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Discussion Starter #1
Model: 2013 Subaru Outback
Engine: 2.5L
Transmission: Continuously-Variable (CVT)
Mileage: 99,000 on chassis, ~10k on new short block (replaced Nov. 2018 for free due to Subaru's oil burn issue)
Issue: After long drives, the vehicle will sometimes stall at idle.

I'm having an odd problem with my 2013 Outback. It seems that after a long drive, the engine will sometimes stall out at idle speeds, but only after a restart. I don't have any problems at all during the actual long drive, but after getting home, parking, shutting it off, and then turning the engine back on within 5-10 minutes or so, the engine will stall out at idle.

Shifting into P, R, N, D all work fine. If I shift into N as I roll into a stop sign and keep the revs high, the engine does not stall out. It will stall if I allow the revs to drop down to normal idle RPM.

All fluids look fine. Engine radiator cap is not abnormally hot (although the car does not have a coolant temperature gauge, only an idiot light. The idiot light did not come on.)

There are no dash lights. No check engine light. No engine codes stored in memory, per my OBD-II reader.

Even more bizarrely, the problem seems to resolve on its own after 10-15 minutes of cooldown.

This issue happened once, after a long drive in March 2019, and then did not recur again until today, Jan. 26 2020.

My theories / questions:
  • Could this be an issue with the engine's deliberately-idle-at-higher-revs-until-warmed-up behavior? I know the engine is programmed to idle at higher revs when the engine is cold, and then drop the revs once it's up to speed. Since I only have this issue once the engine is already hot (after a long drive), I wonder if it could be an issue with that system.
  • Possible bad torque converter? (There was a TSB published for the torque converter back in January of 2014 for the automatics with extremely similar symptoms. But I'm not sure if the CVT even has a torque converter.)
  • Is the problem temperature-dependent, or am I being fooled?
  • Possible dirty / contaminated engine air flow sensor?
  • Bad gas?
  • Vacuum leak? (But a vacuum leak wouldn't resolve itself)
  • spark plugs? (But this would cause engine trouble at any speed)
  • Compression problem? (Hope not...)
  • Leaking EGR valve? (again, hope not...)
  • Air filter clogged? (Unlikely, just replaced w/ last oil change)
  • Leaking head gasket?! (super hope not, short block was just replaced a little over a year ago for an oil burn issue, and the car has no other symptoms, and there's no sign of coolant in the oil).

Would love to hear your thoughts. Thanks.
 

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The CVT transmission does have a torque converter AND your vehicle should be covered by the 100,000 mile extended warranty so get to a dealer to have this checked out before you reach 100,000 miles.

the problem could also be the camshaft position sensor or possibly the crankshaft position sensor or the wiring/connectors going to those sensors. I believe if the ECU does not get a signal from these sensors the engine will stop.

Seagrass
 

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2020 Onyx
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Model: 2013 Subaru Outback
Engine: 2.5L
Transmission: Continuously-Variable (CVT)
Mileage: 99,000 on chassis, ~10k on new short block (replaced Nov. 2018 for free due to Subaru's oil burn issue)
Issue: After long drives, the vehicle will sometimes stall at idle.

I'm having an odd problem with my 2013 Outback. It seems that after a long drive, the engine will sometimes stall out at idle speeds, but only after a restart. I don't have any problems at all during the actual long drive, but after getting home, parking, shutting it off, and then turning the engine back on within 5-10 minutes or so, the engine will stall out at idle.

Even more bizarrely, the problem seems to resolve on its own after 10-15 minutes of cooldown.

This issue happened once, after a long drive in March 2019, and then did not recur again until today, Jan. 26 2020.

Would love to hear your thoughts. Thanks.
I had this problem in my Subaru SVX and I think it was heat causing vapor in the fuel lines. In a modern fuel injected engine this shouldn't happen, but apparently it can. The problem was worst when I stopped the car after a long drive (engine very hot) and then tried to re-start it a few minutes later (like picking up a take-out order). The car would start and 10 seconds later it could stall. I can't remember how long I'd have to wait to re-start it, but it wasn't very long and then it wouldn't stall a second time.

