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Re: knocking in boost, low boost
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Posted by Ari [Email] (#2847) [Profile/Gallery] (more from Ari) on Fri, 12 Mar 2004 09:56:37 Share Post by Email
In Reply to: knocking in boost, low boost, JohnR, Fri, 12 Mar 2004 07:53:54
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If the engine is knocking, then the APC will reduce the boost. The APC is much more sensitive than your ear, so if you actually hear knocking, you know that the APC cut back boost.

Fix the knock, and then see what your boost level is. Knock is caused by low octane fuel, timing too far advanced, hot spots in the cylinder, and excessive compression.

Since you're already running premium, you're doing all you can there. Make sure your timing is correct. Check the decal in the engine compartment, but I remember something on the order of 16 degrees Before Top Dead Center (BTDC). Next, make sure the spark plugs are fresh, and properly gapped. And make sure they're the right ones. If you have to pick, go for a cooler plug.

Check for vacuum leaks. Do the spark plugs look white? If so, you're running lean, and lean running can contribute to knock. Fix those vacuum leaks.

If the timing is right and the plugs are good, and you've fixed all your vacuum leaks, get yourself a compression tester, and check cylinder compression. After 300K+, you may have some significant carbon buildup on the pistons. This effectively increases the compression ratio, which can cause knock.

If the engine is knocking because of high compression due to carbon build-up, the proper fix is to pull the head and de-carbon the pistons. There are some systems that claim to de-carbon the head without having to pull it. I have no direct experience with these, but some folks say they work.

Of course, a simpler solution is the retard the timing. You won't get as much performance, but if the engine is knocking, you're getting crap performance anyway. Anything would be better. Try 12 or 10 degrees BTDC, until the knock stops. Run a cooler spark plug, and try and richen up the mixture. That's not easy on a fuel-injected car, but making sure you've got no vacuum leaks is a good start. I assume your car is Oxygen sensor (02) equipped - can't be sure because with 'Kms', you could be anywhere in the world. If it does have an O2 sensor, and it's original, I'd think about replacing it. It can cause the engine to run lean and knock.



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