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Discussion Starter #1
I've been having intermittent stalling issues, irregular idle and loud humming noise coming from the air intake area.

Been to a bunch of mechanics and replaced a bunch of parts. Check engine light has only recently started coming on. The lastest mechanic my car went to said two codes had been logged being the IAC and the Air Flow Meter. Have just replaced the IAC and the AFM. Problem still exists...

That mechanic also played with the throttle position sensor. Interesting thing is when adjusting the new IAC in position at certain points the humming noise disappears completely. I somehow feel that the problem lies with this noise and the IAC and the throttle position sensor.

Does anyone know whats causing my propblems or the humming noise that disappears when the IAC is adjusted in different positions? With the new IAC position does the TPS have to be adjusted too?

Or can someone explain how the IAC and TPS works?
 

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2001 VDC/SC One of a Kind
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I've been having intermittent stalling issues, irregular idle and loud humming noise coming from the air intake area.

Been to a bunch of mechanics and replaced a bunch of parts. Check engine light has only recently started coming on. The lastest mechanic my car went to said two codes had been logged being the IAC and the Air Flow Meter. Have just replaced the IAC and the AFM. Problem still exists...

That mechanic also played with the throttle position sensor. Interesting thing is when adjusting the new IAC in position at certain points the humming noise disappears completely. I somehow feel that the problem lies with this noise and the IAC and the throttle position sensor.

Does anyone know whats causing my propblems or the humming noise that disappears when the IAC is adjusted in different positions? With the new IAC position does the TPS have to be adjusted too?

Or can someone explain how the IAC and TPS works?
First off, in this case I found your car in another post, but in the future post year, make and model. Or just add it to your user info so its always there.

Vacuum leak or there is a gap between the MAF and the intake manifold, which would also be a vacuum leak. Hose issue between IACV and MAF or Throttle.

The ECM gathers information from the MAF, Crank sensor, Trans position switch and TPS to determine the position needed for proper idle. It is also controlled by direct contact of the base of the IACV with the coolant.

When a leak occurs after the MAF, the computer interprets there may be a problem with the MAF because what it receives from it doesn't add up to what the BARO/MAP is reporting = MAF code.

The IACV was NOT causing a problem. If it was 506 or 507, that just means that the engine would not idle where it should given what the ECM is trying to accomplish.

On how the two work:

The TPS reports the position of the throttle plate in degrees to the ECM so it knows when you push the accelerator pedal and how much. It uses this information to add fuel for expected air flow, alter timing and it also sends a signal to the TCM on automatics to determine shift points and TCC engagement (torque converter clutch).

The IACV, idle air control valve, controls air entering the intake to allow air to bypass the throttle plate when it is closed. These IACVs on your Gen do not fail. It is controlled in two ways. One is by altered current from the ECM and the other is by direct contact with the engine coolant.

There is no adjusting the IACV.

From your other post:My thoughts are still thats it's to do with a vacuum leak. When I squeeze the hose that goes to the IACV the noise goes quieter so that indicates it air related doesn't it?

Does anyone know a good way to check for vacuum leaks?


A spray can of Brake Cleaner or other accelerant sprayed around hoses and their connections will change the idle when you hit the leak or you "smoke" it with a smoke machine. (A smoke machine runs about $900 so you will have to find a shop that does emission repairs for this)

I would check every hose and connection from the throttle to the air filter, and throttle to the IACV. You will find a crack, hole or loose connection.

And out of curiosity, how much did he charge you for the IACV? If you don't mind my asking. Its not a cheap part due to the way it is manufactured.

Find a new mechanic. The one you are using is guessing and throwing parts on the car without actually diagnosing the issue and needs more practice. And to prove it, ask him point blank how the idle control valve is controlled on your car.
 

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Discussion Starter #4
Thanks for explaining how they work.

