90-Quattro

Daily driver runs well with four month old tune up. Drove fine to office last thursday but when I pulled into the parking lot it instantly began running rough like someone flipped a switch. Starts right up, idles rough and reves rough like running on four cylinders. Pulled plug wires while idling one at a time, see spark and idle drops for each plug. Drove home with low power difficulty and parked it. Turned the key this morning, started imeadiately to rough idle...
Next stop is to check my my airbox and intake boot.

Any ideas?

Coupe_7A

Plug Wires? Distributor Cap/Rotor etc? Injectors? Fuel pump? Fuel Filter?

Do a compression check.

bradwilcox

o2 sensor, knock sensor. Both cheap and easy.

EuroGuy9622

CEL at all? Knock sensor was doing this to me. With mine, looked like someone had taken a hammer to it... I got mine for $32 on FCP Euro (Bosch). Very worth it. I had no idea how many running issues a knock sensor could cause.