[skipjobe]

Mine is doing something similar if I run at 70 mph 90 ambient temp ac on it goes over 190 and the check engine lite came on once, if I slow to 65 the temp comes down. I do Have 4.11 gears but the truck should be able to run 70 with a small trailer and no load.

 

[skipjobe]

Thermostat was my problem

Really I will change mine, That makes sence as the temp guage seems to move quickly without conditions changing.
The other strange thing is I tried to put in 4wd low range will not go in.