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What's in the workshop this week? Jim's black Final Edition
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Its that time again :) Jim (The Laird) brought his spotless black FE down for new cam tensioners as they are rattling on start up. I'm going to pull the engine and re-seal everything as it has a few leaks, plus a general tidy up and fettling in the engine bay as it's on 180K miles now.
Here's Jim's (left) next to Stephen's (What was in the workshop last week) before it came in to the workshop: http://forum.a8parts.co.uk/attachmen...1&d=1553983807 First job on Thursday was to remove the bumper and headlights in preparation for pulling the front end. I left the front panel in place as I needed to move the car off the lift on Friday, but then got back on it today http://forum.a8parts.co.uk/attachmen...1&d=1553983807 http://forum.a8parts.co.uk/attachmen...1&d=1553983807 A small pile of bits starting http://forum.a8parts.co.uk/attachmen...1&d=1553983807 >> to today: Split front snubber mount - that's pretty standard so I keep them in stock. http://forum.a8parts.co.uk/attachmen...1&d=1553983807 So this is how its going to be then? :rolleyes: Two snapped bolts taking the front panel off :( I'll deal with these later. http://forum.a8parts.co.uk/attachmen...1&d=1553983807 http://forum.a8parts.co.uk/attachmen...1&d=1553983807 One of the oil cooler bolts in the rad too. This is going to be fun http://www.corradov8.com/pics/banghead.gif I can see a special tool in my future... http://forum.a8parts.co.uk/attachmen...1&d=1553983807 A bit grubby under here after 180K miles http://forum.a8parts.co.uk/attachmen...1&d=1553983807 http://forum.a8parts.co.uk/attachmen...1&d=1553983807 We can fix that +++ |
Amazing work +++ that will be one happy owner when its returned!
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Yeah that's it. The chain tensioners use oil pressure to keep the chain tight between the two camshafts on each head. As they wear inside (I think its o-rings) they develop a tendancy to leak away residual oil pressure while the engine isn't running and they lose their push on the chain so it gets loose. When the engine starts again, the inlet cam is pulled so far by the driven exhaust cam, but then when it gets over-centre on a lobe it jumps forward, pushed by the valve springs. It then pulls the chain tight and slaps it on the tensioner pad - that's the noise that makes the rattle. After a few rotations of the engine there is enough oil pressure to tighten the chain and it stops making the noise.
It starts with rattle on cold start, but Jim's had got to the point where it would rattle on hot-start too so the tensioners are losing pressure quite quickly so need changing asap since the slap of the chain obviously doesn't help the life of the plastic pads. I've tried changing the oil retention valves which keep oil in the heads on shutdown but they have no effect. Its oil pressure leaking past the piston in the tensioner that is the problem. I did have an idea about using a thing called an 'Accusump' which is basically an oil pressure accumulator used on race cars. This would collect oil under pressure while the engine is running, store it while it's off, and then release it all when the key is turned. Theoretically this would put pressure in to the tensioners before the engine fires and therefore stop the slap. This would be cheaper than replacing the tensioners (£576 each!!), but wouldn't actually fix the problem of the tensioners bleeding off oil pressure. I think it might actually mask it such that you wouldn't know there was an issue until they bleed so much pressure that the engine has an oil pressure problem. I did see someone post on FB about rebuilding the tensioners with new o-rings which is worth investigating. I can't see how they come apart in a fashion where they can be put back together but I have some dead ones which I can cut in half. At some point we're going to run out of new ones so we need to find a solution of some sort! |
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One more question before getting back on topic with Jim's car. I'm assuming this is why the tensioner pads wear excessively, and why you replace them (although it doesn't resolve the underlying issue)? |
The tensioner pads wear simply because they're made of plastic and are pressing against a steel chain running over them at considerable speed. It's designed obsolescence - almost all of them made it through their ten year warranty and they're now exceeding their 15-year expected lifespan. Audi don't care if they all explode at this point.
The pads are made of nylon 4.6. If they wanted them to last longer, they could have used a different material like UHMW polyethylene, Acetal, even PTFE, and provided an oil feed to the wear surface to further reduce friction. This would have been more expensive though. Nylon 4.6 lasts longer than the warranty so it will do. Once the pads are sufficiently worn, they crack and fall apart whereupon the engine shreds itself. A loose chain slapping on worn pads makes them more likely to shatter, but is not the cause of the wear in the first place. I replace the pads on non-rattly tensioners to avoid them shattering, but the tensioners will still wear out eventually. |
Great work MJ +++
Good to see my car's twin getting some attention :) I need to arrange to get mine down to you some time in the near future for a bit of TLC and a few relatively minor jobs, but mainly for the tensioner pads, even though I have no start-up rattle .... yet! (130K miles). In my ten years of ownership I have always changed the oil far more frequently than is necessary though ... I mean obsessively frequently. Whenever it goes into the garage for anything, I usually ask them to drop the oil again. So sometimes it'll get 3 or more oil changes in a year, even though I'm averaging at just 5K-6K miles annually. I wonder if that's helped the tensioner seals to last a bit longer? Quote:
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Great to see how you are turning back the clock (metaphorically) on Jim's car MJ. Excellent tread.
Jim, I bet you're looking forward to getting back into it again. I know I would be. |
Mark, I just checked if they fit anything else and they are also used in the D3 so we should have supply for another 5 years in theory.
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