I have a 2004 Discovery SE that strated boild coolant out of its reserve tank after driving about 30 miles to work. Temp gauge remained normal without any fluxuation. Any Ideas?

 

Yes, with no exceptions, stop driving it now.
Are you low on coolant when cold, have you been having to add coolant or have you been too lazy to look?

 

No, Mike I'm a complete idiot and lazzy. WTF. A simple question. I checked my fluids regularly and everything seemed fine. I have added small quantities of radiator fluid in the past but nothing that I would consider abnormal. I am looking for sugestions not sarcasm.

 

Any addition of coolant is not normal.
You have a coolant leak and your truck needs to be blead when cold and then do a pressure test to find the leak. In the mean time, i would suggest you drive this as little as possible or you could toast your engine.

 

Sounds like an air lock somewhere in the system, theres a coolant bleed how to in the tech section

 

When I removed the radiator cap I noticed that an o-ring was broken on the radiator cap. I added coolant and tightened it down as far as I could. Driving it home 30 miles the temp gauge never moved and the truck was not shooting coolant out of the resevior. Could the problem be a fualty radiator cap??? What are your thougts? Thanks again for your answers.

 

I can't 100% attest to the exact vehicle, but yes, if a cap was loose that could cause the fluid to splash out. The question is, how much was coming out and when? Does the temp gauge start out cold and slowly rise to normal and go back down?

 

get a new cap regardless, $32 at the dealer. Phil

 

Temp raises slowly like normal. Coolant was coming out of the relief hose. Tonight it was fine.

 

Yes a bad radiator cap can cause this, the cooling system is under pressure to raise the boiling point of the coolant.
Not enough pressure or no pressure and you will boil over even though the engine is not actually running to hot.
Replace your radiator cap and you should be fine.
If it continues after that then we need to big deeper.