Weird throttle problem, limp mode: 2005 otherwise flawless TT6

[850TurboTurtle]

I just bought this 2005 S80 cheap because it has some major throttle problems.

Original state
Owner was driving on freeway when the engine began losing power and CEL came on. He pulled over and it would barely drive, maybe 5mph, and extremely rough engine. He shut it off and restarted and it ran better though not as strongly as normal. Was able to drive it to dealership. They found error codes 9200 and 9220.
ECM-9200 Throttle position TP sensor - faulty signal. DTC is found active
ECM-9220 Throttle unit, potentiometer 2 - faulty signal. DTC is found active
Shop says repairs are about $1100 (in Seattle area).

I test drove it after it sat for a couple months. After the first start it fell to a very rough idle in about 30 seconds adn the car when put in drive wouldn’t get up past 5mph. Stopped, key off, and restarted. It ran smoothly and i drove it about 100 yards before it fell back to rough idle. Next try got 200 yards. Next try 400 yards. Before it goes into limp mode it runs perfectly. Oil and water and tranny fluid all clean, no metal or sludge. Oil was a little brown from wear which I preferred because it couldn’t be a fresh oil change to hide problems.

So I bought it ($1500) and drove it home (30 miles) with a friend tailing me. First I got 1 minute of driving per restart, then after 5 miles, 2 minutes worth, then 5 minutes. So I got brave and took the freeway and drove 10 minutes without a problem. Stopped and restarted just for safety and drove another 15 minutes home without a problem. Purrs like a kitten on the freeway and the turbos work great.

Once home, I tried hard acceleration from a stop and that put it back in limp mode. Stopped and restarted and it drove fine when I was gentle. Next day I got brave and drove it 40 miles on the freeway without a hitch, and the same on the return trip. (Got 31mpg too!). Then tried a 0-60 floored acceleration which did NOT put it in limp mode. (It nearly put me in limp mode though. Wow!! Twin turbos!)

So, my guess is that the throttle unit is gummy and that if I disassemble it and clean it thoroughly and reset the CEL it may run fine.

My questions:
1. Does this sound like the best place to start?
2. Anything else to do while I have it opened up that far down?
3. Anyone have a link to a good throttle actuator cleaning procedure?
4. Any smart maintenance service to take at 144k?

More info on car:
2005 S80
144k miles
bought from conscientious 2nd owner, who had it since 16k miles, has dealership service receipts
new in last 2 years: mass air meter, turbo control valve, tires, pcv breather box
new timing belt at 118k
needs left upper strut mount and control arm bushings according to shop
radio making weird pop noises sometimes. electrical short?

And, I’ve done the Tracy’s head gasket job on my 850T, so I’m not afraid of much in the way of mechanical work now.


Thanks for the advice,

Rick