The following recall doesn't apply to you, but just demonstrates that fuel vaporization can be an issue:

 

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Discussion Starter #4 (Edited)
@seagrass, that's an interesting theory. Why would camshaft / crankshaft position sensors cause this set of symptoms?

@SilverOnyx, would your car stall only at idle speeds? Having vapor in the fuel lines theoretically would cause a stall at any RPM, and it sounds like the cars affected by that issue would stall without warning while driving. Still, an interesting theory.

After a long night of research / tinkering, here are my top suspects, in increasing order of uh-oh:
-mass air flow sensor (very minor, trivial fix)
-edited to add: idle air control valve (meh)
-dirty / contaminated throttle body assembly (medium)
-torque converter (my poor wallet...)

I'll update this thread as I continue to troubleshoot. One Subaru tech I spoke with ruled out EGR valve, plugs, vacuum leak, head gasket, and compression, so that's good.
 

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@SilverOnyx, would your car stall only at idle speeds? Having vapor in the fuel lines theoretically would cause a stall at any RPM, and it sounds like the cars affected by that issue would stall without warning while driving. Still, an interesting theory.
At idle, you have maximum manifold vacuum, so the fuel pressure regulator lowers the rail pressure, and if it's low enough, it can let fuel vaporize in hot fuel lines.


So I don't know if the solution is to change the fuel pressure regulator or whatever, but this is the theory.
 

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At idle, you have maximum manifold vacuum, so the fuel pressure regulator lowers the rail pressure, and if it's low enough, it can let fuel vaporize in hot fuel lines. . . .So I don't know if the solution is to change the fuel pressure regulator or whatever, but this is the theory.
Since 2006, the 2.5 non-turbo fuel pressure regulator has been integrated into the fuel pump assembly. It's not externally controlled by vacuum or anything else.
 

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Since 2006, the 2.5 non-turbo fuel pressure regulator has been integrated into the fuel pump assembly. It's not externally controlled by vacuum or anything else.
Thanks for correcting. My 1990's SVX had a different system.

It could still be the in-tank fuel pressure regulator.

 

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Even with a vacuum controlled regulator, when vacuum is absent pressure is high, when vacuum above about 5 inHg is present the fuel pressure starts to lower, but not lower than the regulator is designed, as long as it works. At idle, with high vacuum, pressure is at its lowest, the baseline the regulator is set for. Pressure rises with lack of vacuum because that is the time when the engine needs more fuel; when your foot is flooring it. So WOT is at or near atmosphere and max fuel pressure is on the rails.

Also, vapor management is handled through the EVAP system and pressure in the tank is controlled by the ECM. The engine pulls vapors in to the intake with the fresh air, pulling a vacuum on the tank. The vent valve system allows the system to pull in fresh air in to the system to mix with the vapors. So pulling vapor in to a fuel line is difficult. Fuel under pressure won't vaporize in a sealed system. The engine compartment heats up to above 150F at times, but it's not enough to create a problem with "vapor lock".

And, engine vacuum is highest with high RPM and closed throttle, not idle. When the ECM cuts fuel on decel, the engine is pumping mostly air and is pulling against a closed throttle and partially open IACV. On electronic throttles, the ECM keeps the throttle partially open, but the vacuum still increases.

So with the OPs issue:
If your Subaru guy suggested the IACV, you need a new Subaru guy. The 13 has DBW. The electronic throttle is the IACV. cleaning it would be a good idea. If it's carboned up, the motor may be having difficulty moving the throttle plate. Also, if your throttle is gunked your pcv valve is also. I recommend you clean it.

EGR only works under load and if it was an issue the engine would stall on you when you come to a stop. It creates a large vacuum leak when it's stuck open. Does your 13 even have EGR?

It's not the MAF. It would act up after the engine bay heats up and would effect performance across the board. You'd have crappy fuel trims, erratic idle, or stalling. They don't go bad. I've replaced one or two that was damaged by the owner trying to clean it and it got shorted or the heater circuit damaged.
 

