My temp gauge goes up to 210 and then slowly
goes down to 190 then it slowly goes right back up
to 210 and then slowly goes back to 190. I put in a
manual temp gauge and it still does the same
thing. Does anyone know why it does this and is it
safe to run my truck this way?
Sounds like your thermostat may need to be changed but if it's staying under 210 it's safe to drive. Older model vehicles usually run at 210. It could be also be the outside temp, when your fan kicks on the temp goes down, and vice versa. Thermostat is a cheap and easy thing to start with though.