ive put my new motor in and so far it has 400 miles on it. when i start the car and let it warm up the temperature goes all the way to 260 degrees and then drops back very fast to 180 where it stays. ive replaced the thermostat 3 times and the radiator cap twice. i know the gauge is good because its staying right on 180 which is what the thermostat is. ive got 50/50 antifreeze/water. also i have a new stock replacement radiator, new hoses etc. what could be causing this rise and sudden fall in my motor temperature. also when i have my hand on my top radiator hose its cold until it reaches 260 degrees and as soon as it hits this temp the gauge starts falling the hose heats up, so its telling me the thermostat just opened. could my gauge be off ?....any advice will be greatly appreciated.......thanks

 

how long does it stay at 260

 

If 260 is basically the full scale range of the gauge then what is probably happening is an electrical fault. Check that the wire from the gauge to the sensor is in good condition. If you take the connector off the sensor and short it to ground then I believe you would see the same effect, so check the wire, especially where it goes past any grounded metal.

I can't remember off the top of my head which way round it is but, if grounding it doesn't cause a max reading but a minimum reading then its open circuit, and I'd start by replacing the wire.

Hope you get it fixed,

Si.