Hey guys so I join the club of 40-50k water pump failures. I have an 08 Redline. One morning driving to work the temp light and coolant light both came on. By the time I got to the DIC Coolant, coolant light went out and was dropping below 240, and went all the way back to normal favoring 205 to 210. Next stop put water in it and it stayed in the normal temp range. Next day took it to the GM shop, and showed them how it was wet near the water pump. No questions asked, they tested it, pump was the leaking source and replaced it with the 5/100k warranty.
So today (day after getting it back) I was driving home from work (within 10 min starting the car) and the temp light comes on, again it is dropping quickly down before i could get to the DIC Coolant, so I am like great pull over and temp is back in normal range. But coolant is not low enough to trip sensor but lower than it was when i got the car back(1/4-1/2 in below normal fill) So i put some coolant in it i got from above incident drive home. Mixture of stop and go and highway and temps stayed below 210.
So just now I did the coolant "Hand Job" steps and drove the car around. Temps never got above 205 except after i did a short pull coming back to my garage to test the system, let it sit in the garage to see how the system would react. Temps would stay at 210 and let it idle for 5 mins, but never drop lower. Upper hose was hot, lower cold on the radiator. I then brought the RPM's up to 1250 to spin the water pump up a bit, and temps dropped to 200 and then touched the radiator hoses, upper still hot, lower is noticeably warmer then before. Let it idle for 5 and it went to 210 and then shut it off. Fans turned off automatically and will check it tomorrow going to work. Also during the process I listened and felt the fan which would cycle from being at a lower speed about 195 and increasing with temp changes.
So I think the shop just didn't burp the system good enough and an air pocket was stuck in the system, and so when I was driving to work this morning the heat cycle let the bubble work it way up. So when i was leaving the bubble found its way in the wrong place and tripped the sensor the system burped it out.