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QUESTION:

1998 f150 xlt larriat 4.9 str 6, overheating problem flushed out system and changed the thermostat, It boils over in the overflow tank — no leaks — flushed out cooling system and changed thermostat and coolant ran cool for about 350 miles and then started to climb —

ANSWER:

Sounds like the radiator fan is not coming on when it should or not at all. The straight 6 engine do not produce as much heat as the V series engine as they have less side wall pressure on the pistons, so less friction = less heat. That being said, it takes longer for a straight engine to overheat.

Check to make sure the fan is coming on, if it is not check the fan itself and work backwards to the relay, switch, etc.

Testing with a multi-meter

 

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