Over heating

Hey everyone 
 
On the way back from the NSW mr2 meet tonight. I started having over heating issues where i noticed the temp gauge was maxed out. I didnt notice the gauge being at that height at first but what made me aware of this was that after driving for a while, my heater wasnt giving me warm/hot air even on the highest setting. It was then when i noticed the temp gauge. I had to drive about 2ks from when i noticed the temp level to a carpark where i could shut the engine off. Once parked, it was hissing and doing the typical over heating dance. First thing i did was run to the petrol station and bought some coolant and waited for the engine to cool and then topped her up. After all the collant it lost/was spitting out in the car park...it only lost maybe all of 2L. This has me a little confused as there was a substancial amount of coolant left in the system so it couldnt of been a lack of coolant/blown radiator issue. Anyway i had my father come out to assist in my trek from the car park to home. After the car was topped up with collant and was cool, i began the drive home...unfortunately the shortest route was through galston gorge  I made it half way through the gorge with out an increase in temp however through the other half it began rising, to the point on exiting it over heated and shut it self off. However as it increased in temp there was a metallic rattle that increased in volume coming from the engine bay....would anyone know what this is?, it has me very worried! From this point onwards i have hooked up a snap strap and had my father tow me to my local mechanic, where we have left it until tomorrow.
 
One thing i noticed was that the radiator wasnt hot nor warm which im thinking means that the coolant isnt being pumped to it to be cooled. This seems to me that it is possible that the Thermostat has given up the ghost on the way home or that there is a blockage within the cooling system, which is possible as i scraped out some light pink sludge from the radiator pressure release opening whist doing some fortnightly maintenance earlier today.  
 
What are your opinions on this and is there something im missing or dont know about that is causing this?
 
Thank you in advance
 
Declan 

Thanks Rob for such a quick reply.
 
Thats was my best guess but am unsure as it was just such a sudden issue. I guess its about time i had a mechanical issue in my 10 months of ownership haha.
 
With this over heating exercise i also found that the oil that i thought was coming from the gearbox that i was talking with you about earlier tonight was in actual fact coming from the purge tank over flow. Not sure if this has something to do with the over heating issue i have.  

Cheers Nick
 
The thing is i lost all of 2L at most. Having my father driving behind me at the time, he said there was no mist coming from the exhaust which is generally what it looks like when coolant is being dumped out the exhaust. So fingers crossed its not a head gasket. 

Any history of cooling problems?
 
"If the radiator doesn't feel like it's been doing anything, it could well be the thermostat".
+1
The first thing to check. Put it in hot water on a stove to test it.
Should open about 85 centigrade.
Good luck.

The car hasnt had any cooling issues in the time ive owned it nor with the last previous owner. The way i gauged that it was about 2L lost was by how much i put back in before it over flowed. I am unsure of where the oil is coming from...my mechanic assured me it wasnt coming from where i thought it was. So thats still a mystery, though the oil i found was quite watered down compared to engine oil. This has me wondering was this possibly an early sign of this cooling/leakage issue. 
 
My mechanic has had a look at it and suspects that it was indeed the thermostat that went but has now found that gases are leaking into the cooling system as the coolant was foaming up and continuously over flowing through the in-fluctuation of gasses in the system. So its a warped head gasket 

That doesn't sound good. You can try all the normal stuff (cap, thermostat etc), but I'm leaning towards a BHG.

Im waiting for my mechanic to get back to me about the extent of what has happend. If it going to need rebuilding, i guess i need to get moving along with rebuilding the GTE thats sitting in the garage and just do the conversion.
 
 

It has a long way to go unfortunately. I have been stripping it down over the last month. Need to get parts and have her machined/rebuilt. Once thats done its down to getting everything else such as exhaust mani, turbo, fuel rail, intercooler etc since all i have is a long block and then i just need all the conversion parts gearbox, drive shafts/hubs and turbo loom.