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Old 06-24-2018, 07:25 AM   #1
Scruffoon
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Join Date: Jun 2018
Location: Sydney, Australia
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Question Idle Hunting - Assistance Required

Hi all,

This will be my first post on the forum, so bare with me.

I have searched more threads than I can count related to this, on here and other forums. If the questions I am about to ask are already posted and answered here, I do apologize, I could not find them.

OK! Lets begin!

I have a 1994 MK4 Supra NA-T. so consider it an NA for the purposes of this. I had a rear ender the other month, purely cosmetic damage, and this was all repaired and I picked up the car and drove it for a full 3 days without any issues.

Then the other day I was driving and as I stopped at the lights my idle was jumping between roughly 250-300 and 900 (car normally idles at 850).

Car would stall from time to time when braking and stopping at the lights due to the revs dropping so low.

I have been troubleshooting this for the past week. Initially I read that a dead battery can cause the CPU to have issues, and my battery was dead and needed to be replaced just before the accident (got rear ended on way to get new battery), so the car was sitting in the workshop with a dead battery for about a month.

I will list the things I have tried and lets see how we go...

- Changed spark plugs (needed changing anyway)

- Changed spark leads (NA has the distributor and leads)

- Found a bunch of leaks from my intercooler joiners (realigned everything and got rid of the leaks)

- Checked for vacuum leaks (couldn't find any additional ones)

- Reset the ECU

- Checked timing belt marks (no tooth has been missed, timing is in correct position)

- Adjusted TPS (changed the idle higher and lower but was still hunting from very low to around the 1000 mark)

- Cleaned throttle body

Lastly, and my suspected culprit. I noticed there was little to no difference in idle when I disconnected the IAC while the car idled.

After more hours than I would like to admit getting the IAC off due to the screws being super soft and just rounding, I finally got it off, cleaned it as best I could, and cleaned the throttle body.

The IAC gasket looks to be in good shape.

I have tested the voltage of the IAC, which turned out fine, and the resistance of the connector which came up at 22.5 ohms across all pins (not sure what standard is meant to be, couldn't find it in TSRM)

I read that on all cars the IAC is meant to push out and in with the ignition switch on and off. So I tested this while I had it plugged in but not bolted into the throttle body, and it did not move in or out.

I then created a vacuum leak and started the car, and the IAC did move out and back in when the car was turned off. I am not sure if this indicates that it is working correctly since it moved, or if it is still not working correctly as it did nothing with the ignition on and off...

This is pretty much where I am at, and would like some ideas or opinions on whether to replace this IAC, or test some other things on the car first. This is my first time ever looking at an IAC and I want to make 100% certain it needs replacing and is the likely the cause before spending more money.

Currently as it stands, the car seems to be idling a little better, possibly due to one of the other things I did, but it still idles rough. Dipping between 600 and 1000, before stabilizing around 950. If I blip the throttle the dip gets worse and then it seems that the ECU tries its best to stabilize it again around 950, constantly moving up of down by 100rpm or so. The other day the car would start and stall almost immediately when the IAC was plugged in, and then start normally when I unplugged it, so I am not sure if its working intermittently or that was a one-off as it hasn't done that again since.

Thanks in advance for any help, and sorry about the long post, just trying to give as much info as possible as I have run out of ideas.

Attaching pictured of my IAC.

Part No. 22270-46050

https://imgur.com/vNDCQcM

https://imgur.com/rOk2s6c

https://imgur.com/8WSLbJS
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