Yet another fast idle problem - MR2 Owners Club Message Board
 
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post #1 of 16 (permalink) Old January 9th, 2017, 15:32 Thread Starter
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Unhappy Yet another fast idle problem

(91 SW21 2.2L N/A MT)

This is driving me nuts. I have the infamous fast idle with surging. All the standard fixes haven't addressed the problem. Here's what's been done so far:

*The coolant system has been repeatedly burped. There is no air in the system. (This was my first guess.)

* I removed the IACV and it was dirty, but it has been thoroughly cleaned and moves freely. The actuator coils check out with the ohmmeter.

* I unplugged the water temp sensor and used a fixed resistor to tell the ECU that the engine was fully warm. No change. (This also confirms that air bubbles in the cooling system are not at fault.)

* The TPS had a dirty contact on the idle switch, but it was cleaned and checks out just fine.

I can confirm that there are no vacuum leaks. A finger on the IACV port inside the TB reduces the idle to a steady normal value (850 RPM). Unplugging the IACV and apply voltage to the ISCC pins causes the valve to close and the idle drops to normal. So it seems the ECU is wanting to open the IACV for some reason. (????)

I'm stumped at this point.

FYI -- I know what makes the surge at idle in case you were wondering: What's going on is that the TPS idle switch is closed telling the ECU that the throttle plate is closed. However, the ECU sees that the RPM is too high, so what it's doing is abruptly cutting fuel to the injectors. The RPM falls and the fuel is turned on again. The RPM increases. Wash. Rinse. Repeat. This is part of the fuel cutoff system. You can duplicate the surging by unplugging the TPS and shorting the idle switch. Slowly raise the RPM to around 1700. The surging will start.
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post #2 of 16 (permalink) Old January 9th, 2017, 15:43
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When checking the IACV did you check and adjust the TPS according to BGB? Can do this with t on bench or installed to make sure the throttle position clearances are correct according to TPS resistance. Best to also make sure internal butterfly is not sticky as often a lip of debris is formed in TB which prevents it from closing.

Another little tip is to check the idle with diagnostic port jumpered (believe E1) as this should set idle to 750rpm for adjusting timing at 10deg. If this does not work then its a mechanical rather than electrical issue.

jim
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post #3 of 16 (permalink) Old January 9th, 2017, 17:45
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Quote:
Originally Posted by Ryoko View Post
... Unplugging the IACV and apply voltage to the ISCC pins causes the valve to close and the idle drops to normal. So it seems the ECU is wanting to open the IACV for some reason....
Maybe the valve driver in the ECU is burned out, or the connection is broken. You might be able to tell by observing DC voltage at the terminals, or you might need to observe the waveform to tell for sure what is going on.
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post #4 of 16 (permalink) Old January 9th, 2017, 19:33 Thread Starter
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I would love to look at the waveforms going to the IACV, but it's hard gaining access to the pins. I suppose I could nick the wires going to it and clip the scope there to see what's going on. I'd be curious to know if the ECU is trying to bring the idle down.

BTW, I did jumper the diagnostic pins and the idle did not go down all the way. You could here it cycling somewhat. The check engine light blinked the 'all is normal' code.

I do have one question or concern -- I pried the black cap off the idle adjustment screw. It was totally bottomed out. I then put my finger over the IACV port and the idle dropped to about 900 RPM. I've had the TPS out for cleaning and there's nothing hanging up the butterfly. It closes all the way. My question is should the engine be able to idle with the idle adjust fully bottomed and the IACV port blocked? (I already pinching close all the vacuum lines to the manifold include the A/C idle up. None of them made any difference.) I'm wondering if there's another idle bypass I'm not seeing. I'm going to pull the TPS again tomorrow for a closer look.
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post #5 of 16 (permalink) Old January 9th, 2017, 19:59
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Originally Posted by Ryoko View Post
it's hard gaining access to the pins. I suppose I could nick the wires going to it and clip the scope there to see what's going on...
No, no, no! Get needle probes for your instruments. They will penetrate the insulation without compromising it, and they will give you more reliable connections with dirty or corroded conductors also.
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post #6 of 16 (permalink) Old January 9th, 2017, 20:24
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From your description it would seem the IAV is not controlling the air supply as its wide open. This can be because of an ECU fault, wiring or simply adjustment. I seem to recall a lever on the side which can alter spring tension. Think I set mine to full on so it would remain closed under boost. Might give that a try as I didn't see anything on the valve adjust in BGB.

