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Old 09-15-2013, 06:49 PM   #26
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Member#: 364002
Join Date: Aug 2013
2016 wrx


you need a valve adjustment, valves are probably too tight
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Old 10-07-2013, 11:07 AM   #27
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Member#: 367321
Join Date: Sep 2013
2002 Wrx


Is it best to go to a Subaru mechanic to get your throttle body plate cleaned? Or is it simple enough to do it on your own.
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Old 10-07-2013, 02:35 PM   #28
Back Road Runner
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Member#: 60082
Join Date: Apr 2004
Chapter/Region: MWSOC
Location: Minnesota
2004 Forester STI


There is a specific need for the stock air box on these cars. The plenum size on the intake manifold is small. This is done typically to help throttle response. However, you end up getting stronger pulses traveling up through the intake and interfering with the MAF. Even on MAP based cars, the AFR gets pretty messy when the air box is removed. MAP based cars are less sensitive though. The air box is a secondary resonator to damp out the remaining pressure waves and promote smoother flow through the preceding intake tract, including where the MAF is located. Smoother flow means better readings means smoother operation. Now you don't need the stock airbox, but you do need a secondary resonator inline somewhere between the throttle body and MAF. I have not seen ANY aftermarket companies build a short ram or cold air intake with a built in resonator for this purpose, and it's quite a shame. I've seen aftermarket intakes for other brands that have but not Subaru's aftermarket manufacturers. It's required because of the tiny plenum, and everyone seems to ignore the fact.

Also, short intakes pull hot air from the engine bay. Hot air means less power. In fact, you can see a difference on the order of 5-10 ft-lbs lost across the entire rev range from pulling in high temp air. Bye bye power. Pull in the coldest air you can. Stock isn't bad, and you can get slight improvements with a good cold air intake pulling from outside the engine bay. Because stock isn't that bad, you really only see about 5 ft-lbs tops for gains. I should also note that the stock intake has ZERO restriction on the 2.5L NA. I've tested this on the MAP based EJ25. With the MAP, I have the luxury of pulling off all the intake and test right from the throttle body with zero restriction. I've done this and found absolutely zero HP gains, even at redline. I will repeat that, zero as in 0.00 ft-lb/hp gains at redline by getting rid of the stock intake. I did see an improvement on throttle response, but I saw most of the improvement of throttle response from the exhaust rather than the intake. Most of what you're doing with an aftermarket intake is attempting to pull in colder air. That's where the gains come from, nothing else.

Back on topic, there are two main parts that moderately influence engine operation. They are the MAF and the air/fuel sensor. Both of these heavily affect engine operation when they fail. They affect operation differently. The MAF will typically make car starting harder and will often prevent any form of high rpm use. Basically you push on the throttle and the engine barely runs, but you can often let it idle, and the engine seems ok. You pretty much can't drive the car. A test for this is to disconnect the MAF. The car should run better. Plug it back in and the car runs crappy. Typically you can apply throttle and barely get the engine to rev. If you try and drive the car, the engine will barely make power to propel the car. You will have a hard time making revs, keeping revs, and making power. Now MAF behavior varies by car, but this is what I've commonly seen as a byproduct of a failing MAF. You may be able to clean it, but most of the times I've had to replace a MAF, it simply failed completely and wasn't a cleanliness issue. It just failed and had to be replaced.

The second is the air/fuel sensor. When this goes, the car has a hard time maintaining good air fuel ratios. The car will typically idle bad and drive bad under light throttle. Dead cold and during heavy throttle use, the car will drive great. The engine is using the ECU tables during cold start up (warm up mode) and during heavy throttle (open loop mode) the ECU ignores the sensors and operates off the ECU tables. The car will run great when the sensor is not used but run crappy when the sensor is used. The car can be driven with a bad AF sensor, but it will slowly get worse over time. Very bad, the engine will have a hard time idling without the driver adding throttle and keeping revs high to prevent stalling. During light throttle, the car will act as if it is misfiring heavily with strong cut and surging of power. It becomes very hard to maintain a speed without jumping up or down 5mph. You pretty much have to floor the throttle to gain speed and then coast for a while and repeat to drive the car. I had the joy of doing this for 90 miles once, fun stuff, but the car is completely driveable in open loop operation.

