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.
OR your EGR pipes are getting worn (like a hole in your muffler, but then coming from under the hood)
EGR stands for Emissi>
Er .. when I was a lad, it was "Exhaust Gas Re-circulation" - but I suppose the effect is the same!
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