The wastegate is easy to check. Assuming the 300tdi is roughly laid out as the td5:
- Remove the heat shield. On the td5 it's 2 bolts on top, one at the back (the one that makes you swear).
- Disconnect the wastegate arm by removing the clip (see photo). Don't lose it.
- Move the rotating arm from side to side. It should offer no resistance and should go all the way to both sides (metal noise).
- If it resists or doesn't move all the way, spray the rotating part of the arm with generous amounts of WD40.
- Then move the arm back and forth until it's completely free.
- Put everything back together enjoying especially the bolt at the back of the heat shield.
BTW, I suspect a 300tdi without a turbo wouldn't revert to a N/A because it's designed to work with a turbo and the air mix would be completely wrong without turbo.