These are typically the two major sensor systems that affect engine operation. They behave very differently, so it is typically obvious which one it is. Now there are other sensors and controls that influence the car, so don't get the idea that it can only be one of these two. However, these are the two major systems that typically go bad and cause operation problems so they are the first two to check.
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Old 12-30-2014, 02:09 PM   #29
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Very helpful. Great thread!
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Old 08-22-2015, 10:22 PM   #30
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Join Date: Jun 2003
Chapter/Region: MAIC
Location: New Market, MD
2015 Crosstrek


This procedure worked perfectly for me. My 2008 Outback needed a new rear O2 sensor and new oil pressure sensors for the variable valve timing / left system. Every time I reset the ECU to clear the codes the car would have terrible low idle and would stall at almost all stops. Recently this would not go away, until I tried this procedure. Worked flawlessly. If you have low idle or stalling issues I recommend trying this.

Originally Posted by GrundleJuice View Post
After resetting the ecu, turn key to on/run for 20 seconds then back to off. Then start it and allow it to idle for 8-10 mins without touching the accel pedal or putting any significant load on it like a/c, head lights, power windows or even changing manifold pressure by applying the brakes. When engine speeds are rapidly falling toward idle like when depressing the clutch after decelerating in gear, the ecu has to signal the injectors to inject fuel to "catch" it before it gets all the way to idle speed. I assume this is a learned part of the ecu logic since it gets better after some time. If you have a aftermarket or modified intake it may take longer or be impossible to get perfect anytime you disconnect the battery it resets the ecu and therefore clears any codes. If you want to find out what dtc was triggering a cel you should not reset the ecu first.
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Old 10-05-2015, 07:59 AM   #31
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Member#: 153497
Join Date: Jul 2007
Chapter/Region: MWSOC
Location: Fort Wayne, IN
2005 Saab 9-2x Aero


After reading all of this it sounds exactly like whats going on with my car.
I bought the car with mods done to it, It has a EJ205 JDM motor with an STI V7 ECU. If I cut the power to the ecu to reset it would it wipe the tune loaded on the ECU?

I know that sounds like a dumb question, as I don't think it would, but I want to be safe rather than sorry.

Also, is it "needed" to reset the ECU to reset the idle points? Or would cleaning the plate alone fix the problem?

Thanks guys!
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Old 06-09-2016, 02:35 PM   #32
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Member#: 448612
Join Date: Jun 2016
Default Saved me a lot of money

I had to d/c battery this morning, and almost wrote off my legacy coming into a roundabout when the car shut off and took away my power steering and brakes.

Everytime I disengaged the gear the car would die. I used this forum (Grundles post specifially) and it worked perfectly after.

I registered an account just so I could thank you guys for this extremely valuable information.

I can vouch that when you disconnect a 2006 subaru legacy battery, the ecu forgets how to idle, and using these steps (d/c battery for 15 mins,turning the key forward for 20 seconds but not firing,and then starting and letting idle for 10 minutes to relearn idle) 100% fixes this problem.

I didn't even clean my throttle plate.

Thank you thank you thank you guys. Really.
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Old 08-30-2016, 11:02 AM   #33
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Join Date: Aug 2016
Default New subaru owner odd stalling issue

Hi all first post here hope you can help me. I bought a 2004 impreza ts wagon a month ago. Car has about 60k miles only and ran well. Last Friday I changed oil and filter using Mobil 1 synthetic 5w 30. That evening as I pulled into driveway, engine stalled when shifting to P from D. I used car again in the evening and car would stall sometimes coming to a stop or coasting letting go of accelerator. No rough or low idle.

Next day I cleaned and tested Idle air control valve, which was sooty, and also cleaned throttle body which was not so dirty. Car idles well and runs well until I pulled into station to gas up. Shifted to P and engine died. Put key in ignition and no lights on dash. After 2 minutes tried again, dash lights went on and car started normally. Shifted to D from P and engine died.

Waited 2 minutes for dash lights to come on and started in N neutral and shifted to D and had no problem. Drove off car runs well.

The problem doesn't occur when engine is cold. There are no trouble codes, I also reset the ECU. Also when engine stalls, I have to wait for around 2 or 3 minutes for dash lights to come on before I can start the Car. So not sure if shifting to and from P is the problem, or passing thru R (from P or D) causes it.

