Once again, I'm in agreement with Klaus. Recently when I replaced the thermostat, the large hose to the top of the radiator got turned. Once these hoses are on a certain way for a long time and have "shaped themselves" to the pipes they connect, even a slight movement of them from their original location can cause a small leak. A day or two later, I noticed the green puddle too and a loss of coolant in the expansion tank. Took me awhile but I tracked the leak to the large hose in question and noticed it was not on exactly centered where the white mark was on the rubber. This leak came down the side of the radiator on the passenger side and dripped down into the air dam (the plastic piece under the front of the car) and ran out onto the floor, right under the front/middle of the car. Any RADIATOR leak is going to do this. Any coolant leak on the PASSENGER SIDE indicates that it is coming from an area AWAY from the radiator...the expansion tank, one of the hoses from that, the thermostat housing or the water pump as may be suspected here. Like Klaus said, put a paper towl under the expansion tank and see if it gets wet...a good preliminary check. Then check visually for bad clamps on the smaller coolant hoses. I would save the check into the TB area for last, if and when no problems are found elsewhere.
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1998 S70 T5 Emarald Green Metallic, 2004 V70 2.5T Ruby Red, Previous Owner of Black '94 850 Turbo Wagon. My cars have been running so well lately they've got me worried.