MG Car Repair: 1978 mgb, radiator shop, temp guage

Question
Hello,
While driving on the highway the temp guage slowly gets hotter as I drive.  I already replaced the thermostat and changed the oil to 20w 50.  I have 2 fans in front of the radiator and it seems that there is circulation of antifreeze. Next step is the water pump, I have already drained and refilled the fluid and sceptical that the pump is the answer being that I have circulation. I have no issues during casual traffic driving- only on highway!
Thank you,
Chris Calamia

Answer
Hi Chris,
It is no use replacing parts until you know what is wrong. There are several tests for overheating. First and most important is to check for CO in the radiator. Most people don't have access to CO equipment but you can do the next best thing and that is to go to your local parts store and get the dye that goes in the radiator to test for CO. Just follow the directions on the container.

If you do have CO in the radiator, you most likely have a head gasket leaking. Or a crack in the head or block (less likely#

If you find no CO in the radiator you may need to go to a radiator shop and ask if they can check radiator flow and check the lower radiator hose to see if it has a separated inner liner that is closing up at speed or if it feels soft.

Other items that can make an engine over heat at speed are: Incorrect ignition timing, carburetor too lean or wrong octane fuel for the car.

Even though a water pump can have an impeller come loose when hot and show up as over heating on the highway it is very rare on the MGB.

One more thing to check is to take a cooking meat thermometer and when it shows hot release the pressure cap carefully and test the true temperature as the gauge can be reading wrong. If you find that to be the case test the charge rate at higher RPMs #13.8v to 14.5v)

Howard