This one started earlier this winter. I started having the warning at startup around Xmas. Each time it has come on, I've stopped the engine and filled the expansion tank as it is below the minimum. When I get to work it's blowing fluid out the drain pipe, figured I had overfilled, etc. I've done this three times. The last two times the warning has come on, I haven't stopped the engine and light resets within a mile. Check the tank at work and it's spot on perfect height. I'm thinking if there is a check valve in the system that might have failed, but I don't see one in the parts catalogs. I did notice the expansion tank cap has little failures around the part that seals to the tank. Maybe that cap is failing to hold pressure? needs replacement next time I order parts. Any other thoughts? Its a V8, with about 165,000 miles on it and it does not overheat at all.

 

There should be a low and a high level line on the expansion tank. You should set to that mark, keeping in mind coolant expands as it heats up so its possible you have overfilled. You can try filling to the low fill line the next time and see if you are good for a while. With that said, if you are in fact losing coolant over time its going somewhere. replacing the coolant reservoir cap is a good first step - they should be replaced every 5 years or so. Next inspect the hoses and sides of the radiator for any signs of slow leaks. Any odd smells in the cabin when the heat first comes on or dampness in the carpet in the front footwells (ie possible leak at the heater core)? Also check around the expansion tank for any leaks (in case it has a small crack).