MGB: Coolant loss?

I've been posting about this problem a few times already, so please don't get sick of me! What is happening is when I get cooling system good and full, I start motor and take for a drive. Engine will heat up to the point where gauge will climb almost in the red(late opening thermostat?) then when thermostat opens needle comes down to where it should be. Coolant will leak out the overflow when motor is heating up and the needle is climbing towards the red? Why would it leak out the overflow before themostat opens? There isn't any pressure on the overflow until thermostat opens is there? Radiator and overflow should have cold coolant in them until thermostat opens shouldn't they? Eventually after driving it a few times enough of the coolant is gone to where the fans won't kick on because there isn't enough coolant to cover fan switch element in radiator tank. Any input would be greatly appreciated. Thanks...

Hopefully someone w/ more expertise than me will chime in. Assuming your gauge is reasonably accurate it sounds like you at least need a new thermostat. Have you recently serviced the cooling system? I'm thinking you may have some air trapped in the system. When I filled mine I, I first filled as much as I could in the radiator. I then pulled the hose from the heater (where the short heater hose connects to the hard line on top of the valve cover. I used a funnel to fill the hose, until a bit of coolant came out the hard line.

I realize you need to fill from the cap on top of the thermostat housing.

I was going to ask how was your radiator cap until I remembered you probably don't have one?

Good Luck

Paul

When you fill, you have to burp the car by squeezing the lower rad hose. Assuming that all is good, fluid would flow out of the overflow tank if the t-stat doesn't open because there is expansion and it has to go somewhere. If the t-stat had opened, the fluid wouldn't have gotten as hot and there wouldn't have been so much expansion. Change out your t-stat.

And try a "fail safe" one:

http://www.autozone.com/N,16300047//shopping/partTypeResultSet.htm

How old is the radiator cap? How many pounds is it rated for?

Radiator cap is new. I don't know the pound rating right off hand. It is the one NAPA had listed for it. Chris, what I don't understand if thermostat isn't opened yet, the pressure would have to be coming from the bottom of radiator(from water pump) instead of top where thermostat is. Am I not right? Thanks all.