SOLVED: High idle, increases with coolant temp - IAC?

[Spiker Engineering]

UPDATE - It was indeed a bad IAC... Back to original message:

As the title says, I'm dealing with a very high idle that increases with higher coolant temp. It idles at 1000 RPM at startup, then increases to 1200 at 97F, 1300 at 113F, 1400 at 131F, and up to 1700 at 190F (I'm attaching the torque pro data below).

Is this a sign of a bad IAC? I tested the IAC coil resistance (passed), and pinched the air assist hose (idle went down as expected). My next step is to remove the IAC and see if the valve actually moves when powered in open and closed directions.

Could something else be causing this behavior?

 

[spartacus]

Disconnect the coolant temp sensor in the front if the idle drops then the CTS is bad.

 

[brillo_76]

Disconnect the coolant temp sensor in the front if the idle drops then the CTS is bad.

Great thought as they are separate one for the gauge.

I am suspecting is throttle control is bad as he just had this all apart and zinc plated. Being the throttle body was too. I am guessing it has to do with that project. Maybe a vacuum leak or throttle body gadget on backwards.etc.



Possible IAC damaged itself...

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[Spiker Engineering]

Disconnect the coolant temp sensor in the front if the idle drops then the CTS is bad.

I'll try disconnecting the CTS, but my Torque Pro is giving me good data for coolant temp, isn't that the same data the ECM is using for IAC control?

I am suspecting is throttle control is bad as he just had this all apart and zinc plated. Being the throttle body was too. I am guessing it has to do with that project. Maybe a vacuum leak or throttle body gadget on backwards.etc.


Possible IAC damaged itself...

Indeed, having so many things apart all at once (and so many parts replaced) makes diagnostics more difficult. And yes, since it ran fine before the project, and now doesn't, then it has something to do with the project - the question is what?

I did some more tests with the IAC, it moves freely when commanded to either the open or the closed position. Regardless, I have a new IAC on its way to me, so I'll know in a couple of days if that's the culprit.

If that doesn't fix it, I'm not sure where to go next. I can try to get a scope onto the IAC connector and see how the ECM is actually commanding the IAC.

 

[hugho]

As the title says, I'm dealing with a very high idle that increases with higher coolant temp. It idles at 1000 RPM at startup, then increases to 1200 at 97F, 1300 at 113F, 1400 at 131F, and up to 1700 at 190F (I'm attaching the torque pro output below).

Is this a sign of a bad IAC? I tested the IAC coil resistance (passed), and pinched the air assist hose (idle went down as expected). My next step is to remove the IAC and see if the valve actually moves when powered in open and closed directions.

Could something else be causing this behavior?

you are doing everything right. let us know if you fixed it or had to spring for a new unit

 

[brillo_76]

I would focus on the parts you messed with? Being it's idle. It has to be the IAC or the throttle body?

You didn't put the throttle body gasket on backwards?

I clean my IAC with bracket cleaner. It went crazy for like 3 or 4 days then went back to normal.

I mostly idle around 550 rpm...:]



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[hugho]

you are doing everything right. let us know if you fixed it or had to spring for a new unit

 

[Spiker Engineering]

I would focus on the parts you messed with? Being it's idle. It has to be the IAC or the throttle body?

You didn't put the throttle body gasket on backwards?

I clean my IAC with bracket cleaner. It went crazy for like 3 or 4 days then went back to normal.

I mostly idle around 550 rpm...:]

I messed with everything, as you know .

I'm pretty sure everything is assembled correctly. I'm just trying to be logical here - the fact that the idle is perfect at startup but then goes haywire at higher temp suggests to me that it's not a vacuum leak - that would be bad idle even at startup - no?

For idle to get higher at higher temp, I think it has to be a matter of too much air getting past the throttle, which suggests the IAC circuit. So that's either the IAC valve itself, the ECM which controls it (or the inputs it's receiving from other sources), or wires.

I find it hard to believe the ECM crapped out just at this exact time, so I'm thinking the IAC valve, the wires (perhaps one got damaged during the project), or some other sensor.

Does that logic make sense?

 

[Malcolm99]

Could an intake leak not become bigger as everything expands, did you replace throttle shaft seals, and insure the throttle plate was installed in the same position and closes properly. And it does appear your CTS is working as that is what the ECU uses.

 

[HiLife]

Would it be possible to back probe the iac?
That way you can monitor the voltage if the ecm is feeding more and more volts to the iac as the engine warms up.
I'm just thinking out loud...