Jim
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post #7 of 16 (permalink) Old January 9th, 2017, 21:37 Thread Starter
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There is an end plate on the IACV which adjusts the spring tension against the valve. I believe the spring is actually a thermostatic spring.
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post #8 of 16 (permalink) Old January 10th, 2017, 17:08 Thread Starter
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I rechecked the IACV. It's working perfectly. I can find no vacuum leaks. The TB butterfly is a bit loose in its fit, but not terribly. I unplugged the connector to the IACV and put a volt meter on it. The close pin had 6 volts on it and the open pin had 9V on it. So it seems the ECU is wanting to open the IACV. The idle has slowed to 1300 RPM. The car has been in storage with a dead battery for a number of years. I'm wondering if the ECU is having to relearn all its settings.
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post #9 of 16 (permalink) Old January 10th, 2017, 19:16
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If you unplug IACV then the idle should drop to around 750rpm as the valve will shut (default mode). If this does not occur then it is either stuck or needs adjustment as mentioned above.

ECU doesn't 'learn' IACV operation. The only thing ECU will learn and adjust is the VE tables which controls fueling.

jim
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post #10 of 16 (permalink) Old January 10th, 2017, 20:19
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I had a similar problem. What I found was the power wire to the IACV was broken so the ECU couldn't ground the valve making it not work. Mine is a JDM 3SGE bit it's identical. What's your idle like from cold?

Testes to do are, check continuity from
ECU plug D/E (A/T - M/T) pin 10 ISCO red wire to IACV pin 1 red wire.
ECU plug D/E (A/T - M/T) pin 9 ISCC green wire to IACV pin 3 green wire.
Diagnostics plug pin B+ to IACV pin 2 black with yellow wire.

If one of these is broken then the valve won't work quite right. The ECU might have issues with the ISCO and ISCC pins but not likely as I haven't heard of these being a problem.
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post #11 of 16 (permalink) Old January 10th, 2017, 20:24
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Quote:
Originally Posted by benckj View Post
If you unplug IACV then the idle should drop to around 750rpm as the valve will shut (default mode). If this does not occur then it is either stuck or needs adjustment as mentioned above.

ECU doesn't 'learn' IACV operation. The only thing ECU will learn and adjust is the VE tables which controls fueling.

jim
Well not really, the IACV is magnetically controlled and when no power is being applied to it the valve is half open. There are 14 inputs the ECU uses to control idle so can be very hard to find the issue. It has been narrowed down with these tests on other items. I had the same issue when unplugging the ECU temperature sensor there was no change to the idle. I have unplugged the temperature sensor on a car that was working fine and the idle changed and the ECU threw a code.
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post #12 of 16 (permalink) Old January 10th, 2017, 21:07
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Well not really, the IACV is magnetically controlled and when no power is being applied to it the valve is half open. ....
Hmmmm...maybe I adjusted mine to fully closed as I didn't want an open line to atmosphere when building boost. Seem to recall testing it with power disconnected and could not blow air through.
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post #13 of 16 (permalink) Old January 10th, 2017, 22:02 Thread Starter
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I can confirm that the IACV moves to the middle position when unplugged. It's got a magnet that pulls it towards the center of the solenoid coils when power is off.

There is a procedure outlined in the BGB-Engine that describes resetting the IACV idle. (page FI-18, Symptom - High Engine Speed, 1st paragraph) It's not very clear what it's doing though.

The cold idle speed is 2000 RPM.
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post #14 of 16 (permalink) Old January 13th, 2017, 11:58
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Have you considered a MAP sensor? I know you mention no vacuums leaks, but similar symptoms have been checked by simply unplugging the MAP sensor. While its on and surging, pulling the plug on the MAP sensor could steady it out, and Id consider the MAP at that point. My 2c.
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post #15 of 16 (permalink) Old January 13th, 2017, 14:06 Thread Starter
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I don't think it will run very well without a MAP sensor since the ECU won't be able to determine the correct fuel mixture. (It could guess using the O2 sensor and engine speed alone, but it wouldn't run very well.) The surging happens when the TPS tells the ECU that the throttle plate is at idle, but the RPM is still high. I believe this is done as a safety to prevent an engine run-away should a major vacuum leak occur.
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post #16 of 16 (permalink) Old January 13th, 2017, 14:23
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The idea is to completely shut off the fuel injectors when you are coasting down with throttle closed on the highway. This saves fuel and prevents explosions in the exhaust. Below about 1500 RPM, the ECU assumes that you are idling rather than coasting, and turns the fuel injectors back on.
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