Problem Diagnosis - Mercury 150 EFI Service Manual

COOLING

Problem Diagnosis

Condition
Pressure below specification @
idle
Pressure above 5 psi (34.2kPa)
@ idle
Pressure does not drop between
1000 – 2500 RPM indicating pop-
pet valve has opened
Poppet valve flutter/water pres-
sure drop does not stabilize prior
to 2500 RPM
Pressure is below minimum spec-
ification @ W.O.T.
Pressure higher than normal @
W.O.T., but engine still indicates
overheat condition
Page 4B-12
Recommended Range
1.0 – 3.0 PSI (6.8 – 20.5 kPa)
1.0 – 3.0 PSI (6.8 – 20.5 kPa)
4 – 9 PSI (27.4 – 61.6 kPa) be-
tween 1000 – 2200 RPM
4 – 9 PSI (27.4 – 61.6 kPa) be-
tween 1000 – 2200 RPM
12 PSI (54.9 – 68.5kPa)
Maximum pressure – 23 PSI
(157.4 kPa)
Possible Cause
Poppet valve spring defective
(weak, broken, missing)
Defective poppet valve seal
Thermostat stuck open
Severe internal leak
Low output water pump
Inlet restriction
Plugged poppet by-pass pas-
sage or tell-tale
Wrong poppet valve spring
Low output water pump
Inlet restriction
Poppet valve vent hole plugged
or restricted
Severe internal leak
Defective poppet valve seal
Wrong poppet valve spring
Low output water pump
Inlet restriction
Broken diaphragm in poppet
valve
Severe internal leak
Defective poppet valve seal
Inlet restriction
Engine mounted too high on
transom
Engine trimmed out too far
Configuration of boat bottom in-
terfering with adequate flow of
water to coolant inlets
Severe internal leak
Low output water pump
Outlet water passages restricted.
Steam pocket has formed at top
of powerhead due to lack of cool-
ing water
90-883728 JULY 2001