After 90K Mile & Timing Belt Service: Stumbling @ Idle, No Reported Boost?

Looking for some expert feedback here so I can be best-informed before I bring the car back into the dealership - appreciate any of your time reading this (long):

I brought my 2012 Legacy GT in for its 90k mile service yesterday and also had them replace the timing belt while it was in there (ouch to the wallet). I've been running an AccessPort Stage 1 tune for over 7 years now without any issue whatsoever. I picked up the vehicle this afternoon, giving it to them for the multi-day job due to not having the belt in stock.

When I drove the car home today I noticed that it seemed to stumble on gear shifts, as if the throttle was coming on slow or the clutch was engaging late, causing my body to slightly lean forward from unsmooth/rookie-like shifting. I thought it might be a good idea to reset the ECU when I got home to clear any no-longer-applicable learnings, and so I did, but afterward I found that the car had a hard time turning over on the first start, which I seem to recall as a common symptom anytime the ECU is reset. However, while stationary I fluttered the throttle until it hit about 4-5k RPM, and on the way back down it almost stalled out, passing below the 1k mark, bouncing that way and repeatedly almost stalling.

I decided to try re-flashing the ECU to my Stage 1 tune (normal, not high nor low waste gate), but I found that the AccessPort repeatedly asked me to turn the key to the OFF and then back ON before pressing OK. I repeated this process several times to no avail, eventually cancelling out of the routine.

After attempting the reflash I left the AccessPort plugged in and navigated it to the Boost monitor. Surprisingly, it read -0.2 PSI at idle, and did not move when giving the car throttle. Usually I would expect some negative PSI at idle, and a bounce of some kind when giving it throttle while stationary. I decided to take the car around the block to monitor the boost levels, but found that the AccessPort read -0.2 PSI the entire way, even when I got hard on the throttle. When I did get hard on the throttle the car "felt" correct, the turbo seemed to be operating, but the throttle response still felt slow.

Pulling into the driveway and sitting idle again after going around the block, the car seemed to idle fine. However, while stationary if I gave it enough throttle to push up to 4-5k RPM and let it come back down on its own, the car would go back into almost stalling out when the needle came back down, bouncing and almost stalling again and again.

As a double check, I have an aftermarket radio that has an OBD2 connection for showing car information, including the boost level. Coincidentally, this reading also showed zero, despite feathering the throttle.

So to me it seems that either my car's boost sensor has become disconnected/damaged, or the OBD2 port has become damaged and prevents both my aftermarket radio and my AccessPort from reading it correctly. But given the weird gear shift behavior and the stumbling while idle behavior, it seems that the ECU is potentially in a kind of limp mode because it cannot receive and involve boost levels. I do not have any CELs though.

Does this seem like a reasonable assessment? Is there anything easy for me to check (MAF/MAP, etc.)?

The way you are describing the start up after ECU reset is totally normal. The initial start is searching for parameters for the ECU. It also takes a certain amount of time or starts cycles to settle on an idle for the ECU. Mine searches for an idle for quite a while before settling... get to a stop sign/light and it flutters up and down upon clutch in and eventually settles on an idle.

When was the last time you updated the firmware on the Cobb AP?

The way you are describing the start up after ECU reset is totally normal. The initial start is searching for parameters for the ECU. It also takes a certain amount of time or starts cycles to settle on an idle for the ECU. Mine searches for an idle for quite a while before settling... get to a stop sign/light and it flutters up and down upon clutch in and eventually settles on an idle.

 

When was the last time you updated the firmware on the Cobb AP?

I've reset the ECU many times over the course of owning the vehicle but never remembered this severe behavior of fluttering, especially the nearly stalling part. Edit: Started it up for the third or fourth time this morning and found that it wasn't fluttering anywhere near as bad, beginning by idling ~1500RPM and slowly testing its way down to about the 700RPM mark, where it did stumble a couple of times while remaining idle.

I haven't updated the Cobb AP firmware ever - worth trying?

Any ideas on why I don't see a boost level from either the AccessPort nor the aftermarket radio anymore?

Found the problem! This little guy wasn't connected. I could hear the hiss after getting closer to the engine bay. Once I connected it the engine immediately responded. Idle back to normal, boost started showing up again in both the AccessPort and aftermarket radio.

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The issue with your car almost stalling at idle would probably be a vacuum leak.

You would want to check your fuel trims with your accessport to confirm this.

There are 2 reasons that you may not see boost currently, perhaps Subaru updated your rom on the ECU during the service. This would likely have a different address for the boost signal. This should be a question sent to Cobb.

The other possibility is that you need to check whether boost is building while you drive and not just at idle. If you see positive boost when driving, check if you're hitting targets. Maybe you have a boost leak.

I'd say boost/vacuum leak is most likely the cause of both your issues.

Edit: just saw your update, put a zip tie on that vac line so that it won't come off again.

Well, if the boost controller isn’t fed a signal, it’s not gonna do much.

Glad to hear you found it.

Edit: just saw your update, put a zip tie on that vac line so that it won't come off again.

Thanks for all the info - will do on the zip tie!

Thanks for all the info - will do on the zip tie!

Hopefully the zip tie will hold it there for as long as you own the vehicle. Sometimes they don't, obviously because plastic becomes brittle...

Hopefully the zip tie will hold it there for as long as you own the vehicle. Sometimes they don't, obviously because plastic becomes brittle...

If it has a barb, the zip ties should hold for a long time. If the plastic is brittle it should be replace regardless of whether a zip tie is used. I still have zip ties on one of my turbo vehicles holding the vacuum line I put on in 2001.

If it has a barb, the zip ties should hold for a long time. If the plastic is brittle it should be replace regardless of whether a zip tie is used. I still have zip ties on one of my turbo vehicles holding the vacuum line I put on in 2001.

Wowza, 18 years later it's still holding the vacuum line... crazy stuff

Wowza, 18 years later it's still holding the vacuum line... crazy stuff

I used high quality silicon hoses, but now that I think about it, I probably should check them.

I used high quality silicon hoses, but now that I think about it, I probably should check them.

Haha, not a bad idea!