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next time this happens, turn the car on wait a few seconds and turn off, repeat 2x and then see if the car starts. if it does, you might have fuel pressure bleed off and it will be time to investigate.
 

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Any history crank sensor codes?
 

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Discussion Starter #11
Thanks for the replies, everyone. A few updates:

-I can't reproduce the problem. I toodled around my neighborhood yesterday and today (including some highway driving) and I tried hard braking, stopping and starting, shutting the engine off and re-starting, etc., and the engine never stalled. Which, on the bright side, means I guess it isn't the torque converter.

-My Subaru tech (who I generally trust - she got me a new short block free of charge when I was having the oil burn issue) told me that there was probably no point to bringing it in for a diagnostic, because if they can't reproduce the problem, they can't recommend any possible fixes. She also told me that my powertrain warranty is in fact only to 60k, not 100k, and the 100k extended warranty was specific to the oil burn issue -- which I already cashed in.

-I cleaned the throttle body this evening, but it looked clean as a whistle before I touched it. I didn't notice any grime or buildup of any kind. I wiped it down with throttle body cleaner anyway, but the metal was shiny before I began.

Thanks for correcting. My 1990's SVX had a different system.

It could still be the in-tank fuel pressure regulator.
This is an interesting theory. I don't have a fuel pressure gauge so I can't perform this test, but it's a possible culprit. However, now that the problem can't be reproduced, and it happened so rarely to begin with, I'm beginning to doubt it's a sensor, valve, or regulator at all. A problem with any of those components would be persistent, right?

Fuel under pressure won't vaporize in a sealed system. The engine compartment heats up to above 150F at times, but it's not enough to create a problem with "vapor lock".

And, engine vacuum is highest with high RPM and closed throttle, not idle. When the ECM cuts fuel on decel, the engine is pumping mostly air and is pulling against a closed throttle and partially open IACV. On electronic throttles, the ECM keeps the throttle partially open, but the vacuum still increases.

So with the OPs issue:
If your Subaru guy suggested the IACV, you need a new Subaru guy. The 13 has DBW. The electronic throttle is the IACV. cleaning it would be a good idea. If it's carboned up, the motor may be having difficulty moving the throttle plate. Also, if your throttle is gunked your pcv valve is also. I recommend you clean it.
Brilliant. So I don't even have an IACV. That makes sense. I was beginning to think that vapor lock could be the issue, but you've raised compelling arguments negating that theory. Also, I notice you're a two-spaces-between-sentences sort of person; fellow lawyer?

next time this happens, turn the car on wait a few seconds and turn off, repeat 2x and then see if the car starts. if it does, you might have fuel pressure bleed off and it will be time to investigate.
Yep, tried that. I couldn't get the problem to reproduce.

Any history crank sensor codes?
No codes of any kind, per my OBD-II. And no dash lights. No codes stored in memory. But I don't have a full-on scanner, just a handheld OBD-II from AutoZone.

New theory / interesting tidbit:
Okay, so because I'm a #nerd, I track every time I gas up. Guess what? The last time this happened, several months ago, was the day after I filled the tank. This time, it happened the same day as a fillup. On both occasions, I had run the car down almost to fumes (the car took 16.2 gallons the first time, a similar amount this time). Could the issue be caused by running the tank down to fumes, thereby sucking up the sludgey gross crap at the bottom of the tank? But if that's what's causing it, that wouldn't explain why I was able to drive so far on long road trips after the fillup without any issue.

I'm perplexed!
 

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New theory / interesting tidbit:
Okay, so because I'm a #nerd, I track every time I gas up. Guess what? The last time this happened, several months ago, was the day after I filled the tank. This time, it happened the same day as a fillup. On both occasions, I had run the car down almost to fumes (the car took 16.2 gallons the first time, a similar amount this time). Could the issue be caused by running the tank down to fumes, thereby sucking up the sludgey gross crap at the bottom of the tank? But if that's what's causing it, that wouldn't explain why I was able to drive so far on long road trips after the fillup without any issue.
When you run the gas very low, the gas is hot. It's circulating from the hot engine to the tank and back. It heats up the fuel pump (and regulator?)
 

