Well, some blood, sweat and fortunately not too many tears later I have a new timing chain, tensioner and two of the three guides replaced. To change the third chain guide (on the inlet side of the engine) looked to require the whole of the front timing cover to come off, something I wasn’t about to do as the guide had only very slight wear. I connected the new chain to the old and carefully turned the engine, trying my utmost to keep both cams in sync with the crankshaft. A few heart-stopping moments as of course the cams tend to spring forward when a valve is closing. The official MB tool keeps the chain in mesh with the sprockets, but I didn’t have that luxury. Once the new chain was through, disconnected the old chain and rivited the link joining the new chain.
The result is the inlet cam timing mark is again spot on with the crankshaft at TDC, but the exhaust cam is still retarded. Comparing the alignment now in the latest picture below with that in the original post, it is improved to what it was. Just to show it’s not possible to get it any better, the other picture below shows the cam advanced by one tooth on the sprocket – way advanced and out by a bigger amount. Note these two latest pictures only show the exhaust cam.
One thing I should have photographed was old the chain itself. I kept thinking it was my eyes, the chain seemed slightly 'skewed' in the short section between the two cam sprockets as if the side of the chain towards the front of the engine was slightly longer than the side of the chain at the back of the engine. Wear marks on the back face of the sprockets confirmed what I was seeing was actually true. I think that was enough evidence alone to convince me to change the chain and I’d encourage anyone with the M271 on more than 100K miles to think about replacing the chain. The whole job, chain, tensioner and guides didn’t set me back much over £150, so I’d reckon it was a good insurance. I’d bought a chain kit, but it doesn’t include the various expansion plugs you have to sacrifice to get at things like the tensioner, so be prepared to do a bit more shopping. I also took the opportunity to change the coolant.
Regarding the rattle at start up, I don’t think I’m noticing anything different with the new chain. I can understand the adjuster in the sprocket rattling as what little spring pressure there is holding it back against the end stop would be overcome as the cam springs forward when valves close, only to be forced back against the end stop again when the next valves are pushed open, all until it gets oil pressure anyway. I would liked to have changed the sprockets, but without significant wear to the teeth, I just couldn’t justify the cost. If anyone has an idea why it isn’t possible to get the timing absolutely spot on or if this is just typical of the engine, please can they let me know?
Goldbird – that bulletin I mentioned about the rattling cam adjusters, I had sight of the MB workshop manual. I’m not sure how I’d stand over copyright to post it here. Perhaps someone could advise me?
And the bad news: the new chain didn’t fix the intermittent P0020 engine management fault it was getting. I have solved that by other means - details in a separate post.
