I tracked down a small leak on my 98 S90 a couple of months ago to a heater hose behind the engine on the passenger's side. The rubber hose was seeping where it attached to the hard metal pipe. I snugged up the screw type hose clamp and the leak appears to have stopped.

DEWFPO
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1998 S90 083,228 and 1995 964 154,100

If the radiator hasn't been replaced in a while, I'd check that for sure. The radiators in the 960s seem to last about ten years before packing it in. That wouldn't explain coolant on the engine, however.

Do you see the traces of coolant on the front or rear of the engine on the oil pan? If near the front, could be the water pump. If near the rear, there is a coolant line with a rubber to metal connection that goes across the engine block between the engine and firewall that feeds the heater core.

My 960 had similar symptoms that turned out to be a failing and leaking head gasket on the passenger side rear of the head. Coolant would seep out and run down the block. Posts to this site suggest that the head gasket sometimes needs to be replaced when the engine gets 150k+ miles.

I was able to find the leak by listening for the hiss when the car engine was shut down after running at operating temperature. The cooling system is still under pressure and will vent the pressure through any leak that may be present.

Good luck and let us know what you find.