1982 Other Isuzu Models temp guage might be faulty or perhap

I have a 1982 holden Gemini.
i'm not certain if the temp guage is broken, there is a slight oil leak from under the 'oil filler cap' which I thought could be causing overheating, but I put in a new cap and its not leaking now.
the car had been cold for about 5hrs, then turned it on at about 11pm as soon as I turned it the temp guage hit beyond the H position.
any ideas?
i have a radiator fan which works and a 2nd thermo fan but not certain if it works.