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Ah - you're back Posted by Ari [Email]  (#2847) [Profile/Gallery]
(more from Ari) on Wed, 8 Aug 2001 11:47:05
In Reply to: Timing chain rattle ( Greg D.), Tino, Wed, 8 Aug 2001 11:24:12 Members do not see ads below this line. - Help Keep This Site Online - Signup |
I missed the thread. Sorry.
Time for my timing chain rant.
You can't diagnose a timing chain by sound. By 'you', I include Saab technicians. Saab timing chains rattle. So do many other things in the engine. Worn chain guides, noisy injectors, lazy lifters, and worn fuel pressure dampers all make for rattle at the top end. A mechanic has no downside to recommending a chain replacement. A new one is always better than an old one, and it's money in his pocket.
The disadvantage of the 'roll-in' method is that it only replaces the chain. If the timing chain is truly worn, chances are good that the chain guides are also worn. Those can only be replaced by pulling the chain cover, which requires engine removal in a 900.
There was an update to the chain tensioner - putting in an updated chain tensioner is always a good idea. The old style was a cadium plated
plain 27mm flat bolt from exterior appearance. The new is black 27mm with a
12mm bolt on top, considerable taller than the flat predecessor.
The only good way to determine if a timing chain is bad is to inspect it. Measuring the chain tensioner extension just won't do. The inspection is pretty easy - complete details in the 900 Bentley manual. All you do is remove the valve cover. Inspect the chain and sprockets for wear. Turn the engine over to 0 degree TDC for cylinder #1, and check if the alignment marks on the cams line up with the marks on the bearing blocks. If they line up, there is no chain stretch. Chains really don't stretch - the pins and holes wear, and that little bit of extra play adds up over the chain to increase its length. It's that extra length that's a sign of wear, and that extra length that contributes to noise.
If the chain looks OK and the marks line up, the chain is fine. All you're out is the cost of a new valve cover gasket, and chances are the old gasket was leaking oil onto the exhaust manifold and into the spark plug wells anyway. Total procedure should only take about an hour, most of that in scraping the old valve cover gasket off the surfaces.
My recommendations -
If the engine only rattles when cold, and the rattle goes away once the oil warms up, don't worry, be happy.
If the engine rattles at idle when warm, make sure it's the chain. Use a couple of feet of garden hose, one end held up to your ear, as a stethescope. Make sure the sound is coming from the top of the engine on the belt end, and not from the fuel injectors or fuel rail.
If you suspect the chain, use a wrench on the main pulley to turn the engine over BACKWARDS a turn. You can also put the car in 5th gear (if a manual) on level ground and push the car. Much easier with spark plugs out. This causes the timing chain tensioner to reset.
If you're still worried, inspect the chain tensioner and replace it if it's the old style. This can't hurt. A tensioner is about $50 for the part. It is a pain to replace in a 900, though.
You want to be sure? Pull the valve cover. Buy yourself a new gasket, and a new upper chain guide. The upper chain guide is a rubber piece that fits into the top of the valve cover, and you can replace it when you pull the valve cover. Now you can inspect the chain right. If it looks good and the marks align, then you know the chain is OK.
It's about 4 years now that a Saab dealer told me that the timing chain on my wife's 9000 was about to blow. It did rattle when cold, and some when warm. I replaced the chain tensioner and it didnt' help, but going to the new style is always a good idea. Last year I pulled the valve cover since the gasket was leaking, and inspected the chain and replaced the upper valve guide. The chain looked fine, and the lines all lined up great. The chain still rattles, and it's been 60K miles since the mechanic warned me of impeding doom.
That's not to say your chain isn't worn. But there's only one good way to check it, and that can either set your mind to rest or tell you what you need to do next.
Good luck!
posted by 140.157....
, Wed, 8 Aug 2001 11:47:05 <-- Viewing This Message
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