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Fri Jul 04, 2025 7:36 am

Not many hairs left to pull out here. Back story, 2000 subie 2.5 conversion on my 87 syncro, with about 40k flawless miles, until now. All i did was change the oil, check airfilter and retightened clutch clave cylinder bolts. Went to start to check for pressure, toggle engine three times, all good. Them went to start up to drive and all hell broke loose. Went into a really rich running mode, rough idle that dances around 1100-1200 rpm and just pulses. Throwing codes from 1510, 1512, 1514, 1515, 1516, 1116, 1722. Ok, no way tps, ect, and iacv all failed instantly. So what gives? Started from the basics, checked all grounds, good. Check for vacuum leaks, good. Now dig a little deeper, start with tps, back probed signal, .5v-4.5 at wot, good, ground good. Moved on to iacv, all wires good, signals good but iacv is an aftermarket. According to specs, signal pins should be 30-40 ohms, mine are 80-99. I have three different iacv, one oem, two aftermarket, and all are in that 80-90 range,hmm… back to that in a minute. Ok moved on to ect. All wires good, resistance is within specs. Moved on to thottle body, flap is closed, cleaned it up with brake clean all looks good there. So back to iacv. Ok all three units make van behave exact same way, rough idle hunting between 1100-1200 rpms and running really rich. I would expect some variation in behavior between the three iacv, not exact same right? So removed the iacv and plugged the hole, runs smooth but really low idle unless i manipulate the rag to allow in more air, idle then increases until it reaches the point of the exact same symptom, rough running idle, 1100-1200 rpm and rich condition. What gives? Tried to drive with iacv installed but rpms start to go all over the place so i parked it out of fear of it going into limp mode. Then all codes get thrown. I can pretty much clear all of them except 1515, iacv! With swapping all three and gettiing same issue, cant think its the iacv itself. Again, all i did was an oil change and then all this nightmare started. Runs fine with iacv hole plugged but very low to no idle. Very flustered, i triple checked all wiring etc… what am I missing? Lastly, more the iacv, oreillys have a brand new, aftermarket on the shelf, so I went down to check it out and measure the resistance and same 83 ohms on both sides, way above subie specs of 30-40ohms. Can anyone check there iacv ohms and report back their specs?

Fri Jul 04, 2025 8:07 am

I don’t know the EFI system very well, but I don’t think you mentioned MAF sensor. Perhaps you disturbed that when you replaced the air filter?

Fri Jul 04, 2025 8:09 am

Mine is a MAP system and I did go through that as well, and all good

Fri Jul 04, 2025 8:42 am

Any chance you slightly dislodged one of the major engine harness plugs?

Fri Jul 04, 2025 9:51 am

Thought that as well but sadly, no, they see all intact

Fri Jul 04, 2025 9:51 am

Google P1510,1512,1514,1516 together. Seems to indicate a really dirty throttle body or non-OE IACV,bad IACV wiring etc. Good luck.

Edit: My 02 2.5 had a small vacuum hose to the top of the TB. I once tried hooking up the charcoal canister to that hose and it ran like crap. Don't recall any codes though. Maybe check the condition of that hose.

Fri Jul 04, 2025 10:07 am

Have you tried an idle re-learn?

Fri Jul 04, 2025 10:28 am

I'd try the pull battery cable for a few hours trick, and swap ecu if you have a spare.

Also some of those early subies had a weird reaction when you pulled codes and wouldnt start unless you did the key trick... Something like turn ignition on for 15 seconds (not to start),turn off, do that 3 times, then start it.

My 05 outback had to do that every time I disconnected the battery or pulled codes. Otherwise it would not start, and the ecu went through a learn process.

Fri Jul 04, 2025 10:47 am

Cant do idle relearn until i get the idle issue to stabilize. Tried disconecting battery overnight and no love.

