I have a stock n/a 91 ZX with around 60K miles, I have just noticed that while pulling up at traffic or parking etc and putting into neutral or park the engine revs climb to about 2000 from a normal 800-1000 ish, i turn off engine and most times the problem is cured.

 

What is likely to be reason for this on and off problem and what should I try to cure it

Have you got the heating fan on at the time - that does make it rise.

could be your throttle bodies need cleaning.

Geoff.

Check the temp sensor, it has a yellow cover. (it's located behind the fan and in front of the timing belt cover) a dodgy or 'green' connection there can cause an irregulor idle.

 

Mine did this some time ago, would make the Z hesitant sometimes but the idle would be going up and down at idle or in traffic (pulling Z along)

 

Worth a look.

 

Matty.