I cannot stress highly enough that you should have a replacement gasket on hand before opening the IACV. Also, remember that the IACV is attached to coolant lines, so... it's not something you want to work on when the car is hot.
This post explains how to remove, clean and reattach the IACV (sometimes also referred to as simply the IAC). In addition to my own experience (and frustration) with idle problems, this post incorporates suggestions from Unabomber, Overdose, WRX03 and others. I am not a mechanic or WRX expert. This post is not intended to cover all potential idle problems, but is merely a summary of approaches that several NASIOC members have had success with in solving their specific idle problems. Your results may vary.
Feel free to PM me with additional suggestions, edits or corrections. Warning: I would recommend having a replacement IACV gasket on hand before opening the IACV. See below.
What Is The IACV?
The IACV is the “Idle Air Control Valve,” or more fully, the “Idle Air Control Solenoid Valve.” The IACV directly affects idle conditions, RPM in particular. On cold starts, the IACV sends rpm’s higher than on a warm start. Irregular idle conditions may be the result of a dirty IACV sensor. Limited discussion of the IACV is covered on page 36 of the DOHC Engine service manual (at least for the 2002 MY).
Where Is The IACV?
The IACV sits directly above the throttle body, and is affixed to the throttle body with two screws. The picture below shows the IACV. In this picture, I have replaced one screw (left lower corner) with a hex-socket cap screw, while the other screw (right upper corner) has been removed.
Indications and Symptoms
The IACV is exposed to the intake charge, and grime can build up from both (these are my assumptions) a. oil blow-by and b. any impurities that manage to escape the air filter. A dirty IACV appears to cause both too high idle rates and too low idle rates. In my case, RPMs dropped immediately to zero (car stalled) when stopped. I needed to brake torque at a stoplight to simply keep the engine running. My idle problem took some time to manifest, and first appeared and disappeared after one day in January 2005 (bought the car in March 2002). By April 2005 it had returned, and was dramatically worse.
As stated at the beginning of this thread, you should order and have in hand a replacement gasket for the IACV before removing the IACV, which is why I am including the “replacement gasket” discussion before
I include instructions on how to remove the IACV (see below).
The IACV gasket is fragile, but what is more frustrating is that it actually expands under vacuum/pressure and temperature. As a result, it is possible that the old gasket has expanded so much that it is impossible to get it back into position once you have removed the IACV. You cannot know the condition of the gasket before removal of the IACV, so a good safety precaution IMO would be to have a new gasket standing by just in case.
Here is the issue in properly reattaching the IACV (see below): the IACV separates coolant circulated in the throttle body from the intake charge. If you have a bad seal, you will get coolant in your intake. From postings of members who have experienced this, coolant in your intake will show up as white smoke out the exhaust. For members who have performed the throttle body bypass mod, a bad IACV seal would manifest itself as a boost leak.
The part number for the IACV gasket is 22659AA120
. It cost me $6 from Exeter Subaru in NH in person, but I could probably have gotten it for less from subaruparts.com online.
This picture shows how the old gasket expanded over the left-side of the gasket housing. It also shows where I pinched off a section trying to reattach the IACV.
This picture shows how much larger the old gasket is compared to a fresh gasket.
Removing The IACV
The IACV is attached to the throttle body by two screws that are very soft
. These are phillips-head screws with wide slots for a flat-head screwdriver. Given the softness of these screws, Unabomber has suggested removing these screws with a large flat-head and not a phillips-head screwdriver. I used a flat-head screwdriver, and managed to still take a small piece out of the top of one screw.
WRX03 has also found success with this screw removal technique: Attach a small pair of vise grips to the heads of the screws, just enough pressure to slightly indent the metal. Turn the vise grips along with turning a phillips-head screwdriver at the same time. It will break the screws loose with ease. If you're careful and your "small" vise grips have a flat section at the tip of the teeth, you can barely see the indention on the heads of the screws. Larger vise grips work but it easier to eat up the screws. The picture below demonstrates WRX03 “vice grip” technique.
-- I need to locate this picture and fix this link still --
Cleaning The IACV
NASIOC members have used several different techniques to successfully clean the IACV. I would break these into three categories:
a) Unabomber’s alcohol and Q-tip approach
b) variations on Unabomber’s technique using WD-40, carb cleaner, and throttle body cleaner in place of alcohol. Note: it has been reported that carb cleaner eats rubber gasket material; therefore, it may be less desirable of a cleaning solution for cleaning the IACV than other solvents. If the IACV gasket is damaged, the result may include a boost leak and coolant in the intake.
c) WRX03’s gasoline technique
d) Subaru write-up submitted by Jon_in_CT: http://endwrench.com/images/pdfs/ISCSpring04.pdf
I myself have only used the alcohol technique. I have no direct experience with the other approaches.
