You need to fix the code 52 BEFORE anything else. It changes the way the ECU handle a lot of things in a MAJOR way. Look in the MKIII FAQ section for a thread about code 52 and knock sensor rewire. Often the wiring just needs to be replace with better wiring and a new connector is recommended. The knock sensors are very pricey so try the rewire first.
Make sure the TPS is for the right year. There was a change mid '89 and the interface tabs are clocked 90? different from earlier years. This will hold the throttle plate open, but I think it hold it open to a much greater degree than you're seeing.
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