Just about anything that interferes with proper combustion makes a
diesel knock at idle.
Low compression from leaky valves is a possibility, especially if they
are also a bit tight. Bad injectors can do it, so can dirty injectors
or prechambers (carboned up nozzles).
Late timing, leaky delivery valves seals on the IP, etc.
Low compression will give you hard starts cold, so if it fires right
up, that's not the issue.
Get a bottle of RedLine or similar (Diesel Purge is also good) injector
cleaner and pour the whole bottle into a full tank. Drive it out,
should quiet down quite a bit. If it comes back on the next tank of
fuel, it's an engine problem. If not, just carboned up injectors!
Do also check the suction line from the tank to the injector pump lift
pump and the manual purge pump -- if you have a leak there, it will
knock. If the fabric covered hoses are wet with fuel, replace the
hoses, they leak. If the hand pump leaks when you use it, replace it,
air is getting into the IP.
Also, check the timing chain "stretch" (wear) -- if the crank is 8 or
more degrees late with the notch in the cam thrust washer aligned with
the "nub" on the front cam tower, the chain needs to be replaced. This
is a fairly simple job, and will restore milage, performance, and rid
you of the knock.
Peter
