Triumph Repair: 79 spit temp gauge reads hot, fuel level gauge, gauge reading

Question
I have a 79 Spitfire which I am totally restoring.  The temp gauge goes to hot in about ten minutes at idle.....never seems to peg the needle.  I have not driven the car on the road to see if it cools down, afraid to break something.  Radiator and heater core cleaned professionally.  Running twin SUs and a Kent cam.....all brand new.  50K on original motor.  Timing is close to correct and the carbs seem to be correct.....all is new however.  After stopping the engine, I observe the gauge and listen to the fan.  The fan stops running while the gauge is still to the right on hot.  Curious if the fan switch is stopping early or the gauge is fibbing.  Engine doesn't seem to be gurgling like really hot.

You guys are great.....thanks so much......Rory

Answer
Hi Rory,

We often had cars come in to the dealership with a hot reading and the first thing you need to do is confirm that it is really hot or not. We didn't have those thermal guns like they do today but a kitchen meat thermometer works well.

Also, there are other indicators like is it spiting coolant out the over flow pipe? Can you open the radiator cap without coolant spraying all over? If it is not spraying out the over flow pipe and you can open the cap, it is not very hot and here you can just dip the meat thermometer into the coolant and read the temperature and compare that with the gauge reading.

If the coolant is not that hot check the fuel level gauge to see if it is also reading wrong. If it is, then you have to check the instrument voltage stabilizer for a 10v out put and test the charging system to see that it does not exceed 14.5 v at a fast idle.

If the fuel gauge is reading correctly and the charge rate is correct and the coolant is not over heated then you just have a bad gauge.

If the coolant is hot then you need to run tests on the cooling system and the first is to get the dye from any auto parts store that tests for CO in the coolant. If the dye confirms that there is no CO in the coolant you don't have a head gasket leak and you can proceed with other tests. The next test is to remove the thermostat and put it in a pan of water on the stove with your meat thermometer and confirm the it starts to open about 20 degrees before the rating stamped on the thermostat and it fully open by the rated number on the thermostat.

If the radiator was professionally done then it is unlikely the cause. The only other things that need to be looked at is the lower radiator hose and the possible loose impeller on the water pump. These two are rare but I have found a few over the years.

Howard