I have an odd problem that I need some expert assistance with. My SC400 idle is irratic and surges uncontrollably (undrivable). I found that if I disconnect the air sensor -it is driveable. I am guessing this is because this puts it in limp home mode. So if this is the case -and having swapped the IAC, fuel pressure regulator, spark plugs/wires, air sensor and EGR valve -what can cause it to surge like this and then run good in the limp home mode with the air sensor disconnected? Can the ECU cause a problem like this?

Anyone else ever have a problem like this?

 

If you've swapped all those things and not affected the behavior, I would guess ECU. Bad capacitors and board/inputs damage can cause all sorts of wacky problems.

 

Also, just for kicks check your coils.

 

It did turn out to be the ECU -in case anyone ever has a similar problem.