Sorry for long post. Hopefully someone can help.
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Old 08-31-2016, 09:23 AM   #34
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Join Date: Aug 2016

Any help is appreciated.
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Old 09-06-2016, 11:05 AM   #35
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Member#: 454001
Join Date: Sep 2016
Default No power. Undrivable

Any subie experts wanna try and help me, n/a 2.5 idles fine. But under load it spits and sputters and backfires, completely undrivable. Fuel trim sits around -30% (not good). All the sensors are reading on my scanner and look normal"ish". Gonna get used sensors from the wreckers but this has me completely stumped. 310,000km bone stock other than exhaust.

Uel headers tsudo test pipe to custom catback.
Headgaskets done 6 months ago, timing belt, water pump and pulleys done 2 months ago. Brand new plugs and wires, coil seems strong still.
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Old 09-07-2016, 05:21 PM   #36
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Member#: 454001
Join Date: Sep 2016

So I unplugged my upstream 02 sensor...runs like a champ! Best part is it's still under warranty (changed both when I got my test pipe). Getting new one Tomo morning and hope all is well when I hook it up again.

Kinda strange that it went bad after only 4 months
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Old 08-06-2017, 07:07 PM   #37
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Member#: 469482
Join Date: Jun 2017
Location: Texas
2014 Forester


Man it also happen to me. I charged my battery because it went dead, disconnecting both cable for couple of hours and the idle problem show up. Followed your instruction and whala!! Got it solve, thanks SubaruTech84!!!
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Old 10-12-2017, 08:40 PM   #38
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Member#: 90599
Join Date: Jul 2005
Default Won't idle after air bag recall

Bumping this thread to the top. Just got my Outback XT back from the dealer after an air bag recall replacement. If there's one thing that's awesome about Subarus it's the enthusiast following that contributes to forums like this.

Picked up my car and it took just a little longer to start, first time I twisted the key she didn't fire. Tried again with a little throttle and she fired up, put it in reverse and stalled it. Thought "that's weird", gave it more gas the next time and I was on my my, with an incredibly rough idle. I should have turned around but I continued to work. I called the service manager so at least I made contact but knew they'd just try to charge me so I turned to NASIOC and sure enough, there's a simple solution. Thanks guys! F-YOU Valley Subaru of Longmont. Why the hell did you not call me and say "we're done but your car won't idle, it'll be one hour labor to clean your throttle body". Morons. Again, thanks to the memebos who contribute to threads like this!
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Old 04-08-2018, 09:58 AM   #39
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Member#: 346991
Join Date: Feb 2013
Location: Palmer Lake, Colorado
'93 Impreza wagon

Default Rough idle 2000 OB odd injector diagnostic.

Hi All.

Y2K Outback Limited. 130K miles. EJ25, AT. I recently bought this car for cheap because the original owner couldn't get it to run right after a timing belt replacement.

Brought it home, corrected a mis-alignment on the timing belt, and now it runs great. Except:

Idle is very rough, idle speed ~500 RPM. If you touch the throttle to bring the engine speed up to ~1000 RPM, the engine runs smoothly, and, as mentioned, it drives fine, and seems to be getting normal fuel economy. No DTC.

I have a 2002 Forester that runs fine, (but burns more oil than gas) and have been swapping parts with that to try to eliminate the problem. So far, plug wires, coil pack, MAP sensor, throttle body have not affected the problem. Also inspected all of the vacuum lines, etc, but didn't find anything obviously bad.

I recently got a diagnostic interface and FreeSSM software off of eBay. With this tool connected to the car, the only odd thing I have noticed is the injector duration (?) data. This is from memory, but IIRC the software is presenting the injector duration in milliseconds. It displays a min, max, and current value, the minimum being something like 1.5ms. When sitting at idle (rough), the duration was something like 3.8ms, as soon as I upped the engine speed a bit, the duration _dropped_ to minimum, 1.5ms, and the roughness subsided. This is repeatable, and if I use the software to set the target idle speed to 1000RPM, the roughness goes away. I checked the TPS "zero", it was OK. Swapped throttle bodies as mentioned, no change. Currently driving it with 1000RPM idle; probably a little hard on the AT, but seems OK otherwise.

I was wondering if anyone has an idea of what might be causing this behavior, or if maybe it is just a software thing and unrelated to the problem?

When I bought the car, I assumed a failed timing belt job and bent valves, and planned to swap in a nice 1st gen EJ22 I have sitting in the garage. Probably I will go ahead with the swap, unless I find an easy fix to the annoying idle problem. I'll have to admit that I like the older EJ22 far better than the EJ25s; I have yet to see an EJ25 go past 200K w/o major issues :-(

TIA for any thoughts you might care to share on this. Steve
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