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Starts fine, idles for a couple seconds, then stalls itself. No CEL the first time or two. Here's a video of the stall behavior.

I suspect the MAF because I was recently mucking around removing the airbox to access the passenger-side spark plug wires. Prior to mucking around in the engine compartment today, the car was running completely fine.

(For what it's worth, I also messed with the ignition coil. I put everything back where it was when I started, but may have bumped or dropped something.)

  • You need to connect a code reader and tell us the codes otherwise its all just guess... – Solar Mike Dec 31 '17 at 8:31
  • In a car as new as this, I would assume a broken MAF would throw a permanent code. I used to have a '89 Opel Vectra where the MAF equivalent (a VAF) only occasionally threw a code (there were only two of them, too low voltage and too high voltage and I think I had the too low voltage code) and then the CEL turned off automatically. – juhist Dec 31 '17 at 9:26
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I can imagine this could be a bad MAF - giving a valid signal (so the ECU does not throw a DTC/CEL) yet corresponding to a wrong air amount.

Does this happen no matter what the engine temperature is or eg. when cold only?

Anyway, you should be able to isolate the MAF by simply unplugging it - you surely will receive a CEL but for the mixture preparation, the ECU will use some stored values which will keep the engine running (if everything else is OK, of course).

EDIT: I would also check the fitting of the airbox. Could be an unmetered air entering the intake. This cause should also be eliminated by unplugging the MAF.

  • Could it be coil related? – Solar Mike Dec 31 '17 at 9:39
  • Can't get the engine warm enough to tell behavior when warm. Unplugging MAF gives the same behavior (idles for a few seconds then stalls), plus expected CEL. What would cause unmetered air to enter the intake? Insufficiently tight hose clamp? Thanks. – Conrad Meyer Jan 1 '18 at 10:28
  • Yes, you basically need to check that all the connections of components related to air intake you loosened when accessing the spark plug wires were put back together properly. – MaSlo Jan 1 '18 at 12:49
  • I've managed to diagnose and fix it; see my "answer." Perhaps "unmetered air entering the intake" was exactly right? If you have an opinion on which of yours and PeteCon's answer is more correct, I'm happy to mark one or the other for Stackoverflow internet points. – Conrad Meyer Jan 2 '18 at 20:31
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    @ConradMeyer I would suggest marking your own answer as correct. As for the points, you can simply upvote both PeteCon's and my answer... or not, if you feel so :) – MaSlo Jan 3 '18 at 21:37
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Check for disconnected vacuum lines also - there's one that comes to mind which runs from the throttle body to the rear of the engine compartment on the passenger side. Easily knocked off its fixing, and can cause idling/stalling issues.

  • How would I look for it? Thanks! – Conrad Meyer Jan 1 '18 at 10:29
  • Look for a thin runner tube which would run from the throttle body, underneath the air intake tubing, to a black circular box on the rear bulkhead. There's also one which comes out to a sensor located on the passenger side bulkhead, with some electrical connections on top. – PeteCon Jan 1 '18 at 20:11
  • I've managed to diagnose and fix it; see my "answer." Your response was the hint that lead me to looking for disconnected hoses, so, thank you! If you have an opinion on which of yours and MaSlo's answer is more correct, I'm happy to mark one or the other for Stackoverflow internet points. – Conrad Meyer Jan 2 '18 at 20:30
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    I'm here to help, not for the points. @MaSlo's answer is more generic to any car with a similar problem, whereas mine is much more Subaru specific. I think they deserve the tick-mark. Thanks! – PeteCon Jan 2 '18 at 20:38
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Here's where the problem was:

disconnected intake hose of some kind

When I removed the airbox, I inadvertently disconnected a hose of some kind between the engine and the airbox. The connection sits underneath the airbox tubing, after the MAF, which sort of explains how I missed it when I was reconnecting everything. (Also, the procedure I was following from the manual called for leaving that fixture in place, but I had some trouble with the airbox-side hose clamp so I had removed the entire fixture.)

Thank you all for your hints and suggestions!

Here's another angle from the opposite side:

disconnected hose again

With the hose reattached, the car starts and runs just fine. Thank you all for your time and suggestions. It is very appreciated.

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    Glad to hear you are up and running :-) – MaSlo Jan 3 '18 at 21:35

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