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Discussion Starter #13
When you run the gas very low, the gas is hot. It's circulating from the hot engine to the tank and back. It heats up the fuel pump (and regulator?)
Could that be it, then? Have we solved it? Could this problem have been caused by running the tank too low for too long? And if so, why would the problem take so long to occur, and why would it only occur after a restart rather than during the drive?
 

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Could that be it, then? Have we solved it? Could this problem have been caused by running the tank too low for too long? And if so, why would the problem take so long to occur, and why would it only occur after a restart rather than during the drive?
Immediately after shutoff, you have heat-soak. A quick re-start just as temperatures in the engine bay are peaking, could cause more fuel vaporization issues than when the car is running, the fans are blowing, etc. Whether it's the fuel in the line, or the fuel returning from the engine, it's extra hot.

Whether or not this has anything to do with your particular engine is conjecture, but it's a known issue when you run the tank low, it heats up the fuel pump because the gas gets hot. Heat-soak is also a known phenomenon. Bring the two together and maybe it's related. Maybe not.
 

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You are being fed mis-information by your trusted suby tech about the 100,000 mile CVT and torque converter warranty.

Subaru extended the warranty (in the US) to 10 years or 100,000 miles so as I suggested previously, I would get your vehicle checked before you clock over 100,000 miles as it can cost up to $8,000 to repair once you clock over the 100,000 miles

Many members here have had their CVT’s replaced by Subaru under this extended warranty.

Seagrass
 

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When you run the gas very low, the gas is hot. It's circulating from the hot engine to the tank and back. It heats up the fuel pump (and regulator?)
There's one fuel supply pipe from the pump to the engine. The other pipe that runs parallel to the fuel supply line is the purge vapor line, which runs from the charcoal canister to the purge valve at the engine. This line draws fumes from the canister into the engine when the purge valve opens. There's no fuel circulation from the hot engine to the tank. (The usually-stated reason for moving the fuel pressure regulator to the tank is to avoid exactly what you're suggesting, i.e., warmed fuel returning to the tank where it would raise the tank temperature and cause additional evaporation.)
 

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Discussion Starter #17
Okay, folks, sorry it's been a few days. Here's the latest.

Yesterday I tried as faithfully as I could to recreate the failure state. On a recent long drive (not as long as the initial failure state, but close) I let the car get down to almost empty, and the gas light came on; I filled up, kept driving, and at the end of thd drive, shut off the engine and immediately re-started it within a minute. No stall. I tried four times. Couldn't get it to stall. It fired up fine on every attempt.

I'm thoroughly baffled. Having eliminated sensors, torque converter, throttle body, gaskets, fuel pressure, and vacuum leaks, I have no idea what else could be the culprit. And without a reliable way to recreate the problem, I can't continue trying to diagnose it.

I'm taking it in soon for its 100k service, and I'll see if the inspection reveals anything I've overlooked. But for now, I'm going to go ahead and close this bug report under "could not reproduce."

Thanks, brainiacs, for your collective mental effort. I appreciate it.

Honestly, considering selling it and getting something like a Ford Ranger, whose various mechanical problems I can actually understand....
 

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Discussion Starter #18
Update. Everyone, it's happening again. Currently stranded in a parking lot. And the engine immobilizer is active and the key is stuck in the ignition. Drove about 20 miles to work this morning, left it in the parking lot, came out on my lunch break, car won't start and engine immobilizer is active. What's going on?!
 

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Update. Everyone, it's happening again. Currently stranded in a parking lot. And the engine immobilizer is active and the key is stuck in the ignition. Drove about 20 miles to work this morning, left it in the parking lot, came out on my lunch break, car won't start and engine immobilizer is active. What's going on?!
did you find out what was wrong? I have a '15 OB and I am having the same issues as you. Warm restart after sitting for 10 minutes it will go 5000rpm -> 1000 rpm -> 500rpm then dead. flooring it at 1000 keeps it from stalling.
 
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