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Is your engine a 4ZD1 carburettor engine?
Or are you sure it is a 2.3 litre?
Ok, so there is a problem with the carburetor.
Can you remove the air cleaner lid and verify the choke blade is fully open when the engine is warmed up?
Is the primary throttle linkage away from the fast idle cam at hot idle position?
Have you checked for any other vacuum leaks?
How long has it been doing this?
Gradual or sudden change in idle speed?
Unusual for a carburetor to do that in any other way except if the fast idle cam has stuck up in the fast idle position.
It could be binding on it's pivot post.
Can you check to see if the throttle lever is resting on the idle speed screw or is there a gap?