Hyundai Repair: overheating, coolant temperature sensor, elantra wagon

Question
My 2000 elantra wagon overheats while your driving and when you make a stop the needle marker begins to slow down. When you go 30 to 40 miles it begins to go up. We put a new radiator for the engine and it stills overheats we make think that we need a new thermostat but not sure?

Answer
Usually, what you describe is the primary symptom of low coolant flow (but not no coolant flow) caused by a partially clogged radiator.  Since you've already replaced the radiator, we can cross that off the list of potential causes.

In these cases, I usually recommend a cooling system operational audit.  This can typically be done by removing the cap from the radiator and starting the engine (with the heat and a/c off).  You'll want to check for the following:

1.  Proper coolant level on removal of the radiator cap.  If the coolant is low, check for leaks.

2.  As the engine warms, the coolant should begin to circulate.  You should be able to see this circulation through the filler neck of the radiator.  If this does not happen prior to the fan coming on or the coolant beginning to boil, you should suspect the thermostat is stuck shut.

3.  After the coolant begins circulating, both radiator fans should turn on periodically blowing air rearward across the radiator as necessary to manage the coolant temperature.  If the coolant begins to boil without the one or both fans coming on, there's a problem somewhere in the fan control circuit.  If both fans do not run, suspect the coolant temperature sensor in the engine.  If only one fan does not run, the fuse for that fan could be blown, the relay could be defective, or the fan motor itself could be bad.  If one or both fans blow the air forward instead of rearward, the motor is turning backward and, when moving, the fan operation serves to stop air flow across the radiator instead of creating it.


Lastly, and this is my real suspicion here, you could have a cylinder head or head gasket issue.  If all the above checks okay, then this is the most likely scenario.  The faster you go, the greater the load on the engine, and the greater the possibility that hot exhaust gases are forced into the cooling system through a head gasket weakness.