98 Ford Ranger 2.5 4cyl with 344k miles runs good but temp guage fluctuates from normal to zero and back again after warmup. rellaced both coolant temp senders and a new radiator but the problem still exists, no leaks an water pump sems ok. any ideas?
Head gasket, maybe?
Or sticking thermostat
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There may be air trapped in the cooling system.
Coolant temp sensors can’t read air temperature.
With the engine up to operating temperature and idling, loosen the upper radiator hose clamp and slip a small flat bladed screwdriver between the the upper radiator hose and the hose neck.
Allow the engine to idle until all the air is purged from the cooling system.
Remove the screwdriver and tighten the hose clamp.
Check the coolant level after the engine cools down.
Tester
The thermostat may be upside down.