Gauge reading hot

[curly]

Can anyone tell me why, after replacing the head gaskets on my 364 the temperature gauge starts at normal temp and then after 5-8 miles the temp gauge rapidly rises to the maximum and then after 2 more miles it goes back to normal.The radiator seems cool as usual and I have an electric fan controlled by another thermostat on the top hose and this fan does not start so Im thinking there is some kind of air lock around the temp gauge sender causing steam to be trapped around the sender and give these high temp readings.Also when I park the car and stop the engine I can hear a bubling noise in the thermostat or water nack area for 1-2 minutes.Again the engine seems not to hot and you can touch the top tank without being burnt.I wonder if the temp gauge sender needs loosening to let any trapped air or steam out or would the water pump flush the air out.Could it be a thermostat problem maybe.

[BigRivy]

I would start by trying a new thermostat. After the initial temp rise and fall can you drive it for an extended period of time, say 50 miles, without it overheating or any heat related problems such as pinging or run on when you try to shut it off? Usually a temp sender has to have actual liquid touching it to read right, Steam won't do it. So if it's reading too hot it is actually reading the coolant temp right there at that location. Double check your timing as late timing can cause top end overheating.

[Jim Carmichael]

Also make sure you installed the thermostat in the proper direction. The bulb part goes towards the engine.

[curly]

Thanks guys,I removed the thermostat today and it was the correct way up with bulb down.I placed it in a cup of boiling water but it did not open for a minite or so and made a squeeling noise when it did,the thermostat seems very inconsistent and sometimes opens more than others.The timing is correct and the car runs perfectly with no run on or pinking so Im buying another thermostat tommorow and will let you know how it performs.