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First post here, let me know if I need to post elsewhere. I have a stock 05 LLY, 294K miles. I had a shop recently (month or so) replace my radiator because of a leak. They seemed to do a good job. I asked them to check for any signs of a bad head gasket and they said everything looked good visually (no pressure test). The truck would always run around 186-190 under normal load. Up to 210 when towing a heavy load. The fresh radiator seemed to bleed off heat much better than the old one. Today when driving, I noticed on my Edge tuner it was taking longer to reach operating temp than usual. Then, around 180 on the freeway, the temp quickly rose to 225. I let off the accel. and coasted off an exit. The temp fell back relatively quickly to 170. I limped home managing the throttle to keep the temps in range. Idling would hover around 90. Temps went from 140-190 on surface streets back to the house.

I suspected the thermostats or temp sensor, or both. Maybe getting stuck in different positions causing the high and low temps. Hoping to get back on the road quick, I picked up the two thermostats and a temp sensor from Oreilly's and replaced it all in about 30 minutes. I made sure to position the thermostats the same way they were when I took the old ones out. Special attention to the rear thermostat that has the bypass holes. Filled the reservoir with the bleed screw in the tstat housing off until coolant ran out and quickly put the bolt back in. Ran the truck and topped off the coolant. Took it for a test drive on surface streets with the heater on max heat and temp never got too much above 100. I left it idling for a few minutes and the temp dropped and hovered at about 60-70.

What's going on here? Cheapo tstats? Air bubble in the system? Head gaskets? Water pump? I'm really not too sure where to go next. Any help is appreciated!
 
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