[urq] What causes knock ?

Ado Sigal a.sigal at bluewin.ch
Thu May 11 21:59:47 EDT 2006

Louis-Alain Richard wrote:

>The second strange sound is identified, it is the fuel tank collapsing
>after a few minutes on the highway, a big "bong" sound. Today, as the
>fuel tank was nearly empty, I stopped and noticed that the fuel cap made
>the "iiisssssss" sound of air entering the fuel tank. So there are no
>fuel vapors going back to the tank, IMO
>Now, are these 2 problems related ? Can a fuel tank that is under vacuum
>stops the fuel flow to the pump and starve the engine ?
>Then, how can I find the problem with the fuel vapor circuitry ?
>And my title question remains: what causes knock ?
-  On CH side,  the ball valve usually gets stuck, if not, other 
blockage in tank breather lines. Believe the vapour circuit includes 
charcoal canister over there, not sure.

- Yes they can be related. since knock can be caused by lean conditions, 
to much advance, to hot plugs, to high cyl. temperature and combustion 
chamber hot spots. Test it with p/cap loose, and note if it improves.



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