Subaru Outback Forums banner
  • Hey everyone! Enter your ride HERE to be a part of this month's Outback of the Month Challenge!
1 - 7 of 7 Posts

·
Registered
Joined
·
5 Posts
Discussion Starter · #1 · (Edited)
Hi All,
I've been cruising these forums and others a good bit for a solution to my intermittent P0171 problem. This has been going on for abut 2 years intermittently, and occurred both before and after an engine swap (performed by dealer, no mods). The CEL used to come on after an hour or more of highway driving at 70-85mph. This has remained the story for abut 2 years, but I'd say it the light comes on more frequently nowadays. Something interesting is that the light will come on after this highway driving when i use 91 or 87 octane with the standard ethanol mixture found at all pumps. We don't have 93 octane in Colorado. When I use 87 ethanol-free fuel, the light doesn't come on no matter how much highway driving I'm doing. Perhaps I've forgotten what the car should feel like, but I don't think performance is suffering, fuel efficiency hasn't changed much if any, and it doesn't stumble or stall.

I'm pretty savvy with cars but I'm just starting to work on this Subie rather than take it to the dealer, so I have limited experience in the wide world of Subaru. I haven't tried RomRaider nor would I know what the numbers mean, but I do have an OBDII handheld that can log and chart data.

Anyway, some of the things I've already done include:

-Vacuum pressure at the intake is measuring 17psi. I believe this is normal. I followed EricTheCarGuy's vacuum leak test with Propane and found nothing. I am now reviewing this thread regarding common vacuum leak locations and will double check all of the connections mentioned: https://legacygt.com/forums/showthread.php/intake-vacuum-leaks-and-you-pics-inside-235626.html

-Dealership replaced the O2 sensor in August 2017 which did not change the condition. I believe this rules out the O2 sensor.

-I just cleaned the MAF sensor. There was a small bit of residue that came off, but overall it was pretty clean. Air filter looks good.
__

I read on a Saab forum where somebody had the same symptoms and suggested the fuel filter could be clogged up. My car is at about 115,000 miles and I believe filters are recommended at 90,000 anyway. I've never replaced mine. My theory here is that cruising for long periods of time at >3,000 RPM requires a steady flow of a (relatively) high volume of fuel, and if the fuel can't get to the cylinders I'd see a lean condition (P0171). When running straight gasoline without ethanol, I need slightly less volume of fuel to create the same combustion, and so perhaps my filter is clogged just enough so that it can't provide the slight extra bit of fuel needed to overcome 10% corn in the regular 91-octane gas.

What do you all think of this? Any ideas or suggestions on what exactly to test next? I will have a new fuel filter in 2 days and can update after that. I will measure fuel pressure when I open up the back seats to access the fuel filter in a couple of days. However, I don't think this test could pinpoint the filter because I'd need to be measuring the pressure while ripping down the highway for an hour, since the CEL doesn't trigger at any other time.

Thanks!!
 

·
Registered
05 Xt beatin' to an inch of its life
Joined
·
639 Posts
That’s a lot of info! It really sounds like emissions testing failing from when you report the CEL illuminating. Check out those circuits and associated equipment in the service manual. Issues with the evap/gas gap/purge valve can cause other strange stuff. I’d clean the gas cap, filler neck rim, check for pinched tubes on the charcoal box, etc.
 

·
Registered
2005 OBXT Limited, VF37, STI intake, 5MT
Joined
·
1,610 Posts
While your theory about the fuel content has some merit, it needs more data. If you still had a carbureted induction system (or a feedback-less system), you would be correct: ethanol-mixed fuel does not have the same energy density as pure gasoline. Now, you should be running the highest octane-rated fuel available (91 or better), not so much for the quality/content, but the anti-detonation capabilities. Turbo boxers LOVE fuel to keep the knock down. Especially under boost.

However, in your OBXT, the ECU adjusts injected fuel based upon feedback from both (this will matter in a minute) O2 sensors. It can even calculate cylinder-specific compensation per injection cycle. So when it pops a P0171 (Too Lean), it has a long-term fuel trim above 20%. Essentially, the ECU has to add 20% more fuel overall than what should be required, according to it's programmed calibration. In this system, the ECU uses both the upstream (the one the dealer likely replaced) AND the downstream sensor to calculate and verify the Air-Fuel ratio. So while the upstream may be reading correctly, the downstream could disagree and cause the error. Typically, this is done during steady-state cruising (highway), which makes sense why it pops then versus around town.

Your propane testing is a good first step to finding vacuum leaks, but it can be inconclusive when finding non-vacuum air leaks. Since this system is MAF-based, any cracks/leaks/weeping gaskets AFTER the MAF sensor will throw-off the injected fuel calculations and result in a fuel trim adjustment. The common areas are the PCV hoses harden and crack, the turbo inlet splits at the accordion, and the intercooler end tanks can separate, etc. The best way to test those is to do a "boost leak" where they lightly pressurize the turbo inlet with a light smoke, looking for leaks. Ask @traildogck about what he's found...

TL;DR I'd put the type of fuel very, very low on my list of hunting down the P0171 code.
 

·
Master Caster
🖤💔💙 3 Beautiful OBXTs
Joined
·
16,253 Posts
Yes, they are all hard as rock and they typically split at the ends where they connect to something else.



Here is a perfect example of what I used to "******* up" vs spending $35 on one hose from the dealer. But that band new line is incredibly soft and supple when they are new. My set-up is in 5 pieces.

The little 90*-then 90*-elbow down hose that connects the metal crossover tube to the back of the block is impossible to reuse with any motor of any age or mileage. My dealer calls that one ... "the most popular emission hose"
 

·
Master Caster
🖤💔💙 3 Beautiful OBXTs
Joined
·
16,253 Posts
I just read the 1st post in detail now. You definitely need to check for leaks everywhere. Worry about the fuel and fuel flow as separate issue. Checking your fuel pressure would be a good start.
 

·
Registered
Joined
·
5 Posts
Discussion Starter · #7 ·
Thanks for the replies folks! I'll read up on the smoke test in the turbo and will plan to inspect and replace hoses etc. before doing anything drastic with the fuel system.
 
1 - 7 of 7 Posts
Top