Volvo Penta 230 Workshop Manual page 64

valve. A certain clearance is also required to ensure
that the valve does not remain partly open, allowing
combustion gases to pass and the valve to be dam-
aged. Preferably, the clearance should be as close
to zero as possible. This is possible with hydraulic
valve tappets.
The solution chosen for the 251DOHC, AQ171 is a
spring inside the valve tappet cylinder which holds
the valve tappet against the camshaft, but at a lower
pressure than the valve spring, to allow the valve to
expand lengthwise.
A check valve (ball) prevents oil from being pressed
out when the camshaft pushes the hydraulic valve
tappet and the oil pressure in the valve tappet cylin-
der is greater than the engine's oil pressure.
When a valve is pushed down with the engine
stopped, oil will gradually leak out from the hydraulic
valve tappet and the valve clearance will be greater
when the engine is started and until the oil pressure
has been built up. The distance between hydraulic
valve tappet cylinder and the piston is therefore cho-
sen to ensure mechanical safety.
The hydraulic valve tappet on the camshaft's
base circle
The oil for the tappets comes from the camshaft car-
rier and is pressed into the valve tappet via a groove
and a hole in the side of the tappet. Oil passes
through the slit in the top of the valve tappet and into
the piston. As the engine's oil pressure is greater
than the oil pressure in the hydraulic valve tappet,
the oil passes the check valve (the ball).
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The camshaft presses the valve tappet down
The oil pressure in the valve tappet cylinder then be-
comes greater than the engine's, the check valve
closes and the hydraulic valve tappet works as one
unit.
The valve tappet back on the camshaft's base
circle
The engine's oil pressure is greater than the oil
pressure in the valve tappet cylinder, the check
valve opens and lets in pressure oil so that the valve
tappet is pressed against the camshaft.
Fault-tracing valve tappets
The valve tappets are very reliable and faults are
very rare. If the tappets are noisy, the cause is usu-
ally too high or too low oil level. Check the oil level!
Momentary noise may be heard when the engine is
started. This is normal and is due to the oil being
drained from the tappets when the engine has been
stopped for a period. The tappets are refilled with oil
a few seconds after starting.
Noise at high engine speed, but quiet at low
speed: Oil level above max on the oil dipstick – the
crankshaft whips the oil to foam, which causes the
noise in the tappets. Too low oil level – the oil pump
sucks air at high engine speed or when the boat
rolls. Air in the oil can cause noise in the tappets.
Spontaneous noise at idling, disappears with in-
creased engine speed: This is an indication of worn
valve ball or dirt in the tappets. Noise at idling or
when engine oil is hot, quiet at higher engine
speed or with cold engine oil: Tappets that leak a
lot of oil.