Saab 900. 1991. 274,223miles

50/50 Coolant goes in, doesn't overflow, yet dissapears completely.
Even when the car is stationary for days, with a full coolant resevoir, it mysteriously dissapears, never to be seen again. About every two days, it guzzles a complete helping, leaving a dry tank with strange blackish residue in the bottom. Have replaced the cap (the previous one was releasing vapor) but was met with no positive results. I've also checked and double checked all the coolant hoses, gently cleaned the radiator, and the coolant pump was replaced just under a year ago.

Problem started about a month ago.The car ran perfectly up till then.
There are no visible leaks under the hood, on the driveway, or on the floorboards. If i drive the car about half an hour without turning it off, it doesn't overheat. If i start it back up again, it immediately overheats, and will not cool off for a good 4 hours. we're talking night time, temperature reads 0, but within about 600 feet, it climbs to red (if i don't let it sit for a long, long time).
Low idling at stoplights, followed by a sputtering start. Odd smell coming from the vents and resevoir, like aerosolized non-stick cooking spray.

I fear a crack in the engine. Some horrible mixture of fluids.
coolant/oil. coolant/fuel. oil needs a change soon, but there appears to be no coolant seeping into that resevoir. However, the fuel rod began clicking at the same time the coolant began disappearing. I added a bottle of techron as directed, which alleviated the clicking temporarily.

has anyone encountered a similar situation?
any ideas or advice would be welcome.

What does the engine oil look like?

viscosity seems on par. color is fine. smells fine. no condensation.

When refilling the cooling system, have you bled it? (small screw on thermostat housing)

Black residue quite likely just crap in the expansion tank, remove tank and hose it out with a garden hose to get rid of that.

Have you noticed any leakage under the car? Perhaps take a large piece of cardboard (say the box to a big screen TV) and drive on top of it, leave overnight.

Have you noticed any leakage inside the car? Check carpeting.

Have you noticed any traces of coolant being burned in the engine, example white smoke and a "sweet" smell to the exhaust?

i dont mean to sound too grim,but sometimes when a head gasket fails it allows coolant to enter only the combustion chambers.that is, the missing coolant may be going into 1 or more cylinders after the car is shut off. i had one that did that for years,and i found that if i depressurized the cooling system after i drove it i would not loose mutch for quite a while. that car would start in the morning on two cylinders (1and4),then i guess one or the other of the wet ones would pump itself out and catch,and then the other would catch. it was while waiting for this engine to smooth out one morning when i figured what was happening,and i decided not to fix it.i know, its amazing i diddnt bend a con-rod,but depressurizing the system the night before made the smooth-out time mutch shorter.anyway if it is the head gasket you should fix yours. try loosening the pressure cap (slowly )a short time after driving it for a few days and see if your consumption rate drops. this will not prove the gasket is bad,you have to look at your spark plugs they may offer a few clues.you have to do a compression test.that will show you whats realy going on. who knows it could be a hose with a pinhole leak so small that the coolant comes out as vapor or steam,leaving very few obvious clues.the big air bubble theory is highly probable too. please keep us posted.

this sounds probable. thank you much.

by the way, welcome to saab central.

If you depressurize the cooling system, any leak will be reduced, wether it's going into the engine or not no?

right, thats why he needs to do a compression test. loosening the cap is a short term aid, to reduce coolant loss.