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This is my first post here. I bought a 2010 ext cab z85 4x4 with 2.9L engine with auto trans last Aug/Sept and changed fluids, plugs, belts etc for a basic tune-up. Everything has been fine until the middle of Dec.

I drove up onto our mtn(30-45 minute drive on logging trails) and parked at camp for about 4 hours. At that time I left camp, started the truck and quickly realized something wasn't correct, the coolant temp gauge went fully off the scale hot. Haynes manual claims that was a for sure thermostat issue with a cold lower rad hose, so it was swapped for a new Napa stat (motorad 187degree) and a Napa water pump and they didn't fix the issue (Haynes manual is best used for starting fires, lol). Eventually after cleaning grounds, swapping temp sensors and installing a scan gauge II AND reading as many threads here as possible, I was able to trace the issue down to the gauge itself being faulty(pulling cluster fuse reset the cluster temp until it went goofy again).

Now for the current issue,,,,,,,,,,,, After getting the scan gaugeII, I have been watching the coolant temps and noticed something very odd. Coolant temp after fully warming up sits around 200-203degrees f while parked, 197-198 while cruising around town or on the highway, or in the mtns when the engine is below 2000rpms. As soon as I start to accelerate and RPM's are around 2500 the coolant temp drops to 190 and when RPM's get above 3300 the coolant temp drops to 185-187, verifying the thermostat, temp sensor and radiators proper operation. That makes it appear to not have enough coolant flow at low rpms to maintain the temperature of the coolant to the thermostats rated and tested temperature(187). Now those temps do not bother me right now as outside air temps are between 10 and 20 degrees f. But when summer arrives am I going to have issues? Is there anything I should check? Could the NAPA brand waterpump be the issue?

Thank you for any help ahead of time.
Doug
 

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I think you're just fine. Higher RPM's gives you better coolant flow and higher MPH's give you better airflow through the radiator. It's not just RPM's, you have to consider both. Airflow plays a major part in keeping the coolant in the radiator cool as well as air circulation in the engine compartment to keep the motor cool. Yeah, our gauges are pretty crappy. Mine never gets above 1/4 of the scale. I use a scangauge II also for monitoring stuff.
Anyway from all the info and numbers you gave, I'm pretty confident that your cooling system is working correctly. I do understand the close eye you're keeping on it since seeing the gauge go all the way to the top. That'll get your interest pretty quick when you're out in the middle of nowhere!!
 

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187 is not a regulation point, it is where the thermostat starts to open up. You temperatures seem perfectly normal.
 

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Thanks for the replies, it makes me feel a lot better, especially after what else I learned this morning.

Yes it was a tad concerning to see the temp gauge go full hot all the way up on top, it is not a fun walk off the mtn at all.

I did notice something this morning while comparing my recorded ambient air temps and the cold coolant temps before starting the engine(as displayed on the scan gauge) after sitting overnight or even after sitting for several days(3 days with 10 degree high and scangauge read 18 degrees before starting). Yes I went that anal, lol, and without exception the engines cold temps were consistently 8-9 degrees f higher than the ambient air temps. This difference was seen with ambient air temps ranging from zero degrees to 60 degrees f. The small used car dealer I bought the truck from changed the sensor before I bought it trying to fix the occasional full hot gauge display, I don't know if they used a GM sensor or some cheap replacement part.
 

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Wow Dude! You need to lighten up a little bit! I'm sure at some point your truck is going to have a problem that you need to worry about - but this ain't it! Just get out there and drive the thing and have fun. Your temperatures are fine. Even if they're 8 degrees off, they're fine! I'll let you know that I had a '69 Chevelle that had no coolant (In '77 when I owned it, this was called "anti-freeze"). Temperatures that winter got down into the 20's a few times and other that in the reservoir, I had no problems with freezing. The coolant is inside the radiator and inside passages within the engine block, under the hood. The intake air temperature sensor is located inside the intake tube, so it is enclosed also. Your coolant is enclosed and it's nice and warm and "snuggly" and will not drop down to the the actual surrounding temperature. Also, the cooling system is a sealed system. Think of the radiator and engine block of being like a nice Yeti cooler! They work hard to keep cool stuff cool and warm stuff warm!
Switch to de-caff, have a beer, check all fluid levels under the hood, drive the truck and have fun with it. It will have a problem now and then and you can take care of them as they show up. Don't go looking for problems that you don't have.
 

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Yup, temps seem fine to me. My 04 3.5 runs about 190-195 during the winter, and 212 or so during the summer (225 with AC running at full blast when going slow). Should be fine.
 
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