Coolant disappears and car overheats 1990 240 DL

[lightsrange]

I'm new to this forum so I'm hoping someone can direct me to some answers (or to a prior subject section). I have a 1990 240 DL Wagon that is in fair condition. Recently the car overheated. I noticed the coolant reservoir was empty. I filled it but two days later it was gone and the car heated up. There's no leaking, hoses are fine, oil looks fine, all else seems to be running okay. I don't smell antifreeze in the car and the floorboards are dry. Filled it a 2nd time last night. The coolant thief struck again; the reservoir is empty.

I've been chatting with friends on Facebook who have been giving me helpful ideas. My next step is to verify that the thermostat is not broken (would it be if it heats and cools?). I can not afford to have a mechanic look at it therefore I am trying to figure this out and fix it on my own (I may be crazy but I'm trying).

Thanks for your help...Rachel

[JDS60R]

Sounds like a head gasket. Can you check the coolant for combustion gasses. $12 kit at NAPA