Re: Engine knock

Posted by Don Wadge on January 07, 2003 at 18:58:11 from (142.161.0.91):

In Reply to: Engine knock posted by CH on January 07, 2003 at 16:29:41:

All good diesels have ignition knock. That's what a diesel is. OK, that probably got your attention. But, in actuality determining an ignition knock in a diesel engine takes an ear familiar to diesel engines and maybe even the particular diesel engine involved. A diesel engine will often knock ferociously when you first start it cold in "cold" weather. After it warms up a bit it probably won't sound out of place at all. So this may be your problem only when it is cold. In circumstances where you actually have a pre-ignition knock it can be caused by a leaking injector which allows fuel into the cylinder before the "crack" presure is reached and the cylinder will fire too much in advance of TDC, or the injector will continue to dribble after it has closed and you can have the same effect the next time the cylinder comes up on compression and the dribbled fuel will ignite before the injector opens causing again a pre-ignition knock because the cylinder fired too far in advance of TDC.
Usually, if the knock is due to a cold engine all cylinders will sound somewhat the same. Whereas if it is a bad injector that cylinder will stand out from the others and if your ear is trained you will hear that and maybe even know which cylinder is at fault. You can determine which injector is at fault by loosening off the line at each injector until the fuel leaks out at the nut and that cylinder will not fire. If you have one cylinder that stands out from the others and the extra loud knock disappears when you do this, you have located the faulty injector. Sometimes you will only notice a loud injector when the engine is cold and after the engine warms up it will operate satisfactorily.

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