89 240sx high idle IAA problem

[shiro_neko]

89 240sx ka24e. I've read a lot of posts while some helped none fixed the problem. ka24e parts are hard to find here so I cant keep changing parts. It used to idle at 900 rpm which is still higher than normal. car surges on deceleration, slight hesitation, a little rough idle and backfire through the exhaust. idle stays at 1500 rpm for 10-20secs when going to neutral before going down to 900 rpm. Here are the things that I've done:

removed the AIV and plugged the hoses.

checked and cleaned the throttle plate.

checked the ignition wires for arcing buy running it in the dark. replaced spark plugs.

adjusted timing to 15 degrees.

checked the grounds and added new ones. fixed the slight hesitation.

cleaned the MAF sensor with contact cleaner. checked the voltage output: 1.5v to 2.5v while racing

seafoamed it ang changed the oil. improved gas mileage.

checked the air regulator. one wire was cut so I reconnected it. pinching the hose at operating temp doesn't lower the idle

checked the egr. the vacuum hoses were connected to the wrong vacuum lines. egr works now. fixed the surging at deceleration except in 1st and 2nd gear.

corrected the vacuum hoses going to the pr solenoid and scv solenoid

checked for vacuum leaks by removing the hose from intake tube to air regulator and IAA. engine died as soon as I removed it. I sprayed water on all places that might have a leak in the intake system but did not find one.

checked the o2 sensor through ECU. green lamp blinks more than 5 times in 10 secs @ 2000 rpm. output voltage is 0.4v to 0.2v

No ECU codes.

checked the pressure regulator by removing the vacuum hose.

Now, I know there's a problem in the IAA because I cant adjust the idle through the idle adjust screw with the TPS disconnected. I removed the AAC terminal and the car died so I thought it was working. I tested the TPS and the output voltage was way off. so I set it to 0.45v and now the idle went up higher to 1800 rpm. then I removed and cleaned the IAA. couldnt remove the FICD but I figured it was working properly anyway. so I just swabbed whatever I can in the FICD port.

After installing the IAA the idle is still at 1800 rpm and I cant still adjust the idle through the screw. but when I removed the AAC terminal it didn't die and the adjusting screw works and I was able to set it at 700 rpm. It idles a little rough and some backfire but it runs smoother. the 1st and 2nd gear still surges on deceleration but its smoother now which I think is normal.

I checked the AAC. the resistance is 10.5 ohms. but the output voltage is 0.4v which should be 7-10v. there's continuity in terminal b of AAC and terminal 113 of ECU. the FSM says that I should perform an ECU i/o signal inspection test which I cant find anywhere on how to do so. my ECU doesn't have consult. I added a ground wire to the IAA because mine has a gasket but it didn't help. resistance from negative terminal to IAA is less than 2ohms. re-checked the 02 sensor, MAF, timing, pressure regulator and the results are still the same.

I figured I would keep the AAC terminal off for a while. any ideas?

[shiro_neko]

Done some more checking and I overlooked the resistance of the TPS. I think I have a bad TPS. I cant get a continuity between A and B or Im doing it wrong. I tested the terminal side of the female connector going to the TPS with the ignition switch "OFF". A is the one on the left (red wire) with the clip on top. can someone confirm this? And I assume its not repairable.

[Unstock240]

If you are testing to set at .44V then you have to have the ignition key fully on. Not sure but this was on my kade. I took 2 paperclips and they will go right in with the plug connected by backprobing the harness side. It is either the left 2 wires or the right 2 on the grey connector. Also they recommended it be fully warmed up to set the adjustment. I just did this to mine today and my car gained some of my missing power.

[shiro_neko]

Yes its "ON" when I adjusted the voltage and I didnt have any problems with that. I was asking about the resistance.

Now Im confused. If its the TPS then I should be able to adjust the idle through the idle adjust screw with the TPS disconnected which I cant. But if I remove the harness from the AAC valve I can adjust the idle with the idle adjust screw.

And the bad thing with the AAC disconnected is I dont have fast idle during warm up. so I have to plug it back in while warming up and disconnect it afterwards

EDIT:

ok I think my TPS is bad. Looking at the terminal side of the connector going to the TPS (female) there's no resistance between the left terminal and middle, and right terminal and middle. the middle has an output voltage when turning the TPS: 160mv gets my idle to 900 rpm anything higher than that gives me an idle higher than 1500 rpm. The right connector also reads in mv instead of the same output voltage with the middle connector.