I have checked multiple times everywhere for vacuum leaks and so too have a copule of mechanics I've sent my car too. I'm not sending my car to the same mecahnic each time. Its been to a couple of different ones who have all been good and have not charged me for parts they replace which do not solve the problem.

After one of the mechanics said the IACV code had been logged I bought a second hand one for 25 bucks.

Question..If the IAC valve is not adjustable what are the 2 mounting screws that are secured through an elongated slot at the top of the IACV? When I played around with these screws which adjusted the idle speed up or down, at some points along the elongated slot the noise disappeared completely.
 

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Discussion Starter #5
Just read this on a website......a failing TPS can result in random engine stalling at idle, but this can also happen under driving conditions. Most often this will occur under hard braking, since the computer assumes normal operation and continues to deliver cruise-amounts of fuel when the throttle butterfly is shut.

Could a failing TPS be the cause of my intermittent stalling? Has anyone had a similar problem with a failing TPS
 

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Discussion Starter #6
I have ended taking my car to the Subaru dealer who said that a TPS code had been logged. CEL only just started to come on within the last week. The dealership called me yesterday afternoon and said that to be certain that it is the TPS that's causing my intermittent stalling issues they would need to do a two hour test on it. Does anyone know what this two hour test involves?

Has anyone replaced a TPS before on their car? Is it easy to do? I'm just wondering if I could replace it myself but not sure if you can i.e. how do you know the right adjustment with it?

The dealership also said that to replace it they would order a genuine one costing $830 but I looked on cyberspace auto parts and they have one for $200 so I'm just wondering to get one off there and install it myself.
 

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2003 OB 2.5L AT
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I am not a mechanic by trade but I also thinker with my car. Since you are willing to replace the TPS yourself, why don't you try cleaning the throttle body well. Then..

Scribe positioning marks to TPS sensor body, so you have a baseline location where to put it back. Remove and clean inside with throttle body cleaner (I have not touched one in a very long time so I cannot tell you exactly how it looks), then blow compressed air inside where you can. Put it back following the marks you made and see how your car idles. As Cardoc have explained, TPS sensor translates (Basically Transducer)mechanical movements into electrical signal. If you have contact cleaner for electronics like the one used for carbon potentiometers, that could also work.

Do the same with the IACV. If you have the old one, clean it using MAF cleaner, put it back and see how your car runs. You bought a used one, clean it also if you end up using it.

You should also check the return springs on the shaft where the accelerator cables end. That shaft is connected to the throttle plate. Those big, about 1 inch diameter springs could get dirty and bind, giving you false TPS errors. Clean them.

Right now, you have little to loose since you could be spending money to buy parts. Try some clean up maintenance and see if that helps. I don't know the full story of your car issues, I am just suggesting what I might do.
 

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Discussion Starter #8
Thanks whiteOutback.

Throttle body has been cleaned out by a mate whose a mechanic. And the IACV also got a clean when I installed the new second hand one.

Good idea with cleaning the TPS. Only problem is that my car is at the Subaru dealers at the moment. Went in yesterday, they called me yesterday afternoon to get the ok on letting them do the two hour testing on it. Its a public holiday here today in Australia so everything's shut and they will probably start work on it after today. Might have to intercept my car before they start the two hour testing on it.

I just would like to know what testing would be done on it. From what I've read you can test them using a volt meter which would not take 2 hours to do
 

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Sorry, I don't recall doing any test on the TPS on my 96 OB. I did clean it as I suggested when I got a CEL indicating TPS error. Only time I did it. I also cleaned the springs I was talking about. Dealer servicing my car sprayed white lithium grease on it, that grease will eventually harden and cake up. Winter is just around the corner of your world mate. Visited Melbourne for a month a very long time ago.
 

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2001 VDC/SC One of a Kind
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The TPS can be checked in about 5 minutes with a DVOM or similar meter. Either using voltage measures or ohms. 2 hours??? Sometimes I wonder where people come up with justifications?
 
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