First off my car takes forever for coolant temp to warm up..I can drive to work 40 mins away and it will be at 140 which obviously kills my gas mileage..I replaced thermostat a few month ago and it never seems to help for long..Second issue is car hesitates and stutters all the time even when its warmed up but especially if its cold..Any help would be appreciated

 

let the engine warm up as as high as it will reach then will take an inferred temp gun and shoot the engine to get temp if temp gun shows actual op temp then replace the temp sensor

 

Maybe the thermostadt is faulty? I've heard of some of them being bad out of the box.
Take it out, and put it in a pot of water on the stove,
and monitor the water's tempature as you SLOWLY heat up the water. It should open at about 180F

 

Ive replaced it 3 times but the reason I think its not warming up is my gas mileage goes to **** when it happens

 

the pcm controls fueling and switching from open to closed loop through the temp sensor if the sensor goes bad and sends the wrong resistance value back to the pcm then fueling will be incorrect I would change the temp sensor