normal warm-up time

this is my first GMC (2004 Sierra p/u 4800 vortec) and I thought it abnormal when it couldn't warm up at idle. The dealer replaced the thermostat but it
still doesn't warm up at idle. I've driven 5 miles to work and left it idling for over twenty minutes and the gauge reads 175. When driving it will warm up to almost 210 but it takes a lot longer than my other car. If it's cold enough outside, say below zero, it will actually start to cool down if I leave it idling. I've let it idle for 20 minutes in the morning before so I could have a warm car and didn't. Is this normal? Did I get another bad thermostat?
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I'm reading your post to mean you aren't getting any heat from your heater....right? Or is the question about what your gauge is reading?
Spitballing: radiator cap, wrong/faulty stat, valve in the heater-hose line faulty, faulty gauge/sending unit.... Factory guages are nortorious for being less than accurate. The rad cap is the means the system uses to pressurize. Without pressure the cooling system will not function properly. Caps are inexpensive. Possibly the stat is the wrong range. A 195 stat is usually what is needed for proper heater function. I've even seen some come in with the stat installed upside-down.....not good. If you can't do this detective work yourself, then I'd bring it back to where you bought it and tell them to fix it and not to call until it works properly.
Hope something here helps....
Dave S(Texas)
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