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5vzfe, how to diagnose high idle/tests for IAC etc.

Old 06-13-2016, 07:45 PM
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5vzfe, how to diagnose high idle/tests for IAC etc.

I have been reading a lot lately about peoples problems diagnosing their high idle problems, typically people just seem to be cleaning things and then replacing parts...

First off. A high idle cannot be caused by a vacuum leak. Everyone please stop saying to check for vacuum leaks, well, at least external vacuum leaks. This is not a MAP/speed density system. This is a engine control system based off of volumetric efficiency. On a MAF vehicle vacuum leaks will cause a low idle, not high... Not to mention fuel trims at idle would be hectic.

I have some good tests for testing for these high idle issues we so often see. It seems that most of us have disconnected our battery, pulled our EFI fuse or cleaned our throttle bodies and then had an annoying high idle issue.

There are a few good tests to check simple things first. What I like to start with is coolant temp input to the the Ecm, use a infra red temp gauge and monitor the coolant temp from full cold to fully warmed up (operating temp). If you have access use a scan-tool that has graphing capability, if not it is fine too. The temp on the scan tool should only be a few degrees different than what your temp gun reads. These ECT sensors do go bad and cause high idle, it is rare but I have seen it.

If you had this happen after a throttle body cleaning it is unprobable that it is your ECT sensor. There are some instances where the throttle body butterfly valve actually bores out the bore of the throttle body causing too much air to bypass the valve, when this happens the IAC cannot compensate for the added airflow/bypass and, in some instances, you can add a washer to the linkage side of the TB and cure the issue this way, this is only if your TB does not have linkage actuating the throttle valve itself as some have. You may also just have a linkage that is binding up or a bad dash pot, check to make sure full engine vacuum is being received at the one with the vacuum line on it, the one closest towards the firewall. if it is, also make sure the throttle stop is fully touching the screw. Try forcing it all the way closed with your hand, if it idles down spray some penetrating oil on the linkage into the shaft that goes into the TB that the throttle valve is screwed onto.

If this high idle happened after cleaning the throttle body it is more than likely some crud went into your IAC and caused it to stick slightly open towards the engine side port, allowing too much air to bypass and causing a high idle due to the internal vacuum leak that has just been created.

I have seen some people clean the IAC with success, in my case it just got worse, I ended up breaking the spring in the back I believe.

Now. How to test the IAC. This is a three-wire design. It is not a typically closed or open solenoid. With an ohm meter stick one lead onto the middle terminal, with the electrical connector unplugged, and another lead onto the top terminal. Resistance should be 21-25ohms, now take it off the top terminal and put it on the bottom, resistance should be the same. If it shows an open circuit this would indicate a coil winding that is bad...
Next (more accurate test).
The middle terminal always should have battery voltage with KOEO, this is a white/red wire. That is your first check. Now, with the engine running and fully warmed up you should have 6-7.5 volts on the brown/red wire and 8-9 volts on your black/red wire. If these check out go to the next step. Now these are average voltages for the DIY to be able to check. It would be more accurate with a scope, but, in every instance I have run into this works just fine.
If that checks out then backprobe the brown/red wire, take a jumper and ground that wire. The idle should increase/rpm should increase.
Now do the same to the black/red wire. This time the idle should decrease, if the change isn't substantial you have a sticking IAC. I would recommend replacement with an OEM Toyota part, the aftermarket ones work marginal at best. I have also had good luck with an OEM remanned unit.

One last test is to warm the engine up, shut it off. Take the intake tube off and with a piece of duct tape plug the hole for the IAC in front of the throttle plate (MAF side). Put the tube back on and run the engine.This will force the vehicle to run at base idle speed. This will give you direction on weather or not it is an electrical/IAC issue or a mechanical fault with your throttle body. If idle speed is low (600-800rpms) then you can rule out the possibility of being a bad throttle body, stuck throttle linkage or bad dash pot.

If you have access to it the best method would be to alter idle speed with a scan tool that is capable of that, monitor the change. If there is none than expect a sticking IAC.

On my personal vehicle I cleaned my TB and had a high idle. I then cleaned the IAC off my truck and it made it worse. Only remedied by replacing the IAC.

I have also seen improperly set or a bad TPS unit cause a higher idle, but I have never seen a high idle without other drive-ability issues associated with a bad TPS on these systems. I also have never had one have a high idle without setting a DTC. This is more focused on determining if it is mechanical or electrical.

Now, when you clean it or replace the IAC I like to drain all the capacitors in my ECM and do a complete system re-learn. Take your negative cable off of your battery and once it is off tie it together with the positive cable, leave it like that while you are replacing your part. This will drain all the capacitors and force a re-learn.
Start it up and let it idle until at operating temp, then drive it around like normal. It might take 5-6 cycles to re-learn. I have had two that took up to four days later before it idles properly again.

I hope this helps, I know it drove me mad when I first had my initial issues. But once I understood that this IAC isn't a conventional fully closed/fully open solenoid it made a lot more sense on how to test it. They are very finicky so be careful if you choose to clean your throttle body or IAC.

Last edited by i3oykin; 06-13-2016 at 07:52 PM.
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