Hey everyone, officially new to the site but have been a lurker for a long time. On my 2nd LS and 12th year of LS ownership. Anyway, I have an '06 LS, and ever since I replaced the thermostat (used a Motocraft part) I can't get the engine to warm up when running down the highway. The temp gauge will only go up to the bottom of the normal range. When I drive it around town it jumps up to the middle of normal like it should. Go back on the interstate and it drops right back down. I DO get heat out of the heater so I don't think it's related to the HVAC. I thought maybe I put too much straight antifreeze in it when I changed the thermostat so I drained about a gallon out and refilled it with straight water. No real change. The only other thing I can think of is the thermostat is stuck open, but thought I'd throw this out to the forum before swapping it out again.