1_\/\_YOTA

I drive my truck easily 35 miles each way to work 5 days a week. The temp gauge always stays right between C & H. I drove my truck yesterday down the street and back (it was still in between H & C) and I turned the heat on. Maybe 30 seconds after turning the heat on the temp gauge went straight to the red. Possible bad temp gauge? Shouldn't turning the heat on actually cool the engine since its pumping coolant through the engine? Yes the truck has coolant in it and the oil level is good. Any help would be great.

HighLux

Only thing I can think of is the opening of the heater valve....let a trapped air bubble pass the temp sensor...with superheated air...spiking it.

Look to burp the cooling system. I think your heater core releaed a bubble into the cooling system.

1_\/\_YOTA

So it isn't anything to stress over?

HighLux

Is it overheating still? Have you burped the cooling system and drove it?

Gauges rarely lie....if its showing hot at any point...dont drive it till its burped or otherwise diagnosed. Heat kills engines quicker than ...well...poop.

FGZ

Sounds like textbook thermostat issue to me, the colder water in your heater core closed the t-stat which shot the temp up until the t-stat opened again. It's freaky to watch your temp gauge do that, I know. That's exactly what mine was doing, and a dual-stage t-stat fixed it for me.

Here is 4Crawler's write-up on it