Unabomber’s rubbing alcohol and Q-tip approach
Originally Posted by Unabomber
Clean with alcohol and Q-tips; it should take you 20 or so to do the job. Reinstall. Turn the car to the ON position, wait 10 seconds, then OFF. Repeat three times, then start your car. This is the advice I got when my idle messed up this weekend. It didn't work, but when I started the car saturday morning after sitting overnight, the idle was fine. My idle problems were too high of an idle and a wandering idle with the associated P1519 code. This is what I did and what fixed it and since it's a cake fix, you might try it as well. There was a surprising amount of gook inside the IAC. Makes me glad that I switched back to the stock paper air filter over my old Perrin foam unit. Cleaner > supposed performance gain.
I followed this approach with rubbing alcohol and also required about 20 Q-tips. The catch is that you need to apply the Q-tips gently
. You do not want to scrub, but rather gently remove grime. After cleaning, I let the IACV air dry before reattaching.
Variations On A Theme
Other board members have substituted alcohol with WD-40, carb cleaner and throttle body cleaner with success.
WRX03’s Gasoline Technique
WRX03 cleaned his IACV by inverting it and filling the cavity with gasoline, and letting the gasoline sit in the cavity for a time. I would assume that Q-tips could also be used to clean out grime that the gasoline did not dissolve. Here is a picture of WRX03’s IACV after cleaning with gasoline. Note: this is much
cleaner than mine was after I cleaned it with rubbing alcohol and Q-tips.
Reattaching The IACV And How To Replace The Stock Screws With Cap Screws
As indicated above, the screws used to attach the IACV to the throttle body are VERY soft. I stripped one when I was reattaching the IACV trying, and failing, to reuse my old gasket. Thanks to WRX03, who pointed out to me that the screws are 5 mm by 20 mm long, I went to Ace hardware and bought two M5 (5mm) 20 mm long cap screws with hex sockets. You can see one of these in the lower left of the first picture. The stock screws come with both a lock washer and a larger flat washer. I was unable (not sure how they got the flat washers on the screws to begin with) to remove the flat washers from the stock screws, so I used a small lock washer with each cap screw/bolt. I omitted using a flat washer with each cap screw/bolt, which meant that they were slightly longer than the stock configuration. That did not, however, prove to be a problem. This picture compares the stock screws to the new cap screws I used:
The torque specification for the stock screws is 2.1 foot-pounds. Not having a low-torque torque wrench, I used a small hand-held allen wrench until they cap screw/bolts were snug.
Seating And Sealing The Gasket
It is important that the IACV be properly sealed. As you can see from the pictures above, an improper seal will allow coolant from the throttle body coolant lines to flow INTO your intake. Assuming you are using a fresh gasket, insert the gasket in the gasket grooves on the top of the IACV. As explained by Overdose in other posts, the gasket will look slightly too small. This is normal, and it will expand with heat, time and vacuum. Here is a shot of the new gasket ready to go:
Once the gasket was in place, I closed the IACV back onto the throttle body (see the next section below on reattaching the IACV). After the two bolts (I replaced the screws) had been torqued down, I reset the ECU (the manual says to do this; not sure if it is necessary, but figured it would not hurt). I then primed the fuel pump 3 times, for two seconds each time (I read this as part of Unabomber’s IAC cleaning instructions). I started the car, and let it idle for about 10 minutes. This allowed the gasket to slowly heat up with the throttle body.
Following good advice from Overdose, I then took the car on a 10-mile freeway drive staying off
boost, which is harder than you might think. Our cars really want to stretch their legs at all times. But I wanted the engine bay to get warm with highway-rpm’s (yes, it was flush with highway-speed-airflow, but anyway…) while the vacuum of running without boost would force the IACV gasket to expand.
After driving for 10 miles, I stopped and let the car idle for another 5 minutes. I checked the IACV for any outside coolant leaks, and it appeared fine.
I then turned off the engine. After about twenty minutes I returned, then started the car up. I drove back to the freeway, and, to see if the gasket was now sealed, applied boost. Then I applied more boost. After each application, I looked out my rear view mirror for any tell-tale clouds of white smoke that would indicate coolant had entered my air intake. No smoke. I then slowed down a bit, and did a nice healthy WOT run. All was fine, and the gasket was seated.