Hey everyone..so im new here, just picked me up an 89 mighty max 2.6l auto 2wd. truk is in good condition.im doin a tune up since i got it.but sinse the weather has changed my temp gauge stays around cold..even when i let it warm for a bit, then drive,n drive, but it stays at cold. now if i let it idle after driving for bout 5 mins the gauge will gradually move to bout 40%. jump in start driving.....then right bak to cold. any and all help...much appreciated