BrickBoarders,

Several times I have read threads on this forum only to find that they trail off with no resolution. Not this time!

I spent about 2 hours tracing each wire from each fuel injection component in the engine compartment back to the plug that goes into the ECU inside the car. I was just about to give up and replace the entire wiring harness hoping that I had missed something and that the shotgun approach might work. I had only the TPS wiring left to check.

As I looked at the wiring diagram for the TPS to the ECU I started wondering why the hell the 3 wires on the TPS didn't match the colors for the wiring diagram. All of the other wire colors matched perfectly. WTF???!!!

So then in the back of my mind I remember reading a post on this website that said the plug for the TPS and the plug for the IAC are identical and sometimes if someone isn't careful they can switch them. In fact I believe there was a Volvo service bulletin warning of this possibilty becasue it was so common.

Anyway, I disconnected the plug at the IAC and sure enough the wire colors matched the colors in the wiring diagram that should go to the TPS switch!!!

Whoppee! Someone switched the plugs. I reconnected the plugs to the right components, started the car and vrooom the idle shoots up to about 5000 rpm and then drops to around 1000 rpm and then just keeps surging back and forth between 1000 and 5000 rpm. Now what?

Luckily, I had encountered this symptom before in another car and was pretty sure what to do next. I ran off to the Pick and Pull and grabbed a new 511 ECU, brought it home, plugged it in and voila!, the car runs like a dream! No hesitation when cold, no surging, perfect idle, no issues whatsoever.

So why did I need another ECU? I assume that the TPS and IAC switched plugs fried the internals of the ECU and sure enough the new ECU proved this point.

The only drag was that I had had another good ECU spare in the garage but during the initial phases of debugging before I discovered the switched plugs, I swapped the good one in and the screwed up circuitry fried that one instantly so I had to go buy one that hadn't been exposed to the mixed up wiring.

Hope this proves helpful to others out there.

Thanks for everyone's input.