I have a 89 IROC convertible that we are nearly finished on a pretty extensive rebuild process. The car was originally a 305 but we converted it to a 350 5.7 tpi and used the ecm from the same car (89 t-top iroc). I have 2 totally different questions.

First the car seems to surge at idle. The rpms are pretty decent in the 750 range while in park. However the car doesn't really purr it just idles low then revs up then repeats this process. It doesn't ever stall however. I am not getting any engine codes and I already tested the TPS and its reading dead on the .54 volts. However when I tried to perform the IAC adjustment the car will not run at all at idle. Could this mean the IAC needs cleaned, replaced? I have not tried performing the adjustment with the car warm yet as the fans are not installed yet so the car can't be ran for extended periods.

Also on a completly different note we have been working really hard to fix all the splicing that the previous owners have been doing to the car and now have it all pretty much settled except there is a 1 wire connector thats purple and white with a grey connector end on the drivers side under the console that I can't figure out what it is.

 

Check for a small vacuum leak or the egr sticking open from either junk in the port or the egr solenoid is hold the valve open. Those are the most common 'surging idle' causes.