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Truck Overheating - Slightly

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Need some help diagnosing an issue with my 2014 Silverado Double Cab LT Z71 5.3L….


Last week I was towing my boat (boat and trailer weigh under 2K) home on about a 2 hour drive and I noticed that my truck was running a little hot. I was cruising around 75mph and it would intermittently heat up to the next hash mark above 210 degrees and go back down to 210, but stayed in between the two marks on the thermometer most of the trip. Also, my transmission was running about 212-215 degrees and I have installed the new ~156 thermostat on my transmission so I thought it was odd. 

I have 120,000 miles on my truck and no known leaks so my first thought was that the coolant had degraded since I had never changed it. I drained the radiator, and the coolant that came out looked like it was brand new. It was not off colored at all. I proceeded with a good flush of the system with distilled water, then added new Dex Cool. The truck was still doing it any time I step on the gas or drive highway speeds. 

I live on dirt roads, so my next thought was that my radiator might be dirty enough to not let air pass through, so I blew it out with low pressure and rinsed it out with the hose while it was cool. No change, the truck was still overheating slightly at higher RPM’s


I was due for an oil change, so I changed my oil at this point to make sure it didn’t have any water in it. Oil was clean and had no water. Still no change. 

I replaced my thermostat about 6 months ago because the factory one was stuck open and I had no heat.  I used whatever they had available at O’Reilly at the time. I replaced it yesterday with an AC Delco thermostat and housing to see if that had any effect. The truck is still doing the same thing. 

Any advice on what to look at next? I have a trip coming up in 2 weeks, and I am trying to resolve this before towing my boat on a 2 hour trip.  A water pump and radiator will run about $800-1000 and I don’t want to waste anymore money on parts I don’t need. The coolant appears to be circulating fine I was able to watch the overflow tank go from clear to orange after adding dexcool after the flush. 

Will try to post a video of the temp gauge after I drive it tonight. 

Thanks in advance. 


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I think you have a non-problem myself. That is, it operating to design, and you've found the edge of its capabilities in its present state of design.


Now you haven't said if this system is NHT or not, where you live, what the terrain is like this is happening on nor and most importantly what the ambient temperature was/is. 


So, you get three choices IF cooler is what you want. First one is easy. Slow it down. Even 10 mph will make a large difference. Some find that harder to do than others.  Second, more radiator and more transmission cooler. Perhaps even a fan on the aux cooler. Lastly and this will draw some fire, lower the glycol content. You might find this link instructional.


 WaterWetter Supercoolant 3/00 (redlineoil.com)

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If you replaced the thermostat with an A/C Delco one, it's probably the standard temp. I'm curious, what was the outside temperature when driving? Do you have a towing package ? Max towing package ?

The transmission temp would bother me, personally. I did the pill flip on mine & transmission runs cool most all the time here in Western N Carolina. 

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