Hi I'm new here, needed some insight on an issue ive had for 2 years. Bought this truck(2018 LT with the 5,3 4x4) back in 2021 no issues, added some off road tires a 4 inch lift and an exhaust setup. It went fine for months, then one day in traffic my coolant temp spiked to 230 then back down, figured it was a thermostat, so I swapped it for a stock tstat. Since then and progressivley getting worse, I swapped to a mishimoto 180 t stat, new water pump new coolant sensor new belts flushed radiator, any sort of time I use the gas the coolant temp spikes up above 210, and sits around 218-222 for a couple seconds and cools back off. Today though it spiked up to around 235-240(past the 7th dash) on a slight hill going about 65 mph, when I had never had that before and ANY time I use the gas pedal it would move to 215-225 range instantly. I havent lost very much of the heavy duty coolant I put in, but I wanted to figure this all out before I invest in a toy hauler and blow my engine up from high temps. Any ideas to what it might be? Some pics/videos linked below (I can get more if needed)

https://imgur.com/a/x2NAMxn

 

I had to pull the coolant radiator apart from the air cond. radiator and flush out a 14" circle of crap that had completly shut off air flow in the middle area of the radiator. Bugs, cotton looking crap and a ton of sand. I had always washed it faithfully but you can't clean between them without tearing the grill and front off, seperating them enough to get to the plugged area. It made a world of difference in stable temps. It is as you would guess, it's a PITA! Hope this helps you.

 

My first suggestion is to monitor the engine coolant temperature (ECT) using a scan tool. That way, you'll get the exact temperature the ECM is seeing and it's resolution is 1 degC (so around 2 degF). The coolant temperature gauge is hard to read, non-linear, not calibrated, etc. I would also install a thermostat with the proper temperature setting.

The first thing you had happen (spike to 230F in traffic) could very well have been a normal cooling system response. If it was a hot day with no wind, low speed due to traffic, AC on max, etc then 230F isn't that hot.

The only picture I see where the ECT is really high is the blurry one. I don't see an ECT issue in your video.

What @Summit4*$* suggested sounds like a good idea. You can also check the foam seals around your radiator to make sure they haven't degraded or are missing - if they are you can get a lot of bypass air flow that won't go through the radiator. You also need to make sure your cooling system is properly purged and you don't have a bunch of air in the system after the flush.