Is the car in question up to date with all service & maintenance?
Being a car from/for the snow belt, is there rust beneath?
Do all systems function properly?
Do all tires match & have worn evenly?
These are complex cars & with age experience more failures & require more maintenance than newer cars ...

Running rough with CEL flashing during morning commute.Turned around & drove back home.
Parked the S80 & hopped in the Passat for the drive to work.
Upon arrival at home yesterday, I plugged in my cheap Harbor Freight universal code scanner & found #1 cylinder misfire.
removed cover & swapped coils ...

OUTSTANDING!
I do not replace coils or injectors until faults or fatigue sets in. Then I replace the entire set. I have seen original injectors with over 300,000 miles & even original plugs & coils with over 300,000 miles. Miles. Not kilometers.
These were on American trucks. Not the most fuel ...

Happy Birthday Mr. Klumper! :)
Glad you solved your problem.
When I replaced spark plugs on my S80 shortly after purchase, I noticed #6 ignition coil saturated with oil from the nearby filler cap. It came apart in my hand. Still worked but I replaced it anyway. I have driven it 13,000 miles since ...

Do you have a 12v signal to the primary side of the #6 coil?

Each time I log on to scrutinize a new or existing thread, all I read is S80 owners in distress.
As a change, I would like to report that mine has been driven 12,000 miles since acquisition in early October of 2015 and has mainly required replacement of parts that normally wear such as timing belt ...

Replacement should not be too difficult.
It's the bleeding that can be time consuming.
I just bought a hand held vacuum pump/brake bleeder that I will try out on my 2001 2.9 S80 as soon as the sky smiles upon me.

Found it on the curbside between the condenser & headlight assembly by peering upwards from beneath.
Hope I don't have to disassemble the car to reach it.
I just did a brake job (Front rotor, pads, rear pads & rear parking brake kit) that only took me a few hours to complete.
I also scrubbed the ...

Any idea where I might find the a/c dryer on a 2001 S80 2.9.
Blows cool, warm & cold.
Suction pressure @ idle heads south in the teens, then the clutch disengages.
Clutch engages when pressure returns north to 30 psi.
Head pressure holds steady @ a dangerous 200 to 230 psi.
Just ordered a new dryer ...

Could be a deteriorating hose, loose or broken hose clamp a cracked neck on the radiator.
Sometimes as hoses age, the clamp can cut into the deteriorating rubber hose causing a leak.
Good luck with your repair.
I will be in Katy next weekend.
PM me if you need a hand finding the cause.