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Old 05-11-2009, 06:55 PM   #1
rogue
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Arrow Rough idle when warm, 01 RS, yes I searched ;)

Many good threads with much good information.

I just picked up this 2001 2.5RS with 170k on the clock. It needed headgaskets, clutch, timing belt. HG was leaking oil out the bottom, no oil in coolant or vice versa, no overheating.

Did all the maintenance work this weekend.

Car had a "rough idle" before, sometimes. Having completed the work, it seemed worse.

On a cold start it idles well, and works its way up to warm. It likes to idle at about 650 RPM according to the OBDII scanner, MAP showing around 30 kpa.

I *was* having a bad problem with partial-throttle acceleration and acceleration below 3000 RPM today, BUT diagnosed that as a stuck PCV thanks to this forum. That's fixed and the partial throttle is now GOOD.

Still: when it gets properly warmed up, the idle starts to hunt fairly rapidly. Up and down on about a two-second cycle.

I cleaned the IACV (it was gunked up) with carb cleaner, then WD-40'd it. It appears to be working, since I can see the MAP pressure changing on the OBDII scanner while idling. Also adjusted the throttle cables and the TPS to make sure there's a little slack and it's reading 0 at throttle closed. I also reset the ECU and let it warm up until it cycled the fan after hooking the battery back up.

Can not find ANY vacuum leaks (obviously). All hoses hooked up. This is not my first rodeo, I currently own 4 Subarus and a Miata, I've done major work before, this is a bit of a puzzler.

IACV toasty?

Missed a leak somewhere?

Squirrels in my valvetrain?

Thanks for reading and contributing

-Ted
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Old 05-11-2009, 07:08 PM   #2
AcquaCow
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and a 2011 WRX hatch...

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Was the front o2 sensor on my RS...

I even replaced the IACV completely thinking that it was the root cause, but Subaru had a bad batch of fron o2 sensors (I bought two of them)... my 3rd replacement, idle was back to normal with no hunting.

Front o2 is a bitch to get at, but you can replace it w/o dropping the exhaust...

-- Dave
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Old 05-11-2009, 07:17 PM   #3
rogue
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I may try that.
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Old 05-11-2009, 07:21 PM   #4
DiscoGsus
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Old 05-11-2009, 09:52 PM   #5
rogue
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Since the O2 sensors in both WRXes went out and caused problems, I wouldn't be surprised to find that was the problem. I will try replacing the front O2.

Rear O2 is just for emissions, I'm not concerned about that yet.
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Old 05-12-2009, 04:15 PM   #6
BrysImpreza
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Yeah, my 04 is doing the same thing, finally got both a, IACV CE and Front O2 sensor CE. Swapped out the IACV, nothing, changed the o-ring, nothing.

Next going to try the front O2 sensor, hopefully this will cure what ails us.
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Old 05-13-2009, 05:39 PM   #7
rogue
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Front O2 seems to have done the trick. Idle was still hunting a bit after an ECU reset, but I think it figured out the fuel trims.

Ran it up to hot then let it idle back, messed with the throttle a bit, seems like a winner!

Also: the PCV valve. Replace it. It's $3 at Advance Auto Parts and was responsible for bad partial-throttle problems on mine.
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