Is it the TPS? Is it the IACV? What did I do? Idle is now 2K!!

[everpresentnoob]

So my 1990 NA z has been giving me issues for the last couple months. basically its undriveable. Randomly while driving it will start hesitating, bucking surging trying to die. I have gone through the Ignition system. New working PTU, checked all coils chased wiring. All seems good. CHecked fuel system, new filter, confirmed fuel pump is delivering proper pressure. Checked all injectors. removed CAS, opened and cleaned it. car will run fine for a while then BAM back to the same old thing. I had over 300 trouble free miles last weekend, then monday morning at 0400 when I tried to go to work in it I made it two miles down the road and it started acting up. Took 20 mins to get to the next exit and then the three miles back to my house. if I gave it decent throttle it would basically not do anything, nothing was firing. I found that if I just barely touched the throttle, I could slowly drive it.

It has been suggested to look at the IACV. So yesterday I pulled it off. it was black inside, but the two pistons on the electric solenoids that I cant remember the names of were fairly clean and totally free moving. I cleaned it out any way. put it in an ultrasonic cleaner (Just the housing, all electrics were removed) for an hour and it was totally clean, as in the entire inside was totally shiny silver aluminum. put it back on today and reset my idle. Unhook the yellow plug, turn the brass screw under it until idle is at 650 then plug back in. Idle settled at around 800ish and was good and steady.

Then I checked my TPS. I had replaced it early on in this entire debacle and thought I had it set right. Hard idle switch showing continuity with throttle closed, no continuity as soon as I opened the throttle. TPS set to show .44 volts throttle closed and 5volts at full throttle. when I checked it today the hard idle was not set properly and the tps was going from .44 to 3.8v... played around with it for a bit but could not get full throttle to show more than 4.3, and that was with the tps totally rotated up. so that everything was out of spec. got it back to best I could do, Hard idle properly set, tps at .44 closed 3.8 full throttle, and fired up the car. Idle was sitting at 2000 rpm!!

I double checked everything I had messed with but couldnt find anything wrong. If I unplug the yellow plug on the IACV my idle drops back to where it should be, but once I plug it back in the idle jumps to 1800-2000 rpm again.

Does anyone have any ideas? Im about at my wits end with this car. I love driving it, but this is getting frustrating with it randomly being craptastic, now this odd idle issue...

[Destrto]

I was going through the wiki article for setting the TPS, and I believe it mentioned that that might happen after setting it. Have you tried simply unplugging the grey connector with the car running and set to ON a few times? Not saying that would definitely fix it, but it is mentioned in the wiki.

When I set mine, it wanted to spike to 15k rpm at idle until i tried that. Then it came back down and was fine after that.

Also, have you checked the O2 sensors yet? I had a similar problem with mine bucking and losing power intermittently a while back. It ended up being that the O2 sensors were wired incorrectly and were shot. Might be worth looking into if you haven't done so already. Even if it is just to rule them out.

[DCaff300ZX]

Good call Destrto, I'd check this out everpresentnoob.
Also, I'll say that my experience with the ultra-difficult IACV is that there is only one way to deal with it, and that is buy a brand new one. I went through all of the things you mentioned trying to find the same type of glitch/hiccup, and after replacing it with a "good used" one from Z1 (came in similar condition you mention after the ultrasonic cleaning) I got about a day before the same issues again. I finally followed the suggestion of brand new vs. "good used" and my problem with idle went away...but the final straw to fix everything was a new PTU harness. The old one had just enough green crud damage (apparently) to cause intermittent surges and misses and hard idle situations, even after the new IACV, enough that replacement was suddenly an option and the final solution- I'd cleaned it about 10 times over the years and so that was not the solution.
Anyway, keep at it and just be ready to do the usually-unthinkable (beyond going over your previous work, as you have): throw a little money at new parts vs trying to fix/clean/make do with the old OEM in this situation, as I was forced to do and was an eventual solution.

[everpresentnoob]

well the PTU harness Im using is from some strange vaguely trusted member here... although I did cut out the old ptu plugs and soldered the wires from teh new plug straight into my cars harness. had everything plugged in and did one wire at a time so I know i didn't connect it improperly.

As to unplugging hte tps while running, Cant remember if I did that to the grey connector or not but I know I did to the hard idle connector. Kinda forgot to put the wire retention clip back in it and while I was pulling on vac lines in that area to see if I have knocked something loose I bumped the plug and it came off the TPS. Ill try the grey plug tonight a few times. I think it is funny though that once I unplugged the IACV's yellow plug my idle settled down immediately. plug it back in and I jumps to 2K.

O2 sensors... Shortly after I got the car I had a driveability issue and found that i had a code stored for the passengers side O2, so i replaced that. Never touched the drivers side. But I haven't gotten any codes at all with this round of issues. Ill do the ECU diagnostics tonight and have it check the O2 Sensors to see if they are both switching evenly. I really hope thats it... those are a lot cheaper than the damn IACV!

Ill post up again tonight with what I find.

[everpresentnoob]

Well, last night I started the car up with the IACV plugged in. Idle was at 1800-2000. unplugged and replugged the grey connector at the tps a number of times and the idle dropped. But after driving for a while the idle was back to 1200.

Functionally though I put about 20 miles on it and it drove fine. But then last weekend Iput 300 on it,then monday morning at 0400 It messed up at 2 miles... Ill drive it around this weekend and see whats going on. Hoping to solve this so I can just get rid of this car. Since I got it I havent had a month without something going wrong and it being undriveable.

[Destrto]

Just because I'm running through all the steps I would look at if it were me with this problem. Have you checked to make sure the idle is set correctly? I know this takes you back to the IACV, but it could be worth looking at just to rule it out.

[everpresentnoob]

Well I have been playing around with it for a couple days now. setting the idle, unplug yellow connector from IACV, large brass screw to get idle to 650, plug yellow connector back in. This is where my idle jumps back up to 1500, then slowly settles back to 1200. But the car is driveable now so Im counting it as a win. Pretty sure the gasket on the back of the IACV is leaking. Why do I say this? well I never replaced it. Totally forgot to. Thnking I will replace the gasket when I put a new IACV on it. If I do. Im to the point that I just want to get rid of this car. It runs great for a few weeks, then something goes wrong and Im paying 300+ for sensors or parts. Im done with this car. Maybe some day Ill get another one, but it damn sure wont be a $500 dollar beaten half to death one...

[everpresentnoob]

well its back to doing its stupid thing while trying to drive and now the idle is back to holding at 2k.... Im done. Towel has been thrown in. Cars going of Craigslist this weekend.

[everpresentnoob]

well, last night I removed the IACV again and tried jumping 12 volts from the battery to the AAC solenoid. From what i have read here I should hear a click. Nothing. no noise. No anything. checked resistance with my meter and it is showing 10.5ish, but my meter is auto-ranging so i dont remember if that was ohms, milliohms or what... So I guess I am looking at getting a new IACV soon. thats 300 bucks I really dont want to spend...