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Mazda Protege running hot

I have a unresolved problem with a Mazda Protege '99 running hot after about 25 miles on the highway.

I have replaced the 2 likely culprits:

1) the radiator cap

2) the themostat

I’m eye the radiator itself. Has anybody else seen this?

“Running hot.” How are you measuring the coolant temperature?

Has measured by the temperature gauge on the dashboard

The problem might be caused from a partially restricted radiator.

During city driving, the engine doesn’t run as hot as on the highway and the water pump spins slower. During highway driving, the engine runs hotter and the water pump spins faster. If the radiator is restricted enough where not enough coolant can pass through the radiator to keep the coolant temperature under control, the engine starts to overheat under this condition.


I have considered this. I’m considering (and probably will) taking the car to the shop and asking them to disconnect the inlet and outlet of the radiator and do a flow test of some sort to make sure that the fluid under pressure goes in and comes out at the same pressure consistently.

The radiator doesn’t require removal.

Just take it to a shop that has an infrared thermal gun. With this, the radiator can be scanned to determine if any of the radiator cores are restricted, thereby causing the overheating at highway speed.