I have a problem with my 2004 Chevy Tahoe. Long story short I take frequent trips pulling my boat to go fishing. I noticed on the longer trips with lots of hills the engine temp would start to creep past the safe zone. I pulled the radiator and bought a big aluminum 3 core radiator. I also replaced the water pump, fan clutch, and thermostat. The truck ran great after that. Never had an over heating problem. The one day I noticed the heat gauge wouldn't get past 165 or so. I figured the new stat I put in must have failed. so I put a new one in and nothing. still wont warm up. i'm on my 4th stat now from 3 different places and it still refuses to warm up. I now get a trouble code stating my engine coolant is too cold. I was thinking maybe the big radiator and the cold weather is not letting the engine warm up? I'm not sure. If I drive the truck for a while and then pop the hood and look at my coolant reservoir I can see coolant rushing out of the return line from the engine. Almost like the thermostat is open. Is it somehow leaking past the thermostat or am I just so unlucky I keep getting bum stats over and over again.