Still, even though my TPS might be bad I cant understand why I cant adjust the idle with AAC connected and the TPS disconnected. But with AAC disconnected I can adjust it with the TPS disconnected plus it idles and accelerates smoother. doesn't stop at 1500 rpm before going down to idle when shifting to neutral. Maybe both are bad?
Modified by shiro_neko at 2:38 AM 4/2/2010

[shiro_neko]

I think the idle screw doesnt work because it is being bypassed by the ECU through the AAC. So when the AAC is disconnected the ECu cannot control the idle so the idle screw now works.

I figured out how to do an ECU I/O signal inspection test. and terminal 113 should be 7-10v on idle and 4-7v with AC on, defogger on, steering wheel being turned, and headlamp in high position. My ECu sends out 0.2-0.3v on all conditions. Im inspecting my terminals and it seems good. there's continuity in 113 and AAC. is this a bad ECU???

[locoluna825]

Wow. Your really on this issue. I have this same problem but not to this degree. My idle will stay at 1200 rpms and lower down to about 800 after a few seconds. When i shift into nuetral or come to a stop. It Started doing this after I cleaned my IACV. Before that the idle Used to surge up and down at cold start up. and would drop below 500 rpms and bounce back up to 750 ish range going into nuetral or coming to a stop.

Guess i should start probing around too, I had a suspicioun that the TPS might be the culprit.
Modified by locoluna825 at 7:39 PM 4/13/2010

[shiro_neko]

Yep I've been reading the FSM and topics from different forums over and over for months now. It really bothers me how I cant control my car the way I want it to. And its a common issue with different causes and solutions with no definite answers (well mostly) so this would make a good reference to others if I could fix it or someone knows how to fix it.

You could try turning the ECU screw counter-clockwise all the way. My idle stays at 1800rpm warm with TPS at 0.5v and the AAC receives 0.2v and 0.3v w/ AC on. Thats with the ECU screw turned 1/4 counter-clockwise from top. After turning it all the way down idle stays at almost 900rpm with TPS at 0.5v, AAC 5v and 4.8v w/ AC on.

I need to find whats telling the ECU to keep it at almost 900rpm as when I turn the Idle adjust screw fast enough with the TPS disconnected. I could hear the idle go down then it shoots back up to 900rpm. Tried to reset the ECU and nothing happened.

[locoluna825]

did you try adjusting your throttle cable, and the adjustment screw on the throtle body yet?

[shiro_neko]

throttle cable and TB screw is good. The 1st time I Lowered the idle through the ECU screw the idle would go down but the engine shakes too much. After cleaning the IAA and setting the right voltage of the TPS it actually idles at 750rpm on operating temp with the ECU screw turned all the way counter-clockwise. It no longer stays at 1500rpm when going to stop and it slows down much smoother when releasing the accel pedal from 1800rpm up. I have that slight hesitation in 2nd gear from 1800-2000rpm, might be the fuel pressure though cause it feels like its cutting fuel after shifting to 2nd gear quickly. Still cant adjust the idle from idle adjust screw and idle hunts between 900 and 700rpm before settling in 750rpm. But this will do for now while I hunt for parts. I'll check the crank angle sensor next weekend. Does the neutral switch affect idle? it is listed in the idle speed control system but I haven't seen anyone having problems caused by it

[locoluna825]

i believe it does. I swapped my car from auto to manual. and for the first week i had the whole auto shifter assbembly wired up to the fuse box tiptied in nuetral in order to start my car. lol. When i played with the auto shifter, changing it from park to nuetral to drive. the car would react to it.

[WhiteKnight]

Is the AC on?

[shiro_neko]

Nope, the AC is off.

Well if its from AT to MT swap I really think it matters. But this is an MT model, not a swap. But I'll definitely look into it. Last I checked my ECU signal for the neutral switch the voltage stays the same weather in gear or neutral. I'll check its continuity when I have time. Wish I had some jack stands

Edit:
I think why the engine died when I first removed the AAC terminal and engine shakes too much when I turned the ECU screw all the way counter-clockwise is because the idle adjust screw was turned all the way in or almost. Now the idle adjust screw is turned enough for 750rpm (AAC disabled) when I made adjustments

[shiro_neko]

After months (or even a year) of troubleshooting I finally fixed it! Runs like a champ! It was the neutral switch after all. Never did change any parts except for the spark plugs which was due anyway. It was well worth the effort. I'll post a more detailed thread on what I did and checked and reference links for those who are having similar problems.

[locoluna825]

just started my own thread about the s*** my cars giving me shiro. i need your input. i went ahead and messed with the Tps sensor to adjust it. I got to .45v and the resistance seems alright. .94 - 7.4 ohms closed and open. Not exactly what another thread stated it should be but its close. And i tested a second tps sensor and i get the same resisntance. and now the car will constantly ildling at 1400 rpms. If i unplug the Tps sensor it stops, If i unplug the AACV it stops. but it will not go away. really frustrating. been driving with the AACV unplugged