Temp Gauge Rising then falling

[TeamXPLO]

Hey everyone,
Ever since it got cold out, my temp gauge has been rising up to hot and making my check engine light come on. Almost immediately it comes right back down, though. This happens after 10 minutes of driving or so...
I was thinking bad thermostat maybe?

Any ideas?

Thanks,
-Lloyd

 

[Jarrett088]

Yep. My guess would be the thermostat. It may be coincidence that it started happening as soon as it got cold. Not too bad to do yourself, although I'm not sure about the 99's. I wouldn't drive it too much, with it not opening properly. You could cause some serious damage.
Tim

 

[Okrazie1]

T-stat would be my guess, too. I just had to replace mine last week. Mine wasn't allowing the truck to warm up good, and the temp would drop whenever the engine went much above idle.