"Butch Davis" wrote in message
ink.net...
Jeff,

No argument. Consider, however, that the engine is lubricated by 10W40
weight oil. When cold the oil is 10W (mas o menos) and loose rod bearings
would not hold pressure well if at all. When the engine is at operating
temperature the lubricating oil is 40W or so. It's difficult for me to
visualize an engine with 10W at idle cold having pressure much higher than
the same engine with 40W hot at 3800 RPM.

The base stock for a 10w40 is 10 weight oil. To that is added long chain
polymers that unwind when hot causing the weight of the oil to increase. So
when cold the oil acts like a 10 weight oil that is cold. When hot it acts
like a 40 weight oil that is hot. The point is that 10 weight oil cold acts
alot like 40 weight oil hot. The reason 10w40 is NOT recommended for most
engines is that there are too many long chain polymers added to the oil and
they breakdown at high temps and cause accelerated wear in engines. The
recommeded oil is now 20W40. This doesn't apply to synthetic oils like
mobil 1 that has no polymers added to their 15W50, or 5W30.


Seems to me the rods would be banging like the devil at virtually any RPM.
My experience with engines is that it is very rare for all rod bearings to
fail together. It is almost always one bad bearing. I guess an exception
would be when the engine is operated with a very low oil level but that
would also destroy crankshaft and cam bearings, no?

Seems so but the engine I saw act up had 15 pounds at idle and no knock and
when the engine RPM increased the pressure would drop and at about 5 pounds
would knock like the devil (oil starved). You can have all sorts of middle
ground between this case and a good engine. In the case we are discussing,
there is obviously enough oil for there to still be a cushion of oil in the
bearing thus little or no noise. In most engines the sling effect IS offset
by the increased volume of oil pumped by the oil pump at higher RPMs.


Thanks.

Butch
"Jeff Rigby" wrote in message
...

"Butch Davis" wrote in message
ink.net...
Far more likely, IMO, is an instrumentation issue. If the clearances in
your engine were so worn as to cause an oil pressure loss when hot with
10W40 oil you'd not have pressure when starting. As engine speed
increases the oil pump output increases. A relief valve keeps the
pressure from going too high. No offense Jim but you clearly know very
little about engines. Better no reply than a totally unfounded reply.

Butch, at first thought you would think the oil pressure would increse as
RPM increases but rod bearings when very bad SLING oil. Like if you
(after priming) sling a hose around your head it will pump water out of
the hose. This causes a loss of oil pressure! Very bad engines can have
15 pounds of oil pressure at idle and that will drop when you accelerate
the throttle. I've read about it and finally saw it recently. Usually
it's also very noisy.

I'd certanly check oil pressure with a mechanical gage if the engine is
not noisy. If oil pressure is low I'd pull the pan and check clearances
on the bearings and if good suspect a slipping shaft to the oil pump.

Butch
"Jim Goodall" wrote in message
...
I'd be checking the crankshaft and rod bearings. Sounds like when
things get hot, clearances get large and simply let all the oil flow
freely out of the bearings, as the crank is spinning on a layer of oil,
within the bearings. It won't be long before you spin a bearing and
ruin the crankshaft.
Jim

josZ wrote:

The oil pressure in my engine, a volvo penta 3.0gs from 1996, is gone
above 3800 when the engine is hot. I use multigrade 10w40 oil. What
could be the problem?

thanks in advance Jos


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