Onan MJA Series Service Manual page 22

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BLOCk '-REMOVE PUMP
HERE COVER TO DRAIN
(SOME MODELS HAVE
A777
A PIPE PLUG)
DRAIN
FIG .3.1. B - DRAINING ENGINE COOLING-BYSTEM
!3 .1. 1 MAINTENANCE
Cooling system maintenance should include periodic inspection for leaks,
inspection of the rubber pump impeller and periodic flushing and clean ­
ing. See the service chart for inspection and service periods.
The rubber impeller, because of continuous flexing, deteriorates with
time and must eventually be replaced. If, however, the impeller fails
after short service (usually under 500 hours) check for possible defects
(Sect 3.1.3.2).
Only when clean can the cooling system do its work well. Scale slows
down heat abs<;>rption and restricts water flow. Flush the system at
least once a year and more often if operation indicates clogged passages
or overheating. To flush the engine, remove the thermostat, Fig
3.1. C, and the water pump cover. Restrict the pump opening partially
so the cylinder block fills with water. Attach the flushing gun nozzle to
the thermostat opening and fill the block with water, then apply air pres­
sure. Repeat the process until water coming from the block is clean.
3.1.2 TESTING
The COoling system can be tested for 2 abnormal conditions: (1) insuf ­
ficient waterflow and (2) air leaks.
1. To measure water flow, install a tank of kl.1,-wn quantity at the
water outlet. Run the engine until the thermostat opens and thEm
measure the length of time necessary to fill the tank. From this
obtain the flow in gallons per minute (GPM). Minimum values are