 

[brillo_76]

I really doubt it's the ecm. I am suspecting the throttle body or the iac valve.
Leaning heavily with a damaged IAC valve or whatever controls that not getting the proper readings.

All your body grounds good near the intake? As a weak bad ground can do funky things as well..

Does the IAC pass its diagnostic testing?

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[Spiker Engineering]

Could an intake leak not become bigger as everything expands, did you replace throttle shaft seals, and insure the throttle plate was installed in the same position and closes properly. And it does appear your CTS is working as that is what the ECU uses.

I didn't disassemble the throttle shaft, so the seals and installation are unchanged. Everything with the linkage appears to be exactly as before disassembly. The linkage and throttle shaft operation is smooth, so I don't think that's the cause.

Would it be possible to back probe the iac?
That way you can monitor the voltage if the ecm is feeding more and more volts to the iac as the engine warms up.
I'm just thinking out loud...

Yup, good suggestion, that's on my to-do list after the new IAC goes in. The problem is that it's not a DC signal, it's a square wave, so I'll need to use some kind of a scope to read it.

I really doubt it's the ecm. I am suspecting the throttle body or the iac valve.
Leaning heavily with a damaged IAC valve or whatever controls that not getting the proper readings.

All your body grounds good near the intake? As a weak bad ground can do funky things as well..

I agree, I doubt it's the ECM. The grounds look good, I can double check them of course.

I really appreciate all the thoughts, this is really helpful in working through this logically. For now I'll just wait for the new IAC valve, should be here by Wed. Then we'll go to Plan B (or hopefully won't have to!).

 

[spartacus]

I'll try disconnecting the CTS, but my Torque Pro is giving me good data for coolant temp, isn't that the same data the ECM is using for IAC .

You are correct. I suggested it as one of the possibilties. A while back on a corolla trouble shooting à similiar problem. It ended up being the cts.

 

[Spiker Engineering]

You are correct. I suggested it as one of the possibilties. A while back on a corolla trouble shooting à similiar problem. It ended up being the cts.

No worries, I appreciate all suggestions! I didn't mean to dismiss your suggestion, I was just thinking out loud when I said that I didn't think the CTS is the cause :thumb:.

 

[phattyduck]

For idle to get higher at higher temp, I think it has to be a matter of too much air getting past the throttle, which suggests the IAC circuit. So that's either the IAC valve itself, the ECM which controls it (or the inputs it's receiving from other sources), or wires.

I find it hard to believe the ECM crapped out just at this exact time, so I'm thinking the IAC valve, the wires (perhaps one got damaged during the project), or some other sensor.

Does that logic make sense?

It almost looks like the positive and negative drive wires for the IAC are backwards.

I bet right around 1k rpm is the 'default' idle if the IAC is unpowered.

-Charlie

 

[brillo_76]

It almost looks like the positive and negative drive wires for the IAC are backwards.



I bet right around 1k rpm is the 'default' idle if the IAC is unpowered.



-Charlie

Is that even possible to do?

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[Spiker Engineering]

Is that even possible to do?

Probably not without repinning or flipping wires, which I didn't do.

But Charlie got me thinking, and so I ran an experiment, which was quite interesting.

Recall the original data of RPM vs coolant temp, with the IAC connected:

So today after thinking about Charlie's post, I disconnected the IAC, and fired up the truck. Things looked pretty familiar until about 131F, when the engine speed dropped like a rock to under 1000, and stayed there until full warm up:

That makes no sense to me. I thought maybe the IAC fixed itself, but as soon as I reconnected the IAC, the speed jumped back up to 1540.

I did score a code in the process, P0505, but that's an IAC code that is probably not surprising given I ran with it disconnected.

So what does this tell me?

 

[brillo_76]

Your IAC is bad and is in need of replacement.....[ that's my guess anyhow. It's probably sending bad data to the ecm ]

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[Spiker Engineering]

Your IAC is bad and is in need of replacement.....[ that's my guess anyhow. It's probably sending bad data to the ecm ]

I hope you are right, and my new IAC is coming in a couple of days.

I don't think the IAC sends any data to the ECM, it's just a dumb actuator, which the ECM commands to open or close based on other data it's collecting. But I hope that my IAC is just not following these commands, and a new one will fix everything. Fingers crossed.

 

[brillo_76]

Yes. I am hoping that resolves it. Was the AIC plated ?

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