2000 Cadillac Deville, 4.6L, Surging Idle and Stalls

2000 Cadillac Deville with 4.6L engine was sent to my shop for surging idle. The engine has been changed. The IAC valve, TPS and MAP have been replaced with oem parts. The IAC valve harness connector did have the terminals spread and there was poor contact between harness connector and IAC valve. Replaced the connector and surging was improved but still there. Checked data with Tech 2 Scanner and found IAC counts at or near zero, MAP was fluctuating with idle variations. Checked data further and the only thing that seemed out of place was the spark timing was shifting from -10 to +25 during surging idle condition. The higher the swing in the spark numbers the worse the surging was. Using the special function feature I found that the surging would almost go away with +10 degrees of timing added. The next step was to scope the cam and crank sensor patterns. Cam and crank patterns were good. What now? Research found that the computer can base its timing programs on either angle or time data and this works with the engine having an A and a B crank sensor. Maybe the sensors were plugged in backwards? Nope, different plugs. Well I was there anyways lets see if computer will recognise a missing sensor. Disconnected sensor A and cranked engine, SES light on, idle problem is gone but can tell base timing is off due to flat engine sound. Great, finally found someting that would make a difference. Went to ACDelco.com and looked up part numbers. Parts are listed as upper and lower and pictures are available. The upper had been disconnected and it was black in color. How come the picture of the part is grey? No it could not be! Removed both sensors and exchanged positions. Upper = Gray, Lower = Black . Cleared codes and cranked engine, PERFECT!!!
Although the plugs cannot be connected to the wrong sensor the sensors can be placed in the wrong hole. After fixing this vehicle and reviewing data I realized that the dead give aways for this condition are erratic spark timing and IAC counts that are not between 40 and 80 at warm idle. Hope this helps some one else.

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