2000 Forester L, 125000mi Car started stumbling off idle 5000 miles ago. Eventually got so bad it would barely run missing and jumping all the time. Brought to independent mechanic and said he couldn't pull codes so thought computer was fried. Bought used ECU and replaced to run excelent for the next 4000 miles. Recently started stumbling again and surging at idle. Decided to get replacement (used) computer straight away and did not make any change. Still no CEL. Went back to mechanic: Replaced IACV (aparently checked IACV and wasn'tstepping properly to determine need) and Knock Sensor, And another used ECU. Also "blew a bunch of gunk out of the cat." NO CHANGE. He then "re wired a diode" in the ECU that seemed to be burnt. He thinks IACV has fried ECU and need another one. Finally have a CEL: Got code read and result is PO136 (H02S 12 circuit condition Heated oxygen sensor bank 1 sensor 2) and PO170 (Fuel trim bank one) Can IACV fry out the ECU? tried two used ECU's already, do you think they could have been bad? If not bad, would they fry out within the first minute of idle? Could o2 sensor be behind all this from the begining? (I've put $150 into 3 used CEU's and would rather not put $800+ into a new one) Any suggestions? Thanks folks.
IACV as in, Idle Air Control Valve? Its a freaking solenoid, that simply opens and closes when at idle to allow the motor to breathe by bypassing the flapper in the throttle body. (sorry, substituted flapper as I'm forgetting the technical word). The only thing I have experience with happening when the IAC clogs, is the ECU goes into "safe mode" because it thinks the MAF is malfunctioning. (at least thats the issue I had on my 2.5rs back in the day). You can easily fix that by unplugging the battery, and pumping the brake to reset the ECU. If the car is idling poorly, and stalls out at idle, I sure wouldnt blame something thats hard to track down like the ECU first. I would go with cleaning the throttle body, getting a new IAC valve, a new Mass Air flow sensor, and since you have a new knock sensor the only other thing you can replace is the TPS (throttle positioning sensor). At least "sensor" wise, thats what I would target first. The ECU is going to ACT messed up, only because the data going into it is completely inacurate and the computer being a logical, man made creation, cannot make sense of it.
and cleaning the MAF can help, but if its broken, its broken, and just needs to be replaced. same thing goes with the Idle air control valve solenoid, and the passageways in the IAC. Just take off your throttle body, and clean everything on it very well with brake cleaner or rubbing alcohol. Ensure a good seat between the throttle body and the intake manifold as well when you reinstall it. Otherwise you'll find out the hard way how frustrating it is to track a problem down when you are just creating MORE of them as you work, lol.
So you got a Front A/F sensor code? Have you replaced it yet? The P0170 is saying the system is lean, more than likely caused by the faulty A/F sensor. I would replace that STAT! Do yourself a favor and buy a Subaru sensor. Part # 22791AA00A $97.08
Hey guys, thanks for the quick responses. Been tracking down info so haven't gotten back to you all. Most of this info is not mine, it has been located in posts elsewhere and this is my interpretation. I had previously looked into a MAF problem but couldn't locate one in my engine compartment. Found this post regarding 2000my "2000 was a MAP ...... No MAF present. Your only air filter is in a box directly attached to the rear of the throttle body." So I don't think cleaning MAF will help. I wish I did have a MAF so I could clean it and it'd be fixed Also code P0136 says sensor #2, which I thought to be rear (found posts validating that it is rear sensor) which according to other posts won't cause driveability issuse. As far as resetting the ECU, I've done that multiple times and no change (keep in mind I've switched it out to different units 3-4 times)
it is a map on the older now turbo models...dont know where it is located but i know the only air filter is located in a box right behind the throttle body...i think im no mechanic but i think thats where the filter is....not too sure where the so called MAP is tho
MAP Manifold Absolute Pressure (or something like that) It seems to be on the manifold (go figure). Don't really know how it effects the big picture of what I'm seeing.
Update: Reset ECU and ran 10 miles, not CEL. Straight up unplugged front 02 sensor and drove a bit more. CEL came on (guessing it says my 02 sensor is wacked), but car runs great. What are the chances that replacing the front O2 will take care of it? Just looking for input, thanks in advance. JED
