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Guys,
I have 99 civic with vtech SOHC engine And the car got overheated at night and somtimes at the day time just when i speed up not more than 100km/h , the coolant is fine and no leakage , fan is working on time , car overheated just when im driving only ,
i dont know what is fhe reason , could anyone help ?

 
Rep Power: 518

When driving at highway speed, the air flowing through the radiator is supposed to be enough to keep the engine cool.

Did you suck up trash or leaves in the bumper opening, blocking the radiator?

A zillion miles worth of bugs, dirt, sand, and garbage built up in the tiny fins of the radiator can block enough air flow to cause overheating at speed.
Same if the fins have been bent flat so no air can pass through them.

Put a bright light behind the radiator in the fan area, then look through the front bumper openings to see if the fins are clear or obstructed.

You can usually use a garden hose and spray nozzle to wash the junk out of the fins...but you need to spray from behind to push the junk out the front.
I've seen them bad enough it was necessary to remove radiator and condenser from the car and use a pressure washer to clean the crap out.

Be super careful with the water, sometimes you can fold the fins over with just a strong stream of water.


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A thermostat could cause the same problem if it doesn't open wide enough.

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I already cleaned the fins before , let me try the second option , the thermostat i will go change it tomorrow in workshop and i will see ,

Many Thanks

 
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Try changing the thermostat I had the same problem and it got solved by changing it

 
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One car I had overheated because a lot of the radiator fins had corroded and fallen out although the radiator was not leaking.

Another possibility is that an aftermarket waterpump with a plastic impeller has been fitted and the plastic impeller is no longer fixed to the shaft so is not spinning. Common problem with the cheapest water pumps.