Ive been battling an issue with my Mk2 (BP4W engine) on and off since I purchased the car about a year an a half ago. The car would drop to around 500 rpm when coming to a stop e.g. at a junction or slowing down to a crawl while on the brakes. I did the obvious thing and clean the IACV and set the base idle, although I quickly found that I could not get the idle to go below 950-1000rpm even when the screw was all the way in and driving the car like this was just as bad as before if not worse. The only way to make the car more drivable was to back the screw off enough that the revs wouldn't drop but this meant that the base idle was far too high, around 1400 rpm.
This seems to be a vacuum issue but I feel like I've been round pretty much everything on the intake side, list follows:
Replaced all vacuum lines including port caps.
Vacuum tested the brake booster and check valve.
Vacuum tested the VICS valve and check valve.
Cleaned manifold including all the ports and refitted with new OEM gaskets and injector seals.
Checked that the mating surfaces on the inlet manifold were flat using a proper straight edge, not just an engineers rule.
Cleaned EGR valve, checked that it was seating properly and replaced the boost solenoid.
Tested the TPS.
Changed the IACV to a new aftermarket unit.
Swapped the complete throttle body, TPS and IACV from a working Mk2.5 vvt (which uses the same Throttle body, TPS and IACV), I even put my throttle body onto the later Mk2.5 engine and managed to set base idle, no problem.
Ive also tried changing the MAF and a new coolant temp sensor although I was doubtful it would fix the issue.
After all this I am still unable to properly set the base idle and I have to back the screw off enough to stop the revs dipping as previously mentioned. Is there anything else Ive missed that would be worth checking and is there a possibility it could be an electrical fault causing a component to open when its not supposed to or am I on a wild goose chase?
Any help or advice would be greatly appreciated.