Engine overheating, coolant disappearing but no visible signe of leaking

[mduff]

I have a 2003 Kia Sedona. It's been working fine until today when driving home we noticed the heater blowing cold air in the rear of the van, then the engine temp started to redline. We pulled over and turned off the car for a few minutes. When starting up again, noticed the engine was running loud but the temp. gauge went back to normal. Then heat stopped working completely and air conditioning fan and light would not turn off. After driving a few minutes the gauge stared to redline again. The air turned off and the gauge came down a little but not much. When we arrived home (which was only 5-6 minutes after this started happening), we checked the coolant levels (even though we check and fill these regularly and had just done so a couple of weeks ago) and noticed the engine coolant is very low. Strange, because it was just done and we haven't noticed any leaks. Topped up the coolant and started up the van again, same things happening and the coolant disappeared by 1/4...no visible leaks anywhere! Radiator is cool, no smoke, no discharge anywhere....could it be the thermostat stuck open or closed? HELP! My warranty just ran out last month, of course!

[ESKORT27]

could be the thermostat stuck closed but check the rear of the van for leaks. those vans are extremely bad for the rear heater lines leaking. Just in case. check your oil, make sure the level is on and make sure it isnt milky. You need to be very careful when you redline the temp guage that you dont blow a head gasket. If you were able to fill it up that means that the coolant is going somewhere obviously. If i had to put money on it i would say rear heater lines. They dont leak horribly all the time, alot of the time they leak little by little and then all of a sudden there isnt enough coolant in the system to allow pressure to rise which in turn doesnt allow any heat to be blown out the vents. Good luck, and let me know how you make out.

[woodchuck]

There could have been some air trapped in the system when you added coolant the first time thus the big loss so quick. Check it again. You may still have a slow leak.

[anwarr]

may have rear heater tubes leak, if your car have those.

[mduff]

Found a leak in the coolant line...replaced them all, but coolant is still disappearing. Garage ran the checks and couldn't find the problem.

[Highlander]

With having your underside pipes changed there could still be air in the system. It can take ages for all the air to find its way out. Are you getting any smells from the engine bay? I was losing coolant soon after buying my car and was getting a smell like curry in the engine bay it turned out to be a small leak at the hose connection on the bulkhead.
Are you still having overheating problems?