Strange Idle Issue

[holyghost]

Hey guys, I have a 1993 300zx N/A, and have had it for about 5 months now. In the past few days I have experienced an intermittent high idle issue. On my way to work, I noticed that at lights/stop signs my idle was at about 2000 rpms. I continued to drive to work, but eventually decided to pull over and restart my car. Upon restart, the idle dropped down back to normal range (for me it has been 900 rpm, maybe a bit high, but it works for now). Weird part of this is that it doesn't go to high idle all the time, and on the days it does, I will just restart my car to set it back to normal.

Another thing to note is that the idle has always seem to have a "lag" before it got down to lower rpms. After coming to a stop, after 5 -10 seconds it would drop from 1500 to 900 rpms, however I am not sure how quickly it should be dropping to idle, or if this is normal. Anyway, I was hoping if somone could point me in the right direction of which areas to troubleshoot.

Thanks!

[DCaff300ZX]

Welcome!
You unfortunately have the Idle Air Control Valve (IACV) issue that is a bit notorious for our cars. Either the valve itself is sticking or the connectors are corroded, or both. The connectors are easily broken and even if not are not weather tight, and the harness should be replaced. While servicing the harness be sure to check your idle circuit hoses (the larger vacuum hoses) for cracks/wear, and open the one end closest to you into the IACV itself and spray in a decent amount of WD-40 and let that sit for awhile.
Hopefully both jobs will net you a correctly-working IACV, if not you may need to replace it which is pricey, and DO NOT buy a used unit to replace it as it will most likely also cause issues, as was the case for me.
You can get this all at Concept Z performance or Z1 motorsports, and elsewhere.
Good Luck!