CAV MK3 CAMBELT RUNNING OFF LINE

[nms11]

hello there. Can any one help please? I have a 2 litre 1993 mk3 with 225k on clock. runs beautifully but the cam belt runs approx 3-4 mm further away from the engine than it should. it recently pushed the water pump belt retaining edge off which lodged in the belt plastic cover and burned a hole in the cover. I am lucky it didnt do worse things.

I have recently had a new pump twice, a new adjuster pulley twice and had the cam and crank pulleys off to see if anything is amiss. nothing showing as obvious. I believe it is the camshaft pulley that is pulling the belt off. If i leave the tensioner slack and push the belt right back to the engine and then turn it over by hand it is the camshaft pulley the leads the move to come off line.

? i think it is the camshaft bearings or the thrust plate holding the camshaft endfloat and this then allows the camshaft pulley to tilt and the belt runs out of line. If it is the thrust plate wear then I would expect the belt to just be further on or off the pulley because I think there would be no tilting. but something definitely drives the belt off line and it stays so.

any help welcome. I have to go soon but need to get answers asap but can get back tomorrow. thank you.

regards nms11

[btcctroy]

Never Heard of that one tbh.
So if you push the belt forwards the engine it works it's self back out.
Use a straight edge across the pullies, you should see the cocked pulley

[nms11]

i have put a straight edge as you say but it does show anything. if there is any thing to see it is very difficult because it is so small an amount and there is nothing to line the straight edge up against. sounds a bit naff that but the tilt must be very very slight as it takes perhaps a dozen hand cranks to start the move off the pulley.

i can,t see any threads anywhere on a similar topic. had several very high mileage cavs with this engine and not had this one.

could you think of any other source of a reason for the belt running off?

many years ago had an hs chevette that did it and even rally experts couldn,t solve it. we left it offline but it didn't foul anything cos it had plenty of pulley room and space offline. this one is different.

appreciate your interest.

[Robsey]

That is so bizarre -

You have already replaced the water pump and tensioner multiple times, so we can assume that they are okay and also fitted flat against the face of the engine block.

This leaves the Crankshaft and Camshaft.

Crankshaft I cannot see being an issue as there are several bearings around the journal, and for this to skew enough to cause the sprocket to tilt - well your bottom end would be pretty much scrap.
The sprocket itself should be mounted securely with four external torx bolts.
Not much chance of working loose.

So last man standing is the camshaft sprocket or bearings.
The sprocket is held on quite securely with M6 hex head bolts so not an issue...

So it must be the camshaft bearings or the bores having excess wear.

Have you tried releasing the timing belt, and then trying to waggle / move the camshaft up and down within it's bore?.

If my theories are correct, I think the only "proper" fix would be a replacement camshaft housing.
I would expect some excessive noise if any bearing or bore is worn.

[nms11]

i am at exactly that place you describe but i only have a very slight whine when the engine is running that could be the last water pump that i am currently replacing. i have tried to move the camshaft but it doesn't do anything i am assuming because of the pressure of the valves pushing it upwards???? can't get it to move in or out either.

I am very frustrated cos the only solution seems to be to take off the carrier and hence the cylinder head. maybe then a new camshaft and carrier is the cure unless I find out different to that. It it were to be the thrust plate It doesn't seem to be able to replace it without again taking off the cam carrier and ehnce again the head?

Are new/ refurbished carriers available and camshafts? if so are they mega bucks? I would be ok to do the head but not spend loads on the parts if it possible to refurb them etc....

appreciate your thoughts.

[Robsey]

My thought is.

The bearing / bore at the sprocket end is constantly being pulled downward by the tension of the belt.
If wear is sufficient, then this would allow a very small amount of drop on that end of the shaft, but I have no idea how much drop you would need to cause the belt to run off line.

There could also be a proportionate amount of upward force and therefore upward wear on the distributor end of the shaft bearing / carrier / housing bore.

At 200-odd thousand miles of wear, who knows.
I had no play whatsoever on my 225,000 mile C18NZ camshaft and housing before I changed up to a C20NE four years ago.
I had only changed the seals, nothing else within that 16 year period.

Regarding replacement parts - I can only suggest getting a compatible camshaft / housing from a scrapper if working to a budget.
Others may have better information on suppliers of new parts (if there are any) - bear in mind however that the newest Cavalier is about 19 years old.

[nms11]

I am with you very much on your theory - unfortunately!!

I think I can source a cam carrier off a 60k engine that was in a calibra.I could use the cam if it is the same? I was hoping beyond hope that you guys could give me another line. or alternatively confirm the problem before the dismantling begins.Which you have very kindly done. Thank you.

i would appreciate your view on whether you would replace the camshaft as well as the carrier?

I would have to see where the wear was I suppose, but I am assuming the cam runs in the bare aluminium? this would make that wear first?

As a similar aside, do you know if the cam thrust plate will come out without taking the carrier off? Are you thinking the same as me, that if it was the thrust plate the movement in the cam sprocket would just be in and out and not tilted?

I very much appreciate your thoughts and discussion, most of my family glaze over on car talk!!! thank you.

[nms11]

Sorry, forgot to ask, do you know if the cam carrier can be refirbished, say by boring and sleeving? thank you.

[planetc]

If the carrier was worn enough to make the belt run off I'd imagine that you'd be able to measure the movement in the cam. If the crank was it would be knocking it's bollards off. If you have replaced everything else then the only other thing that springs to mind is the possibility that the head has been badly skimmed giving the whole top end a subtle tilt, but it seems far fetched.

[nms11]

yep i understand the way u are thinking but as far as i know the head hasnt been off and it did run on line approx 10k ago so thinking that it was ok.

have put the car offline at mo cos mot due. will report back when i have mot done and maybe replaced the cam carrier and maybe the head.

as a note - when got the car it had done 100k plus and the oil was very very old.two cam lobe holes were bocked and the oil breather was too. it had done most miles on the motorway but had not been loved. I change oil and pamper them but I think that the damage was done in its early life and i am now paying for it. maybe even the cam bearing feed is/was blocked. will see when i dismantle it.

thank you for the ideas.

[colin1]

...runs beautifully but the cam belt runs approx 3-4 mm further away from the engine than it should....

I've never heard of this issue before but which specification is the above 'error' based on? I ask because I've never seen (or heard of) a GM spec that stipulates how far from the engine the cam belt should run.

Pull-down on the camshaft will be negligible; the belt is tensioned via the water pump for small-block and tensioner for big-block and I'm willing to bet the GM design team got that right; there is no way that you can impose enough tension on a camshaft via either method that would incur destructive pull-down forces. Don't forget that the shaft is bolted to the carrier equi-distantly along its length. You haven't mentioned oil everywhere so possibly not a worn out cam oil seal though I doubt even that would incur any pull-down.

I don't really know what to suggest.

When the belt moves this 3-4mm what happens next? Does it continue to sit there and 'run beautifully' (your words) or does it continue its migration towards the outer edge and threaten to jump off the pulley?

I'm just guessing here but if it continues to remain at a distance of 3-4mm from the engine and run sweetly, I think your belt is telling you that it's quite happy there. Are there any features of contour or relief on the block (ie non-rotating parts) that would push the belt out once the engine starts cranking?

Could you explain this water pump part that detached and lodged in the casing? I'm not sure what bit you mean.
Has it happened since? Just wondering if it's a red herring..

[Ads]

id be very careful with parts coming of the water pump, the outer ring came off one of my water pumps and fell down into the crankshaft sprocket, jamming the timing belt, resulting in 8 bent exhaust valves, ouch, Ads