My 85 200 has a plug on the passenger side of the radiater. This plug kinda steems a little and stinks. It is brought in threw the vents. With weeks going by before I notice that the fluid level has gone down.
I simply went to the hardware store and got a "lever boat style" drain plug, of the size and pluged it up. All is well and did not need to spend 200.00 on a radiater.

Depending on the type of oil you use it could be leaking on the exaust/header and stinking. Getting inside the car from the vents.

It could also be the heat control valve leak, That is easy to fix.
Good luck.
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Kevin * HONDA spanking,1985 240DL * VOLVO ON!!!!

I'd vote for a small leak in the heater core. Very carefully test the passenger and driver side carpets for dampness. Better yet to look under them. Replacing it is very straigth forward, but is truly a "Journey to the Center on The Volvo". Take lots of digital photos as you go...funny how many wires and hoses there are that seem to have no home I use lots of masking tape labels (1 goes back to 1, 2 to 2, etc.)! Absolutely remove the dash (much easier than it sounds), and don't be afraid to cut up the housing to get it out without removing the car from the frame (just kidding on that one). I hope its not, but all us brickers seem to get to do it as sort of an initiation rights thing. New blower motor and resistor while you're in there, also.

It wouldn't take a very big leak at the heater control valve or in the heater core, and you'd smell it all day. Kinda gives a sweet taste as you breathe it in- unmistakable odor.
Pull off the driver's side kick panel and look for the tell-tale green drips... or a wet carpet.
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Rob Bareiss, New London CT ::: '87 244DL/M47- 225K, 88 744GLE- 209K, 91 244 183K. Also responsible for the care and feeding of: 88 745GLE, 229K, 88 244GL, 146K, 87 244DL, 235K, 88 245DL, 236K

How old is your catalytic converter? Could be on its way out...