Fri Jul 04, 2025 5:02 pm

Yes every time you clear codes you have to cycle the key to on then off before restarting.
Other people have reported this group of IAC codes online. Coincidence that it happened with your oil change. I wonder if it's the IAC or TPS connector and wiring.They appear to be somewhat bent or stressed and with age something has given.
I take it you are following the diagnostic criteria for these codes in the service manual.
Stick with the OE IAC.

Fri Jul 04, 2025 7:07 pm

wesitarz wrote: Yes every time you clear codes you have to cycle the key to on then off before restarting.
Other people have reported this group of IAC codes online. Coincidence that it happened with your oil change. I wonder if it's the IAC or TPS connector and wiring.They appear to be somewhat bent or stressed and with age something has given.
I take it you are following the diagnostic criteria for these codes in the service manual.
Stick with the OE IAC.

Yea all wiring is backchecked all the way to ecu so confirmed good. The only question mark is the three iacv with questionable resistance. Even a new one at oreailly was 83ohms, higher than spec. Curious what others iacv resistance is. New oem ones are $400. Ill spend that if it is indeed my problem, but dont want to throw money at parts. Tps is tested and within specs. And yes, the coincidence with oil change is killing me

Fri Jul 04, 2025 7:15 pm

what is your base timing looking like? are ALL of the timing marks lined up?

don't argue why, just confirm please

Fri Jul 04, 2025 7:30 pm

skills@eurocarsplus wrote: what is your base timing looking like? are ALL of the timing marks lined up?

don't argue why, just confirm please

Ill have to dig in to confirm. But curious if all is running smooth for 40k+ miles, could timing all of the sudden go out off. Not arguing, just trying to understand. Also, would that throw some codes if it did. This whole iacv thing has me crazy

Sat Jul 05, 2025 1:52 am

Diagnostic procedures here for 2002 Impreza.

https://jdmfsm.info/Auto/Japan/Subaru/Impreza/2002/Service%20Manual/2%20-%20SOHC%20Engine.pdf

Sat Jul 05, 2025 4:29 am

Does it have the upper guide Subaru used to keep the timing belt from jumping a notch on manual transmission cars?

Sat Jul 05, 2025 10:32 am

dobryan wrote: Does it have the upper guide Subaru used to keep the timing belt from jumping a notch on manual transmission cars?

I thought SODO was going to chime in with that... But my thoughts too. Rolling off ramp after oil change, put in gear (reverse), bam, jump a tooth.

Sat Jul 05, 2025 4:11 pm

dobryan wrote: Does it have the upper guide Subaru used to keep the timing belt from jumping a notch on manual transmission cars?

Yes, absolutely does!

Thu Jul 10, 2025 7:25 pm

Just updating an ending here for anyone who cares;

Did a lot of troubleshooting, backprobing all circuits of ect, tps and iacv. While backprobing the iacv saw the signal wires were not responding as if the ecu is not talking to it. Replaced the iacv with multiple known good ones. All behaved exact same way. Decided to throw a hail mary and look at the ecu. I have never, ever in all my multiple vws or any other cars had a failed ecu, so I was never even considering it to be the problem. Again only codes thrown weere related to iacv 1515 and ect and tps. But not seeing a signal coming from the ecu to command iacv, decided it was worth investigating. Opened up the ecu and saw what appeared to be a burnt transistor. Took a pic for AI, and it confirmed THAT transistor controls the signals to the ecu. Did some diode testing on it and found bad values according to AI. Bingo. Called a wrecking yard,luckily got a $50 exact replacement ecu, put it in and fired right up and went to proper idle and has been smooth as butter! 3 weeks of agony and dead end diagnostics got it! So now, all codes cleared themselves and super stoked. Id like to replace that transistor on my old ecu and see if i can fix it. Anyone have any skills board repairs?

Fri Jul 11, 2025 3:46 pm

That’s great news! Thanks for posting the solution.

Fri Jul 11, 2025 4:48 pm

Took a pic for AI??

That's wild. Nice work.