Hello, all. I have a 1996 Z-28, which recently has been running hotter than usual. In cold weather, around freezing, I'd sometimes get the low coolant light, but it would go away in warmer weather. Since it would heat up to well over half gauge, then come back down to 1/4 gauge (roughly 205F I think) when I had some airspeed and turned the a/c on, I was thinking the thermostat was sticking. The overflow tank was a couple of notches below full, so I filled it back up. But then yesterday the temp guage got close to the red zone so I shut it down and limped home. So I picked up a replacement at the store, and was getting ready to put it in when I removed the radiator cap to look at the gaskets. The coolant level was low, I've put in a bit more than a half gallon of water and am now going thru the usual cycle of fill and check. I check the coolant level in the overflow tank about twice yearly. Last service was a flush and fill at the Texaco place back in 2001, about 10k miles ago. No visible leaks, no abnormal amount of vapor in the exhaust, oil looks OK.

My question is, assuming the Texaco guys didn't screw something up, how can the radiator lose coolant without sucking the overflow bottle dry?

pin hole in the core
I would get that radiator out and checked at a radiator shop.
at least back flush it to remove scale etc.
and DONT use tap water in the coolant system use distilled water.
or else itll corrode all over the place

Also, do mix the anti freeze in as specified. If your running more water then suggested. It will evaporate with the heat absorbed. Fill the radiator, not the overflow.

Thanks for the replies. I'm just filling with water temporarily, yes distilled, so I can diagnose the problem. If I can resolve the issues, I plan to get another flush and fill. But I don't want to waste new coolant if its all got to come back out.

I live in a much warmer area (80F-85F)and with little traffic and without the help of the fan (A/C on) the temperature will go over half, but not during the cold season, which here's 60F. Check the radiator cap opening pressure, it may be blowing some of the water as vapor when hot if the spring is damaged.