Hey guys I had the same problem most of you are having with cold starts found a forum that was spot on for a fix the r31 air regulator has to be replaced by a r33 air regulator you can buy second hand but i recommend paying a bit more for a new one see link below
https://www.efisolutions.com.au/air-regulator-rb25-r32-r33-wc34-c33-c34-c35-a31
you will also need a new connector plug see link below please note you have to make sure it fits there are different kinds that look the same it has to be the blue one
https://www.efihardware.com/products/2156/Nissan-2-Pin-IAC-Front-Connector-Blue
the r31 air regulator uses a butterfly type setup which is not working any more because of age the r33 air regulator uses a slide type setup which is very efficient and a better design
this conversion is very straight forward it fits exactly the same as the factory one fits and you only need to modify the pin clips on your wiring very easy to do i cannot tell you the difference the r33 air regulator has made it runs perfectly now no more back firing sputtering having to rev it to 2 thousand so it runs this will solve your cold start problems
