I changed the radiator today and drove to work, same thing: overheating and boiling. Don't see a leak from the radiator or anywhere else. The thermostat was changed and still no change. I am stuck, so I need advice. The plug to put the diagnosis machine on under the dash seems to not read. I know it works but no codes even come up. What might be going on with the cooling system?

  • 1
    Do the fans come on? What's the car (Year, Make Model, Engine)? Not all cars throw codes for overheating or cooling fan circuits.
    – Ben
    Feb 21, 2017 at 21:44
  • If it's boiling and you have a 50/50 water/coolant mix, there must be a leak somewhere.
    – Spivonious
    Feb 21, 2017 at 22:00
  • 2
    @Spivonious - If the system is sealed, but not flowing, the system can still boil inside the engine block, pushing coolant out through the overflow. It just has to have more pressure in it than what the cap can handle and it will boil over without any other leak present. Feb 21, 2017 at 22:05
  • Your radiator cap may not be sealing under pressure. Try a different one?
    – tlhIngan
    Feb 22, 2017 at 0:08
  • How many miles on the car? A family member riding a Pathfinder has a overheat issue that's winding up looking like a bad head gasket which is like a worse case scenario for an overheat :( Sometimes you have to burp the radiator with like a Lisle 24680 Spill-Free Funnel. Fan clutch could be bad too and not ramp up the fan speed appropriately.
    – jxramos
    Feb 22, 2017 at 1:10

3 Answers 3


Next test steps are:

Test for proper fan operation. The fan should come on when the A/C is selected on regardless of coolant temperature, this is a easy way to ensure that the fan works. The fan should come on when the coolant temperature exceeds 200 Deg F and run for around 20 seconds. The run time depends on ambient air temperature and humidity. Fan on time should be less than 1/3 of fan off time. A short fan on ratio indicates good heat transfer in the radiator. Test radiator temperature: It should get hot, over 190 deg F.

Test for a failed head gasket, a common problem on some years of Civics. This is generally considered a test best done by an experienced diagnostician. If one gets all the air purged from the system and more keeps coming the head gasket is likely the cause of the problem.

It can be helpful to use the diagnostic plug and a scan tool to check the system temperature but it is not required. Trouble codes are not provided for overheat problems specifically. A temperature sensor over limit code can set but this does not tell us anything we did not already know.

  • Good answer, would suggest also checking for operation of the water pump and that the system isn't air-locked. Feb 22, 2017 at 10:29

Check the water pump - is it moving the coolant - sometimes the impeller becomes dislodged from the shaft and the driving pulley and shaft work fine but don't rotate the impeller to move the coolant. And, yes, have seen and experienced this on both customer and personal vehicles.


blockage of radiator fins can cause overheating, water pump system are not working properly can cause overheating, blockage of water jacket can cause overheating , electric fan are not working properly

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