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Discussion Starter · #1 ·
I have a 2000 OBW, EJ251 engine, Auto Trans that is experiencing what appears to be the fairly common "rough idle when in gear" problem. The engine never stalls and otherwise runs great. I feel fairly certain that the idle air control valve is causing the problem. I have removed it and cleaned it very well with throttle body cleaner. It almost looked new when I was done. I also cleaned the air passages in the vicinity of where the valve mounts to the throttle body. Everything seems very clean, but the rough idle problem persists.

All other components that I think might be related to rough idle should be in good shape (new plugs, new plug wires, new air filter, new PCV valve, solid electricals/grounds).

My next guess is to replace the valve, but a new one is $300.00 (yikes). I can't really justify spending that much on the problem or a car with 200K miles. I have checked with several local junkyards and no one has a used valve.

So my question to those with much more parts experience (grossgary ?) is, how often do these idle air control valves actually go bad ?
 

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Discussion Starter · #5 ·
The idle appears to be within specs when in gear or park/neutral. It never seems to want to stall, it just gets really rough sometimes with a fair amount of vibration. It comes and goes. Sometimes I can come to a stop and the idle will be smooth, but most of the time, about 3-5 seconds after coming to a complete stop, it will start with the vibration/rough idle.

I removed the IACV a third time and cleaned it really well, but it did not improve the symptoms. I doubted that these actually fail very often since I have rarely heard anyone replace it. A good cleaning seems to be enough.

I guess I am just really skeptical that aftermarket axles would cause this type of vibration/rough idle. But, I am starting to think that may be the issue. I had the drivers side axle replaced back in Sept due to a torn boot. I am fairly certain the prior axle was not a genuine Subaru and I never had a problem with it. I think I may have started to notice the idle problem after that. I figured it was just the car getting old. But after that new axle I noticed a knocking noise when turning and accelerating such as when pullling into traffic from an intersection. I called the shop and complained and they installed another axle. The knocking seemed to be better, but still seemed to be present, but less so. Then a few weeks later, I decided to have the drivers side axle replaced as well before winter set in. The rough idle has remained and maybe even got worse after that. I let it go up to now since it really didn't seem to be hurting anything, it's more of an annoyance.

But now, the knocking noise when accelerating while turning seems to be getting worse. Something is not right. I am taking it into the repair shop tomorrow morning to go for a test drive with the tech.
 

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Discussion Starter · #7 ·
Yes, the idle does seem to smooth out if I put it in Neutral or Park. Strangely, it doesn't really seem to be rough when in reverse.
 

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Discussion Starter · #9 ·
Yes, that's the impression I got from searching for this problem in the newsgroups. I didn't want to believe the axles were the possible problem, so I was focusing in the IACV.

I will report back after I take the car to the repair shop tomorrow.
 

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Discussion Starter · #11 ·
Nearly all of the normal tune-up related items you mentioned have been recently changed including new plugs and plug wires (genuine Subaru wires and NGK plugs). Vacuume hoses have been checked. New PCV valve. New air filter. Engine runs great, excellent gas mileage and plenty of power.

It's at the shop today and they said they would replace the drivers side front axle, which is the one that seems to make the knocking/clicking noise when turning left and accelerating. I went for a test drive with the tech this morning when I dropped it off and he heard the knocking noise. I also mentioned the rough idle/vibration at idle problem as well as the possibility of it being related to the axle and he didn't necessarily shoot that idea down.

I think the problem in this case can be more accurately described as, "a pronounced vibration at idle" not really a problem with the engine idle itself. The engine actually idles just fine and at the recommended rpm's. It just seems that more of the normal engine vibration that is typically present at idle is getting transmitted to other parts of the car, presumably by the aftermarket axles. It would seem that it could be the motor mounts, but if that were the case I would expect other noises during shifting or other things that torque the motor around in the mounts.

We'll see how things are when I pick it up tonight.
 

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Discussion Starter · #15 ·
Well, the repair shop installed a new aftermarket axle on the drivers side and as skeptical as I was that it would fix the idle vibration problem... it did correct the problem. Idle is now quiet when in gear :)

I really didn't think it was related to the axle, but apparently in my case, it was.

I'm glad it's fixed.
 

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Discussion Starter · #18 ·
Based on the searching I've done on these newsgroups, I think the general theory is that certain aftermarket axles are either too stiff or don't have enough freeplay, or maybe they have TOO much freeplay, and they therefore transmit too much of the normal engine vibration. To the driver, this SEEMS like rough idle, when in fact the idle is normal. My engine idle is at the proper speed and is very smooth. Mine did improve after I had the shop install a different axle, but I still do get the occasional idle vibration. So, in hindsight, I would call it an idle vibration problem more than a rough idle problem.
 
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