fingers crossed.

Hopefully it'll be something daft (read cheap) to fix, but there could be a silver lining to this... You never know...

I've already poured about 300.00+ into trying to get this KR motor to run sweet, so it's last chance saloon for Kjet 1.8
as I'm reluctant to spend anymore money on it as I only managed about 1100miles in last 18months

[Thom89]

Sticking valves
Camshaft seizing
Lack of sufficient oil feed to the cam
High lift cam incorrectly fitted
Water pump seizing (If driven)
Crankshaft pulley slipping
Crankshaft pulley key worn
Inferior tensioner fitted
New tensioner required
Debris getting under the belt
Belt contaminated from oil leak
Cylinder head over skimmed, valves hit pistons on initial start up
Pre ignition where the engine can momentarily run backwards
cant think of anymore atm, but I'm sure the lads at work can come up with a few more

Thom

[danny_p]

Sticking valves           ( they  normaly stick open )
Camshaft seizing        ( strips teeth off cambelt round cranck pully or snaps belt hardly the belts fault  )
Lack of sufficient oil feed to the cam   (causes sieze same as above )
High lift cam incorrectly fitted             ( ether causes contact to head casting so same as above or causes valves to bash incorrect fitting )
Water pump seizing (If driven)             ( strips teeth or snaps belt )
Crankshaft pulley slipping       ( thats not the belt slipping teeth thats the pully moveing )
Crankshaft pulley key worn       ( as above )
Inferior tensioner fitted             ( incorrect fitting )
New tensioner required              ( incorrect fitting )
Debris getting under the belt       ( there should be gards to stop that so incorrect fitting )
Belt contaminated from oil leak     ( belts are toothed but oil contamination may cause premiture fail but oil leack should have been rectifyed before fitting hence incorrect fitting )
Cylinder head over skimmed, valves hit pistons on initial start up  ( thats stupidity turn engine with spanner first hence incorrect fitting )
Pre ignition where the engine can momentarily run backwards       ( incorrect fitting as the timeings got to be out for that )

a correctly fitted cambelt dosent just jump teeth,  ifa belt dose jump teeth or appaers to have jumped teeth it menas 1 of the following.

A: it was fitted by a muppet   
and or
B: it's a symtom of something else haveing failed or failing

[Thom89]

Sticking valves           ( they  normaly stick open )
Camshaft seizing        ( strips teeth off cambelt round cranck pully or snaps belt hardly the belts fault  )
Lack of sufficient oil feed to the cam   (causes sieze same as above )
High lift cam incorrectly fitted             ( ether causes contact to head casting so same as above or causes valves to bash incorrect fitting )
Water pump seizing (If driven)             ( strips teeth or snaps belt )
Crankshaft pulley slipping       ( thats not the belt slipping teeth thats the pully moveing )
Crankshaft pulley key worn       ( as above )
Inferior tensioner fitted             ( incorrect fitting )
New tensioner required              ( incorrect fitting )
Debris getting under the belt       ( there should be gards to stop that so incorrect fitting )
Belt contaminated from oil leak     ( belts are toothed but oil contamination may cause premiture fail but oil leack should have been rectifyed before fitting hence incorrect fitting )
Cylinder head over skimmed, valves hit pistons on initial start up  ( thats stupidity turn engine with spanner first hence incorrect fitting )
Pre ignition where the engine can momentarily run backwards       ( incorrect fitting as the timeings got to be out for that )

a correctly fitted cambelt dosent just jump teeth,  ifa belt dose jump teeth or appaers to have jumped teeth it menas 1 of the following.

A: it was fitted by a muppet   
and or
B: it's a symtom of something else haveing failed or failing

First of all, I was not referring to one type of engine, but engines in general
Secondly, from long experience, Ive learnt that anything can happen!
Third, If you take your car to have the cambelt changed at a garage and 10k miles later the fanbelt snaps, and throws the cambelt off in the process, destroying the top end of the engine in the process... who would you blame then!
Fourth, Neo asked in the title of his post "What cause a cambelt to jump a tooth" I was merely pointing out that other scenarios are to be given thought, before blaming the garage that fitted it
Lastly, Isn't your answer "B" exactly what I said!
Thom

[VeeDubGTI16v]

i bet they fitted it a tooth out, if its only one tooth out it will still drive ok, it just runs approx 35bhp down and feels sluggish.

you have to get the belt tensioned on the right side of the cambelt when turning the tensioner to tighten the belt, otherwise you can move the belt out one tooth even if the marks line up

the woodruff keys can also shear off on the top pulley on 16v's

[jezza16v]

Tension is more critical on the 16v I would say, it needs twice as much effort to turn two cams hence it has a wider belt than the 8v iirc. Also on high mileage engines the teeth on the pulleys can get very poilshed and a bit 'rounded off' which can make them more prone to slipping. Timing should not go out on 16v as the dizzy is driven direct off the camshaft, unlike 8v.

[AudiA8Quattro]

I tend to agree with DannyP, the chances are the problem is down to incorrect fitting.
Thanks Thom for the extra explanations, i never considered so many scenarios were possible!

[Neo Badness]

Interesting stuff. Cheers for the extra possibilities, will see what comes out of their jumbo mechanics excuse bag when I speak to them tonight

 Just for your info the engine has only cover 114k from new. Has a pretty good history file to confirm mileage but couldn't see when the cam belt was last done so as good practice got it changed along with waterpump/ tensioner/ bolts ect from VW dealers. Just some gave some general mechanical love hoping I would get some reliability.
 
The garage said that it was just as well I'd put it in as they said the bearings in the water pump were on their last legs. Also there was no Thermostat fitted!?

Rectified now but indicates that some mongo has been fiddling. The heater had never worked in the car but it now does, albeit I do need to replace the heater matrix with the mk3 one I bought, but that's another thread...