Faulty Thermostat or Hidden Cooling System Crisis?

Had the exact same symptoms on my 2013 Sierra with the 5.3L last winter. After diagnosing, it turned out to be a defective map thermostat causing irregular coolant flow. The thermostat would stick either open or closed, leading to those inconsistent temperature readings and warming issues. The cooling system inspection revealed the water pump was fine, but the thermostat was definitely the culprit. Fixed it through a complete thermostat replacement including housing and gasket. Wasn't a major repair but definitely not something to ignore, engine overheating can cause serious damage if left unchecked. Quick tip: While the thermostat was being replaced, had them do a full coolant system flush since everything was opened up anyway. That cooling system maintenance helped ensure no debris was floating around the new components. Your symptoms match my experience perfectly, the unusual overheating and slow warm-up are classic signs of a failing thermostat. Would definitely get it checked soon before it potentially causes bigger issues.

those thermostat symptoms sound exactly like what I dealt with on my Silverado last season. Had the same frustrating hot/cold issues. Turned out my thermostat housing was cracked too, which wasn't obvious at first. Since you got yours fixed, curious what the total damage was at the shop? My repair included the thermostat, housing, and fresh coolant, but wondering if costs vary much. Also, has everything been running smooth since the fix? My cooling fan would sometimes act up before the repair, but that all sorted itself out once the new thermostat was in.

Just checked back in, glad my earlier reply helped identify the issue. After seeing the mechanic, it was exactly what I suspected: the thermostat cooling circuit was defective. Got it all fixed up with a new map thermostat, and the truck's been running perfectly since then. The repair ran about 350 total, which included parts and labor. Pretty standard price for this type of cooling system work. They did a thorough job, replaced the thermostat housing gasket and checked the water pump while they were in there. Best part is all those temperature problems completely disappeared. The cooling fan works normally now, and engine temps stay rock steady. No more of that annoying slow warm-up or unusual overheating. It's been about 8 months since the fix, and the coolant system's working exactly like it should. If your symptoms match what I had, you're probably looking at a similar repair cost. Just make sure they do a proper cooling system flush when they're done, helps prevent any future issues.

Took your advice and got the Silverado checked out today. You were spot on, bad thermostat was the main culprit. Mechanic found a tiny crack in the thermostat housing too, which explained the occasional coolant leak I'd noticed. Bill came to €375 total, pretty close to what you paid. They replaced the thermostat, housing, and did a full coolant system flush. Even threw in a pressure test to make sure everything was sealed up tight. Truck's running like new now. Temperature gauge stays steady, and that annoying slow warm-up issue is gone. Fan kicks in normally too. Such a relief not watching the temp gauge like a hawk anymore! Thanks for sharing your experience, helped me go in knowing exactly what to expect. Definitely worth fixing before it turned into a bigger engine overheating problem.