There is an O-ring on the neck of the fan switch that shrinks and causes a slow leak. You will see something in the lower radiator hose with a small wiring connector, this is the fan switch. Look straight down on the passenger side of the radiators at the lower radiator hose.
You need to check your coolant when cold and make sure you use the float properly and do not overfill the expansion tank with coolant or you WILL be replacing the expansion tank in the new few days! Many times you need to pick the float up and drop it back down to make sure the float is not stuck so you can properly gauge the coolant level.Īlso while you are under the hood, check for this very common slow leak a the fan switch in the lower radiator hose.
The first thing you need to do is understand these cars have an expansion tank and there is a float level indicator inside this expansion tank.
I have seen this problem often on E46 where the coolant light does not come on until the outside temps dip close to or slightly below freezing, but then do not come on if the temps stay above freezing.
Note that the coolant will expand and contract quite a bit do to temperature.