IAC Valve 300 ZX Idle Air Control question about if it needs replacing

[my1990300zx]

Question how do i know if i need to replace the IAC Valve / Idle Air Control unit.
Reason I ask my high idle won't go down or up when i turn the set screw with the wire connector off or on. Should I get a code or what??
Also another question even if i have a vacuum leak causing the high idle shouldn't the Idle Air Control turn it down its running around 1100 to 1500 in neutral and 900 in drive . thank you

90 300zx non turbo

[DCaff300ZX]

You should definitely check for any codes first and foremost, as this can possibly pinpoint your search and at the very least point out a definite problem even if that also reveals other problems, and with your issue is a real possibility. The manual code pulling procedure is pretty easy- there's a wood panel under the carpet on ps under the dash, remove it and you see the ECU. There's a small screw on the side you turn following those directions and you look at the check engine light and count how many times it blinks including a pause- long pause...blink-blink-blink-blink-blink, pause, blink-blink-blink-blink-blink, long pause...is 55 and no code, any other combo is the fault code (ie 32, 24, etc.). Read it two or three times to be sure the order of blinks.
Saves you $90 bucks and you can do this any time, be sure to return the ECU screw (potentiometer) to the off position when done.

I'll start off by saying I have been at war with my IACV and idle for quite some time, and after my recent engine upgrade work and a talk with my mechanic his evaluation is that a new IACV is really always a need for the VG30DE and DETT engine due to their age, and the way the IACV works. As they age and get buildup inside of them, they just naturally become less accurate and can become stuck up somewhat, to completely locked up in addition to their OEM connector corrosion issues. Replacement of the harness with an upgraded sealed connector and harness can fix that issue less expensively if the IACV still functions well enough.
HOWEVER, there are quite a few things to look at, and do maintenance on before you go to that drastic and expensive step.
1. Check ALL connectors for corrosion and clean/replace, I replaced all of my connectors and the harness itself. This helped a lot but not completely.
2. Check and replace the larger hoses connecting to the IACV as they are often split/cracked due to age and removal of balance tube for maintenance, using silicone replacements for easier removal/use later. Silicone is less expensive than the rubber replacement hoses, and lasts longer as well.
3. You may also want to replace the PCV's (tough job, but doable) which can be clogged up and be an idle issue.
4. The coolant temp sensor and temp sensor (the two plugs with leads on the coolant hardpipe) plugs are also known to fail or give indifferent readings due to age and corrosion or both, replacing the connectors and also the sensors is the best way to service that area, inspect and clean at minimum. Also while you're there inspecting and cleaning if necessary the larger two plugs for the PTU harness is a good idea as they can be an idle issue as well, the PTU is mounted on the engine timing belt cover.

If all of this fails then a new replacement IACV might be in your future, but still maybe not. Be sure to use the OSM for testing procedures for the IACV and idle issues, as there is a LOT of things involved with idle in a VG30DE engine (vacuum issues are a real PITA) and especially with AT added in...good luck, hope this helps you get started!

[my1990300zx]

wow a lot great information thanks for taking the time explaining the mechanics of those units ...