740 sounds like a diesel

Suddenly my -92 740 B230f has developed a rather loud knocking
or tapping sound when cold. It completely dissapears above 2000 rpm.
It only lasts for about 2 minutes, then completely gone.
What could that be?

Thanks,
Ben

 

Piston slap is one possibility, caused by worn bores or pistons. The
noise goes away as the engine warms up because the ally pistons expand
more than the iron bores.

Here's a test for it I found a while ago - I've never tried it, so
can't comment how effective it is:

With the engine cold (first start of the day), remove all the
sparkplugs and put a teaspoon of oil in each cylinder. Replace plugs,
and start engine. For twenty seconds, the noise will have disappeared.
Then it will return until the engine is a little warmer.
--

Stewart Hargrave


For email, replace 'SpamOnlyToHere' with my name

 

OR your EGR pipes are getting worn (like a hole in your muffler, but then
coming from under the hood)

EGR stands for Emission Gas Reduction and the idea is that gasses coming
from your engine get partly redirected back into your engine for more
complete burnup. You'll see metal "hoses" (pipes) coming out of the engine
on the side where things go to your exhaust-pipe and then leading back to
where the air-intake goes into your engine.

 

In an earlier contribution to this discussion,

Er .. when I was a lad, it was "Exhaust Gas Re-circulation" - but I suppose
the effect is the same!