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'07 Cobalt, 70K miles

I need to replace the water pump, and possibly the timing chain. (The Cobalt pump is on a separate chain, with the balance shafts.) I can get parts with or without sprockets.

Is sprocket failure a thing? My inclination is to use the old ones, unless I find some obvious injury.

  • Rule of thumb, If you're going to change any chain, then change the sproket along with it, the new chains usually come with new sprockets... if the difference between your new chains dimensions and the old sproket are off by even a tiny amount you'll be changing the same thing over again, the new chain was manufactured to the new sprokets specs for your car. – hello moto Oct 1 at 5:29
  • A water pump failure at 70K miles is not all that unusual but a timing chain? That should be good until at least 100K, probably longer. Does the owners manual give a recommended service interval for this chain? Unless it says to replace it at or near 70K or you know there is a problem with it, I think this falls into the possibly unnecessary repair category. – jwh20 Oct 1 at 13:20
  • Running a water pump with a chain, how stupid is that GM. – Moab Oct 1 at 15:59
  • @jwh20 re the timing chain: There's a 'tinkling' noise on cold start. Plan A is to just replace the tensioner. I see a lot of talk about the original tensioner design being flawed. It's also a lot easier to do than the whole chain kit. – George Oct 1 at 17:42
  • That's not uncommon with many engines using an oil-pressure driven cam chain tensioner. Until the oil pressure comes up the chain is a bit loose. If it stops after a few seconds, then that's not a problem. – jwh20 Oct 1 at 18:53
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The timing and balance shaft chains on these Ecotecs are very long-lasting, and usually the sprockets don't need replacing - what happens is that the chain stretch plus wear on the guides goes beyond the capability of the tensioner to accommodate. Usually by that point the stretch isn't sufficient to wear the sprockets, so I'd be tempted to just look at the sprockets plus the guides carefully for signs of wear, and re-use the sprockets. There are chain kits including all the sprockets, guides and tensioner, but I doubt that the quality of the sprockets is as good as the originals. At 100,000+ miles, my sprockets appeared in perfect condition. The balance shaft chain has an easier time than the cam chain, and isn't a common wear issue, other than the guides.

The tensioner on the camshaft chain has both a spring preload plus an oil pressure loading, and the 'new' tensioner moves more of that balance to the spring, so that the tension on startup, before the oil pressure has come up, is better - reducing the possibility that the chain will lose tension as the lumpiness of the cams overcomes the preload applied by the tensioner. That's what causes the rattle. There's a staircase type travel limiter built into the tensioner, so it can't be pushed back by more than a certain amount.

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