Our maxima developed an engine problem some months ago . On start up the engine revs to approximately 1500 rpm from cold. As standard with this model , but the fault  happens when the engine full temp is reached around 90 deg. Then it goes into a up and down revving mode ,  800 to 1500 revs continuously. Revving the engine higher will not ease the revving once the accelerator is released. The knock sensor went open circuit , it was changed out, these engines require a maxim amount of patience . So the ECU shows no faults , but the revving continues.

I desperately need some help from a Maxima expert. Any realistic help would possible steer me in the wright direction.

We love the car , great to drive had it since 2003 ,  looks like new. But this fault I need to rectify.  

The first thing I would look for is a vacuum leak somewhere in the induction side of the engine.
Look also for any exhaust leaks as this can throw the sensor to the maximum limits then overfuels as it senses a lean mixture.

Lastly look at the coolant temperature sensor readings as if this is out then it can be sensing engine is cold