Technical Temperature gauge or heating problem

[alfapop]

The temperature gauge on my 1974 Spider slowly climbs almost to the red line and just as I feel the need to pull over it rapidly drops back to normal. My guess was that the thermostat was sticking but then I checked the records and found the previous owner put a new one in so it's probably not that. Any ideas? Thanks.
Rob G.

 

[wetminkey]

"Burp" any air from your cooling system,...that's usually the culprit.
A 'T' fitting, placed in the engines' heater outlet hose (on top of the engine) makes it easy to purge the air from the cooling system,...
You can place the thermostat in a pot of boiling water, on the stove, to check for proper function,...if there is any question.
Good luck, Rob!

 

[s130]

Well you could try the old left hand / right hand temperature test on the top and bottom rad hoses. Until the stat opens the top hose will warm up due to the thermostat bypass hole. Will get hotter and hotter. Bot rad hose will remain cold/cool till the stat opens and then quickly warm up.

You will need an assistant / mirror / camera feed to monitor the gauge during the process. Also a temperature test on the top hose / thermostat will help clarify if the temperature is actually going to excess before the stat opens and then settles to normal. This would indicate a sticking stat.