Rough idle, surging, erratic but only when stopped at lights

[zch91]

Received my replacement O2 sensor today, waited for the ehxuast to cool down and fitted it just now.

Reset the ECU and took it for a 20min drive, all seems well so far! Will see how it fares driving to work over the next few days.

Here's hoping

For the record, part numbers are the following.

22641AA220 = Old style O2 sensor (prone to failure from what i've read)
22641AA391 = New style O2 sensor (revised by Subaru to fix the above issue)

[maj]

Looks like I should have touched wood since my last post.

Car started idling erratically, reset ECU, didn't help. No codes either.

Today checked the numbers on the MAF sensor, was telling me 3.8g/s idling but occasionally would start fluctuating between 1.1 and 4, which is when the car idle rev would go between 400-1100 and occasionally stall.
Pulled the MAF and cleaned it with CRC MAF cleaner, put it back in and it told me 1.98g/s idle, so back to normal (or so it seemed).
Took it for a drive and saw as much as 4.3V from the MAF readout, so it appeared fine. IAM was 1.000 car felt normal and boost was solid.

Pulled back into the driveway and it started going erratic again and stalled. The MAF readouts were doing the same thing, is this normal? When it doesn't idle erratically the g/s readout is solid at 1.98.
Once the missus gets home I'm going to have a look at her Impreza's MAF to see if it's the same, and swap it over for a test if it is.

Failing that looks like I'll be ordering a front O2 sensor. $92.79 + shipping on Amayama. I honestly cannot say when the front O2 sensor was last replaced so I'll do it anyway.
If it ends up being a vac issue I'll just take it into Subaru to deal with if I can't spot anything obvious.

[SUM12C]

I had a rough idle issue ages ago, it turned out to be a broken down banjo filter. It was worked out after the O2 sensor was replaced and the injectors.

[Victripper]

A mate of mine had this same sort of issue and ended up throwing a ton of money at it and ended up being his alternator was throwing out dirty voltage that was throwing off the cam sensor etc. When driving normally it was good.

Maybe get the Alternator checked - the voltage output was spot on so might need some fancy analysis tool to check it out?

Edit: He has a 01 B4.

[maj]

Actually the battery recently went flat and I recharged it but it has never been quite as good, though I expect this is also due to the Canberra winter always punishing batteries in near zero temps when starting on a cold morning.

You might be onto something here though with dirty voltage readings, I'll get Subaru to look at it I think.
Voltage is usually mid 10's with the engine off but appears to be holding fine once started.
The O2 sensor is cheap enough to replace without too much worries but I guess if It's possible i'm up for an alternator overhaul or refurb'ed replacement then I should save my pennies for now.

I'm not getting the issue constantly, only randomly, but it's very clear once I pull up at the lights if it's going to happen or not, and it usually ends up with a stall.

[maj]

Ended up being the MAP sensor vac hose again. This time at the manifold side it had cracked and eventually fell off.