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IACV (Idle Air Control Valve) on 2003 Outback, H4 (2.5l) engine

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I recently troubleshot some IACV issues in my 2003 2.5l Outback. I ran into some apparently defective replacements. Here are some notes about what I learned.


Symptoms included stalling at idle, hesitation when shifting, and a P0519 code, all suggestive of a "sticky" IACV. Cleaning the air passages only helped a bit. Replacement IACV are horrendously expensive - $300 from most after-market vendors - so it was worth trying a bit harder to fix the existing unit.



This model IACV consists of a small stepper motor whose rotor drives a lead-screw in or out. A plastic cone at the end of the lead-screw controls the amount of idle air allowed to bypass the closed throttle plate.


The stepper contains 2 coils, which I'll call 'A' and 'B', each of which is center tapped resulting in 6 connections: A1, A2, Atap, B1, B2 and Btap. The two center taps, 'Atap' and 'Btap', are connected to +12 and the ECU grounds one of the sides in each coil. Grounding A1 makes the current in coil A run one way while grounding A2 instead makes the current go the other way, thus reversing the direction of its magnetic field.

So there are 4 grounding combinations:

A1+B1 A1+B2 A2+B1 A2+B2

sequencing through these left to right makes the motor turn one way, reversing the sequence turns the opposite way. To move the valve the ECU steps through this sequence at 200 steps/second.


When you turn the key on to the Run position the motor takes 8 steps outwards (extending the lead-screw to reduce the idle air).


When you turn the key off the motor

first takes 176 steps outward at reduced (150/sec) speed

then takes 156 steps inward at full (200/sec) speed.


A stepper motor driven valve doesn't "know" its own position - it just stays where it was last left - so there is a need to place it into a known position. I think the shutdown logic is as follows:


1) first keep stepping outwards long enough that the valve closes completely and the motor stalls as it tries to step beyond. This ensures that the valve is completely closed in a known "home" position.

2) then step inwards to the desired idle position (mostly open). (the ECU 'learns' the valve's behavior so it might not always step inwards exactly 156 steps)



If you cycle the key on and off with the IACV removed from the throttle body, the lead-screw never hits the stop (fully closed) so it will extend 20 steps further out after each cycle. Eventually it will run out of threads and the lead-screw, plastic cone and spring will 'sproing" out - they can fly several feet, as I now know from experience...


This provides a way of removing the lead-screw for cleaning and re-lubricating. I strongly recommend enclosing the IACV in a plastic bag before trying this so you don't waste time searching for the flying parts...


It also provides a way of reinstalling the lead-screw - while a helper turns the key back to the off position gently hold the lead-screw in place until it gets drawn back in during the second half of the shutdown cycle.


Cleaning and lubing the lead-screw seems to have solved my problem. However I don't see any way to relube the rotor bearings themselves if they get sticky. If (when) that happens I'll try flushing the motor with solvent to remove the old lube, then flood it with a very light weight spindle oil; might even work...



Before reaching this solution I tried buying a surplus stock IACV being offered for $128. The part looked absolutely identical to the original - same blue sticker, same part number, etc. It seemed to solve the problem when installed, but within a week the ECU was throwing codes again.


Examination showed a critical difference. In the original IACV the shaft is not free to turn - it is keyed. The replacement is missing the key so the shaft can be turned, and unscrewed all the way out. The vendor sent several replacements, each had the same problem. I have a feeling that he had received a batch of defective units, or (more worrisome and the reason for this long note) that someone is manufacturing counterfeit units which may now be widespread.


They can appear to work. If the motor takes only a few steps then the inertia of the lead-screw / cone may keep it from turning. However, if a long sequence of steps is made, or if the lubricant is cold and stiff, the lead-screw / cone will start to turn with the motor and thus not move in or out as desired.


The attached picture shows:

far left: part number sticker, identical on old and new

middle: defective part, red arrows indicating the keyway and key which does not match and allows rotation and a crude red sketch of the missing part

on the right two views of the original piece with a proper key - which appears to be a stainless steel shell around injected plastic.




Bottom line - if you buy an IACV. check it before you leave the store. If you can turn the cone and unscrew the lead-screw you've been given a defective or counterfeit part...

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