Overheating / Boiling over? So lost/fed up!

Hello my friends! I did a search and read what was previously posted, but don't think it pertains to my issues.

2001 Mitsubishi Eclipse, 90,*** miles. 4Cyl. man. 5spd. A/C. Black (w/ fading paing:))

Problem(s)- Winter Time - While driving, the heater works GREAT!, when idle/stop sign/red light/ traffic. the heater blows cold air.

Summer Time - 1)Car runs ave. temp for a few days/weeks. then will start to get hot. I check the coolant and it is low(sometimes very). I fill it up and am good for a few more days.
2) once I stop driving, the car boils over into the resivoir. It looks like steam is comming out of the motor into the resivoir. I feel the upper hose and I can feel the steam comming out from it to the overflow.

Car - This car has NO thermostat. I just discovered this a few weeks ago and havn't put one it(was told its not that big of an issue) the water pump is appx 6mo old. the sytem wasn't vacuum drained, only gravity. The radiator cap is newer (1/2mo). I have tried with straight water, and have tried with 50/50 mix. both perform the same. Looking into the radiator via the top, it looks like very light brown mud is in the radiator.

My thoughts - when the motor is running, when I feel both the upper and lower hose, they are squishy (more than they should be) both too hot to touch, and both feel like they have air in them. Even after I burp the system, they feel like they have air in it.

I used to work at a Firestone as a mechanic (not a very good one) so I'm comfortable around cars, this one just baffles me.

Any help is great. thanks in advance guys!:smokin:

http://img683.imageshack.us/i/dsci1644b.jpg/http://img20.imageshack.us/i/dsci1643.jpg/http://img17.imageshack.us/i/dsci1642n.jpg/
here are some pics of my radiator from the top, hope it posts ok. :)

A thermostat regulates the temp properly. Running without a thermostat leaves parts of the engine hot since there is no restriction in the cooling system to even out the temperature. Just becuase the temp gauge isn't running into the hot range doesn't mean that the engine isn't overheating. Boiling into the overflow is a sure sign of overheating. Your problem probably lies with a bad headgasket.

Maybe this will help a little.... http://www.automotiveforums.com/vbulletin/showthread.php?t=587560

@Rhax - Would the T-stat create back-pressure as well to help purge the system of air? and/or help with cooling?

How can I check to see if I have a blown HG? I read the thread from Sloe and didn't see any oil->water or water->oil. If it is the HG, thats the one on top right(the easy one)? nm, The valve cover gasket is the easy one, the HG is the one in the middle of the engine right? the pain in the ass one. =[

thanks again for everything!

The headgasket requires head removal. It can go bad a couple different ways. One way gets oil/coolant contamination. Another gives you rough running coolant loss and overheating. Taking out the thermostat is a bandaid solution and a bad idea. Air shouldn't be an issue since it would take quite a bit to cause trouble and Mitsubishi cooling systems burp easily. You can get combustion test strips that you dip in the coolant to see I there are any combustion gasses present.