Okay, so lets start here. First my car started overheating. We changed the thermostat but it keeps on over heating. Let me explain my problem. I am driving on highways my heaters work perfectly blowing hot air. But as soon as i come into a city where the limit is 50 my car overheats... Okay now motor side, top hose from the rad is smoking hot,bottom hose from the radiator is ice cold, top hose from heater core is hot, bottom heater core hose is cold ... I went to a garage to do a compressor and fluid test, which were all negative . I have to put Prestone anti-freeze regularly in my car, but it is not a head gaskett as there is no milky oil or any white smoke. Just to make sure I put radiator seal into the head in case there is a small leak ... So, here i am still with my problem..

I cleaned out the radiator and the heater core by plugging a garden hose into it and cleaned out every thing and then let is sit over night. I put in CLR and let it sit over night and cleaned it out again... So nothing is clogged but i still have my overheating problem...

I changed my heater valve control and I still have the problem. In addition, my water pump is not noisy or leaking.. The only reason why I'm losing Prestone is because I'm building too much pressure and heat. The Presotone is coming back up and out from my radiator reservoir. Please help me .

  • Maybe use an infrared camera and look at the radiator when it's running, Flushing doesn't always clear a plugged heater core/radiator. Make sure the electric cooling fan turns on when the coolant reaches 200+*F. And I don't think it's possible, but make sure the thermostat is installed correctly and that it actually opens at it's specified temperature. Also when you say fluid test do you mean they checked for hydrocarbons in the coolant?
    – Ben
    Commented Jan 30, 2016 at 0:12

4 Answers 4


I'm in general agreement with Ben.

  • Cooling fan not working. The description of high vehicle speed = okay and low speed not means your fan isn't working for you. This could be a problem with fan controller, fan motor or temp sender used to turn on fan.
  • Note, the fan shrouds matter big! Ensure they are sealed off to radiator correctly.
  • Please check the radiator cap, ensure that BOTH seals are clean and working correctly. My guess is you've got a bad cap, and the small seal isn't working. The small seal has a very light spring on it. The small seal lets coolant back into the engine (from the overflow bottle) after you shut the engine off and the engine cools down. If that small cap has no spring force on it, that means the water in your coolant system will boil at 212 deg F. If both seals work correctly the coolant boils at a much higher temperature, in the range of 240 deg F. This allows for MUCH better heat transfer and allows your car to cool off better. The other thing that small valve does is help the system purge air bubbles from within the top of the engine.
  • Flushing doesn't always clear heat exchanger. Hmm, make that flushing rarely clears a plugged heat exchanger. Has this car only had distilled water + antifreeze in it? If not, why not?
  • In fact from your description, it definitely sounds like the coolant isn't getting cooled off in the radiator. That could be a plugged heat exchanger or could be a water pump impeller issue. (If you removed the 'fan' belt can you spin the water pump by hand, does it seem to tighten up with the more fast turns you make, or does it spin way way too freely?)
  • I'm not positive your thermostat is working correctly. Its a pretty simple test. Start with cold car. Place it in Park, start engine. Hold on to the top radiator hose. It should start cold. After a few (10? 15?) minutes, the hose should jump from cold to hot. The thermostat acts like a switch, it either open or its closed. If the hose goes from cold, to a bit warmer to a bit warmer to a bit warmer than the thermostat isn't working correctly.
  • For your testing focus on the radiator, not the heater core. The coolant always runs thru the heater core, there is no coolant control system there.
  • Agree with this diagnosis. Looks like the issue is with heat rejection. What HandyHowie says is also plausible
    – Zaid
    Commented Jan 30, 2016 at 13:06

I would check the water pump, it sounds like the blades may have corroded away. From your description it certainly sounds like the water isn't circulating.

  • 1
    I've had this happen before with similar symptoms. Seemed almost like a fan issue until I wired up a manual switch and it didn't cure the problem. The impeller was PLASTIC and had cracked.
    – rpmerf
    Commented Feb 1, 2016 at 11:23

its either the water pump or the thermostat. the bottom hose being cold means the water is not being sucked into the engine (the bottom hose is the suction). It goes directly to the thermostat housing, which connects to the water pipe on the back of the block which leads directly to the water pump. So, its either the water pump isnt sucking the water, or the thermostat isnt opening to allow the pump to suck the water...


Hello guys i have had a similar problem to where my 01 civic would start to overheat due to revervoir shooting out coolant towards the cap .. and collant kept dissapearing from radiator so i had to keep on refilling. Replaced everything from radiator, thermostat and thermostat gasket and even flushed the coolant system after almost giving up and thinking it was my head gasket checked reservoir and it was completely full so i emptied it and refilled it right in the middle of min and max and problem fixed no more antifreeze pouring feom reservoir.. hopefully this helps and i learned my lesson never overfill engine liquids

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