What if your cam belt was "out by one tooth"?

[theelanman]

While I don't disagree with Gareth about the engine running like a bag o nails, I've got a slight issue with the "length of belt" description.
As long as there's enough adjustment in the tensioner to keep the belt as tight as it should be, then it makes no difference if it's 5" or 5 miles long!
If it's one tooth out, it will always be one tooth out. The important bit is that the camshaft pulleys are ALWAYS twice the size of the crankshaft pulley.
It still needs to be checked and corrected, but unless the tensioner is allowing the belt to slip then it shouldn't get any worse.
As the CAS runs off the exhaust camshaft, if you can apparently time the engine up properly and get good compression readings, then my suspicion would be that either the inlet camshaft is out of sync, that the CAS itself is on it's way out, or that it's coil packs.

Neil
its not so much the length of the belt but the distance between the teeth....
otherwise why dont we all use one belt..no matter what car....(thats a rather big generalisation but I'll try to explain)

if you have a belt 100 long and it has 100 teeth then the distance between teeth is 1...
however if its 100 long and has 99 teeth then the distance between the teeth is 1.01...whilst this is only 1%...it will then take 100 revolution for this to become 100% (ie back to the start of the cycle again)....does that make sense???.....

I also agree with what youve said about the other stuff....Im just running with jackos initial theory of wrong baelt and what happens......

cheers
G

[GeoffSmith]

I think that GST were saying that the alignment between inlet and outlet cams was "one tooth out" rather than the belt being wrong. As Neil says, the length of the belt is largely irrelevant (don't forget that those with AirCon have a longer belt) as it's the distance between the driving shaft and the driven shaft that determines the phase difference between the two. The relative speed of the shafts is fixed by the relative diameter/number of teeth on the various shafts.

As Gareth says, the technical term for the effect of being one tooth out is "to make it run like a bag of nails".

[dapinky]

Just to try and sum it all up....

There is some confusion about the 'one tooth out' and the thread can be read in one of two (or more) ways.

I read it as being the correct belt, with the cams (or perhaps only one of them) being slightly incorrectly possitioned when the belt was fitted. If this is the case, Gareths 'bag of nails' all the time would result.

However, if the belt caused the cams to rotate at a different speed than the crank, then the other scenario of the valve timing changing with each revolution would apply, but i can't see that being the case as it would be the number of teeth on the pulleys which would cause that, not the number of teeth on the belt.....

....so far?

I can see that if one cam were out by a tooth then we would get erratic running, but from the description, i would initially check that the CAS hasnt slipped, or that the base timing was set correctly to start with.

If they're okay, perhaps there is play in the CAS which is causing the problems.....


....if not, then there are about a hundred and one other things to work your way through, in no particular order i would look at the TPS, IAC, injectors, leads, earth lead which fits under the expansion bottle, all other earth leads to the engine etc etc.



and i think Geoffs analogy of the Air Con belt being longer just confuses the issue - that belt is a drive belt (PAS pump, AC), not the timing belt we are talking about here (although his principle is correct - belt length is irrellevant - it is pulley size which makes the difference in rotational speed of the respective shafts).

Does that help, or just confuse things more?

I'm sure that the belt can be checked quickly by Specky, and then who knows how long it will take to issolate an Elan gremlin

[GeoffSmith]

and i think Geoffs analogy of the Air Con belt being longer just confuses the issue - that belt is a drive belt (PAS pump, AC), not the timing belt we are talking about here

How dare you Sir! My point was that even with AirCon, the PAS pump still revolves at the same speed. Try this one if you're still confused: Think of it unwrapped like a rack but with many pinions. In the Wiki animation, the driving wheel would be on the left and the driven wheels would be placed on the right (because you can't push things with a rubber belt!) The only thing that can change is the distance between the contact points between belt and wheel which is constant once set up (but can obviously be set up incorrectly). The diameter sets the speed of rotation and the returning belt can be any length you want as it ain't doing a thing. With the timing belt, it is placed under tension to stop it lashing about.

[theelanman]

ah...another variation....

geoff is correct too.....Id not read it like that...but actually that makes more sense...
correct belt but one of the cams has rotated slightly when the belt was put on and as such will run like a bag o nails permanently......sounds more probably now that Ive reread the one tooth posting...

title should be..... 'what if one of your cams is 'out by one tooth''?

[Jackonicko]

It's Ok, Gareth, I heard you the first time. Pardon?

[theelanman]

It's Ok, Gareth, I heard you the first time. Pardon?

pc crashed and went off on one just after I pressed submit.....
sorry
extra posts deleted

[Jackonicko]

I preferred the second one.

[mnemonix]

I think that GST were saying that the alignment between inlet and outlet cams was "one tooth out" rather than the belt being wrong

That's exactly what I read the problem to be, in fact I've never heard of a case of a belt with the wrong number of teeth.
If that is the case, then you have every right to be very cross with the garage that fitted it.

Chris.

[techlectic]

I replaced the cam belt myself when I got my car and know that I followed the procedure in the manual correctly. The car ran great on a first test drive, but then it ran like crap on subsequent drives - like a normally aspirated engine. The boost gauge moved, but there was no "ooomph". The idle was not bad though. A check with a timing light revealed that the timing was off and could not be set correctly. Apparently the belt jumped a tooth, presumably because I didn't get the tension right despite my best efforts (I used weights from my weight set as I was never able to track down the tension gauge mentioned in the manual). That was 2 years ago and the car has run great ever since resetting the cam timing.

[Jackonicko]

Sounds, familar.

I'll await Dr Specky's input with interest.

[DeeAitch]

Never easy to get the timing marks lined up - a lot of tolerances involved - always seems to move a tad when you tension the belt and no doubt moves again after a few miles and the belt settles in
Important thing is to get the valve timing right and before you ask no, I've never checked mine
Now whatever happened to the vernier pulley thread

[ovi]

Hit the pistons the valves if the belt breaks?
Also important to know!

[ovi]

Hit the pistons the valves if the belt breaks?
Also important to know!

Nobody ???

[ailwyn]

Some people say they won't, some people say they will, some people say you'll be ok if it breaks at low rpm, but will do damage if it breaks at high rpm. Nobody has a sure answer.

[Jackonicko]

Thanks to Dr Specky, we know that it's the right belt, but not, of course, if it's on the right teeth!

We took some ElanScan readings, which will be winging their way to someone who can make sense of such things.

Driven hard, it does overboost a little (no engine warning light, though, and not like it used to), and there is some roughness on acceleration.

Thanks again to Dr Specky.

[Specky]

Ok...Here's Jon's results

Could an expert cast a discerning eye please

The idle run was carried out whilst engine was slightly warm, from about 80 degrees to when the fans cut in

The second run is a normal drive....

A bit of additional information,

when I drove the car a little more spirited there was overboost of 0.75, I'll sort the waste gate when I get a chance...

TTFN

[Jackonicko]

Thanks Dr Dave,

MUCH appreciated.

As a techno gumby with a Mac, I'm very much in the hands of any more capable folk who can do something with these.

Geoff, perhaps?

[Jackonicko]

I'd be most grateful for any analysis of these.

J