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I do see alot of problems with the thermostats on those engines but they usually will cause engine overheat. Just double check the thermostat housing where it bolts to the block as I see leaks there that are hard to see. A pressure tester works good in finding the leaks. If no leaks are found then replace that thermostat. When refilling the system you need to fill the block first by removing the thermostat housing cap on top.