cam tensioner analysis

the cam chain tensioner on my 1.8t aeb engine was worn badly, so i did an analysis to see what causes the rapid wear and how it could be prevented.
the tensioner is of the hydraulic type, using engine oil pressure to force the shoes apart and tension the chain, i will explain its workings and why it failed as i go, bare with me.

here is the tensioner in place


here it is removed
the oil flows into the unit and forces the top piston up, tensioning the chain, its important to note that its only the top shoe that moves up, the lower shoe stays in place.
there is a valve inside the unit which stops the oil flowing back when the car is turned off, if this happened the top piston and shoe would drop thus letting pressure off the chain and it would be slack on startup.


here is the top piston and shoe removed, also showing helper spring, this spring is to keep the shoe partially up when the unit is being fitting, before the oil pressure reaches it to give it the final tension.


here is the one way valve


now another function of the unit is to oil the chain, unlike the 16v chain which sits in a bath of oil the 20v chain does not, so it needs to be oiled, here you can see the tiny oil hole under the bottom shoe, oil flows into the bottom chamber too but does nothing with regards movement, it just flows out the small hole, through the shoe and onto chain.


REASONS FOR FAILURE
1. the gasket between head and tensioner can leak leading to lack of oil pressure to the unit, this in turn causes the the piston to drop, the chain then starts to slacken causing the chain to hammer on the guides and wear them out very rapidly.

2. and this is the main reason for wear from what i can tell from taking the units workings and everything else into account
as we all know the 1.8t suffers from sludge build up, well this is a tensioner killer, for one reason, the tensioner has its own tiny filter(15mmx10mm stainless gauze) located between the unit and the head face/oilway. sludge builds up fast on this filter clogging it, therefore stopping the flow of oil to both oil the chain and push up the top shoe, causing instant failure. below you can see the hole/recess where the filter sits and the filter left on the book below it, its completely clogged with sludge making an oil tight seal.....not good


i think the design is completely stupid in such a critical area, why have a tiny filter there after the main filter, the gauze is extremely fine, i mean really really fine, and there is a good flow of oil through it after all....it does oil the chain.

so thats it guys, im off to get a new one, it is tempting to remove the tiny filter before i fit the new one, ill have to think about it a little more before i do, i mean the oil hole that supply's oil to the chain is plenty big(1.5mm) so i dont see a problem there.

i hope this helps as i reckon it was worth thinking it out totally for a simple looking unit its workings are complex enough and it has 2 main jobs to do, relying on an oil supply coming through a tiny badly designed filter....

so if you have a 1.8t i suggest pulling the unit and cleaning the filter and renewing gasket before its too late and the part wears out.
shoe wear also effects performance as the two cams go out of sync as the the chain gets slack.

Brian.

ps, this is the non variable valve timing adjuster, the newer 1.8ts use a servo controlled tensioner which alters cam relationship at higher revs by tighting or slacking the chain, but its workings are very same and it has the gauze filter too, the variable valve timing adjuster is over 500euro from vw so it is something to look into as its a serious matter and not one to turn a blind eye too, you have been warned.

 

i see your point, but the gauze could only effect flow and not pressure, i think

 

well your a fat lot of help so and there was me thinking you were the online hydraulics expert

 

i think ill leave it in to be sure, i just pulled the dissy plug and fitted a bit of garden hose to the oil pump shaft, with the tensioner out and the cam off i pumped about a pint of oil up the oil ways, so im hoping its cleared any ****e in there, im after rebuilding the bottom end so thats fine.
worth keeping in mind i got the a6 for 300euro(it has all the toys, real bonus for once, even electric windows ), so the over all costs arent high, happy days

 

I am away to do the tensioner on my Passat and was going to replace the chain as well.
Can you get the chain off the exhaust cam without unbolting it from the carriers?
Found this how to guide http://www.passatworld.com/forums/showthread.php?t=215732
but didn't see why the exhaust cam retaining caps had to be loosened if the original chain was being kept?? Haven't been inside the cam cover of an 1.8T but i am sure it will all become clear when i do.

 

i wouldent bother changing chain, they dont wear much, and ive NEVER seen one to snap, if it looks good it is good.
and no it cant be pulled without lifting exhaust cam, not even if you slacken caps, as ya know this makes the job bigger as the timing belt has to come off and the car put in the ''service position'' unless of course your changing t.belt anyway, two birds with one stone as they say....got my new tensioner today, febi part, 176euro[:^(] and 5euro for gasket

edit, the exhaust caps dont have to be loosened if its just a tensioner change, just the inlet and swing the whole assem up

 

Thanks for the info
Just finished a marathon trawl on the net and yes it looks like the chain should be ok
Quoted 101 from the dealer for the tensioner.
There does seem to be a major issue over the pond with 1.8T and sludge in A4's and Passats...!!

 

101? damn we get screwed for everything in Ireland
no bother fella on the info, glad to shed some light

 

Pretty sure mine has gone on my 97 A4 1.8t... rattle metallic noise coming from that area at around 2k revs... sounds like its a fairly common problem though! so will get it sorted asap now.

Is the Tensioner & Gasket available from GSF?

 

i dunno mike, i got mine in otto, same part as main dealers and price too, but otto was nearer, gasket is main dealers only, the half moon rubber gasket out of any vw engine can be used, i used a 16v one as i have about 10
the noise your hearing is the tensioner, sounds like diesel rattle, i turned pump just to be sure when i had it out as the aeb is shaft driven, bit of hose and a drill, i got a few bits of ****e up the oil way which i was happy about.
just fitted new one there, the cams had gone outa sync about 2-3mm from my original marks, the shoes were worn that much, so hopefully the noise will be gone, and all my power restored
out of interest i left in the mini filter, johnny had me paranoid

 

right, new tensioner in place, check out how much the cam timing was off with the tensioner worn, theres a good 3mm in it(intake cam)
i had set the cams and marked with paint before i stirred anything, once refitted with 16rollers of the chain between sprocket dead centres the amount it was off showed once the new one went in place, so it should be good to go now.

check out the amount it was off, look at mark on cam cap in relation to sprocket mark


and heres a pic of the tensioner compression tool, it comes fitted to the new part, but incase anyone wanted to remove the unit for any reason and wasent renewing it, heres the tool needed to compress it, it could be made easily if needs be, the basic dimensions show up in the picture, the bolt is of an m6 thread.

 

can you remove just the noid and clean it..my motor is running too good to tear apart just keep getting the p0012 code

 

thanks good info and just where I have got to with am AGU I am rebuilding. The friction pads were scored, so I went to the dealers and guess what, you have to buy the whole tensioner. Strange.
You can get pattern ones though.
I found the same filter, didn't like the look of it for 1 minute so I'm not fitting it.
As I understand them filters have a nominal diff pressure across them, clean and for a certain flow. The flow into the tensioner will be through the lube oil spray hole, if we knew the diameter some meche eng far cleverer than me will be quantify the flow.
Filters are usually over designed so partially blocked doesn't immediately cause a large DP rise
As they get more blocked, so the DP increases and flow reduces
Thanks for the pic of the fitting tool, diy for me.
This stuff I have picked up from work
Jon