Hello! I have a 1993 240sx, the car is mostly stock other than the an aftermarket spectre air intake that was halfassed and duct taped to the maf. I bought the car like this with the high idle. I have cleaned the IACV and changed spark plugs and wires thinking it would help the car idle better. The adjustable screw on the IACV is also turned all the way to the right.
It has very high idle that stays around the 3k rpm range on startup and then proceeds to go stay in the 2.5k range and sometimes hesitate. While driving it will stay about 2.5-3k rev range without putting my foot on the gas pedal. Turning on the ac also makes it idle a little higher but that should be normal.
1. Do you have any suggestions on checking for vacuum leaks?
2. Can the air intake impact the idle in this way and should I go and buy a new one? Also recommend some if you can.

Lots of things like a vacuum leak could cause a high idle. Some stuff to check (timing/TPS);

https://www.nicoclub.com/archives/240sx ... iming.html

https://www.nicoclub.com/archives/ka24d ... -idle.html
https://www.youtube.com/watch?v=V-Mwxicmpg4&t=216s
https://www.youtube.com/watch?v=rKK7QdochkI&t=40sv
https://www.youtube.com/watch?v=28opzJ4hALQ

vacuum diagram for places to look for vacuum leaks.
https://www.nicoclub.com/archives/91-94 ... aters.html

I've heard of people using carb cleaner and spraying around while the car is idling. if the idle changes then the leak is in that area. Personally, I find it easier to do a boost leak test. The throttle body on a KA24DE is 60mm or just under 2.5" You can make an easy pressure tester by getting a 2.5" OD PVC plug at a place like lowes or HD and then drilling it out and installing a valve stem and using a compressor to pressurize it. Just make sure you turn down the pressure on your compressor's regulator to 10psi or less first. It's easier IMHO to find vacuum leaks with the motor off and using a bottle of soap/water mix and spraying around. The leak will blow some nice bubbles for you and you get to clean stuff in the meantime. If it isn't so big you can hear it outright.

A quick dummy test to tell if you have a vacuum leak; in the picture of the manifold below, there's a tube coming out from beneath where the upper radiator hose connects to the manifold. it'll connect to your intake. Pinch it closed and hold it while the motor idles. If the motor doesn't stumble and die you've got a vacuum leak.

Lastly, check your throttle body. It's possible someone adjusted the screw underneath that controls how much the throttle plate closes. It's an 8mm retaining nut and a small allen wrench to adjust it. If your throttle body is propped open you'll never be able to idle down. Here's a pic with the adjuster I'm talking about circled in red.



Best of luck!

Thank you for all the helpful information!! Definitely going to be trying all these!!

Quick update: So after doing the dummy test, the car did stumble and die and after readjusting it my rpm has dropped to 1.5k. Honestly i feel way more comfortable with that range than the previous 3k range. Next I'll check out the TPS and throttle plate

You may need to clean out the IACV too. That tube you pinched off? I disconnect it at the intake (charge pipe in my case) and pour a little seafoam down it and let it sit for an hour or two. Your car will smoke like a chimney for a couple of minutes after you start it back up but that stuff will clean out the IACV pretty well without having to remove it from back there. It's a real PITA and unless you have troubleshot EVERYTHING else first and know your issue is the IACV, I think it's a bit overkill to completely remove it just for s*** n' gigs. You may be able to remove the adjuster screw though, taking care not to drop it or lose anything, and spray some carb cleaner in there to clean out the passages.

https://www.youtube.com/watch?v=5SzpXE0lTuM

vid of some dude removing his IACV and cleaning it if you decide to go nuts. If nothing else, it'll give you a view of what's going on back there. I'd definitely buy a new gasket BEFORE removing it if you get a wild hair. I wouldn't reuse the old gasket or trust that it isn't going to break into several pieces when removing the IACV.

I actually did clean the IACV when I first got the car but the gasket was stuck on (not on the IACV but the on block?). So I ordered a new gasket but wont get here until a week later and I'll have to scrape it off somehow. I didn't remove or clean the adjuster screw which I should have done, but didn't because I was scared to strip or break anything. I am however going to replace my alternator and will clean the throttle body since it will have to be exposed for more space. I'll try the sea foam trick too!

That adjuster screw should turn freely and easily. If it doesn't, there is a problem.

Throttle body is same story as IACV; we don't get a rubber gasket but a dense wax-paper material. I'd grab a Felpro replacement at local auto parts store before you pull it. Also make sure you get it torqued in sequence and to correct values when you reinstall it. The FSM on here has those values/sequence.

Small narrow 2" putty knife works WONDERS removing these old gaskets. I keep a hand-me-down in my parts washer and get it out for doing this. Don't go nuts and gouge at the mating surface or you will damage it but you can usually get the blade underneath and 99% of the time it'll just lift right off, sometimes in pieces. Gasket scrapers are nice if you have the space to work with but I find myself using that putty knife in the engine bay more often than not. Also it's a little less hazardous to my fingers than a razor blade. Don't ask how I know... Be aware that your coolant runs THROUGH the TB on the bottom. Lots of people, myself included, just re-route the coolant line to bypass the TB permanently. Here's a shot of mine when I cleaned it a few weeks ago. I used a cutting wheel on my dremel to shave the lines off flush with the throttle body. I completely removed my intake manifold and redid all of the vaccum and emissions parts when I got the car. I don't really think that's necessary to do for a N/A car. The manifold wasn't designed to be boosted though and I take exception with the way the PCV system is routed through the intake runners when positive pressure is applied to the intake mani.




Don't try to remove the 2 screws holding in the throttle plate unless you have an impact screwdriver and a vise to hold the TB. You'll just round them out. Yeah they look easy enough but, trust me, they aren't.



That's what I'm talking about. Very handy to have around the garage for stubborn stuff like brake rotors. You can find them on Amazon or Harbor Freight for a good price.