I have an NA S5 and it keeps surging idle between 700-300 until it stalls and dies after getting up to temp. Had to replace water pump+ Rad after found puke looking stuff in coolant. Flushed system and now after that it keeps dying after given throttle once warmed up. Just a tap and the car will go from a solid steady 1200-700 rpms to surging at 200 rpm at it’s lowest up to 700 before eventually stalling out. I had only driven the car once after I got it running for the first time after replacing Rad/WP And it misfired under load @ 3-4k rpm, turned out to be a coil pack not plugged in 100%. Plugged coil pack back in and car sounds more smooth (as smooth as it can w a shitty welded rattling exhaust) but haven’t been able to test drive underload since it keeps dying at idle and won’t stay running w/o a foot on the throttle after it gets to temp. Car has new spark plugs and a PS delete. What could be causing this issue? I heard having AC on increases load on the engine but I have no working logicon to test this theory, maybe TPS? ACV? ECU? (Stock ecu)

 

welcome to the board.

first check for vacuum leaks. you can also check the TPS and the coolant sensor at the back of the water pump.

if you need reference material, you can get a copy of the FSM at Foxed.ca

 

Did you check the ecu’s coolant temperature sensor, mounted behind the water pump housing. Could be disconnected or faulty.

ecu uses coolant temperature to determine the amount of fuel to put in.