frequency valve, 200tq '89 MC2

[isham-research.freeserve.co.uk at pop.pol.net.uk]

> what is cavitation?

The fuel pump can happily deliver 10 bar.  That means it could generate
10 bar of vacuum, were such a thing possible.  If the screen gets blocked,
the pump is quite capable of achieving near-vacuum conditions between
the inside of the screen and the actual pump mechanism, and fuel will
boil into vapour, which the pump then recompresses into liquid.

Cavitation is the process of vapour formation at the low-pressure side
of a pump or propellor.  Short-lived vapour bubbles.  Noise - this is
how nuclear submarines are tracked underwater.

If the screen isn't _quite_ so badly blocked, there will occasionally
be enough liquid fuel between it and the pump proper for the pump to
deliver liquid fuel without cavitation.  This gives rise to short
breaks in the pump noise.

Another way to check for this phenomenon is to use a stethoscope on
the system pressure regulator.  If you hear a steady sloshing of fuel
despite the pump noise being intermittent, this is it.

I'd still follow others' advice and check the relay first, because it's
a much easier job and the odds are quite good.  But be prepared to go
into the tank.

--
 Phil Payne
 http://www.isham-research.freeserve.co.uk/quattro
 Phone +44 7785 302803   Fax: +44 7785 309674