You are seeing what you SHOULD see. The reason for the variation is simple, when sitting still or going under 30mph the heat will build up more. When on the highway, you get super chilled air going through your radiator and that will magnify the fact that your thermostat isn't ever closing (they fail open mostly unless you are me :eeps. The reason you don't see this other times is because your gauge has a wide range of temps that show "normal". Normal could be 186 dgerees or 220 degrees, either way it will read middle. With the therm messed up, it is reading really cold and shows you. You need to replace asap because having your car in the blue is actually bad for your car. Needs heat to run at optimum.Now, i am wondering, is this exactly what i should see if i have a failed thermostat? And why the highway/ weather dependence.
Piece of cake. Bleeding the system properly is the hardest part. Park the cold car nose-up (on ramps if you only have level ground, but the average driveway has enough slope) turn on the heater full-blast, and start the car. Crack the bleed screw in the upper radiator hose to let air out of the system. Once you get nothing but water coming out, tighten the bleed screw. (Not too much, it can break!)Is buying and replacing a t-stat expensive and hard? I've never had to do it before