2015 Outback-Running Rich Difficult Diagnosis

[JoeDirt41]

Any diagnostics experts or Subaru specialists here? My mom has a 2015 Subaru Outback 2.5i Premium with 157k miles has been experiencing issues. About a month ago, a P0172 showed up along with the CEL light. No drivability issues. She previously experienced the same thing about 2 years before and it was a bad upstream O2 sensor. Based on history, and fuel trims that were all over the place, I replaced the O2 sensor, and immediately the fuel trims normalized and within maybe 100 miles, the CEL went out. Problem fixed.

But no. Now, about 1000 miles later, the CEL came back on. Now the car is hesitating and almost feels like it is going to stall when taking off from a stoplight, and it seems like the longer you’ve been sitting at the light or the harder you accelerate, the worse it is. I plugged my scan tool in, and there are 2 codes, both “History” status- a P0172, and a P1603, engine stall history.

I looked at the fuel trims, and they are all over the place. Short term is on the high side of acceptable, but long term is way off.

I did a thorough visual inspection and I found no loose, chafing, or broken hoses or wires. At this point I’m suspecting the MAF sensor may be going lazy, but I can’t tell. Here’s the live data graphs showing the MAP sensor reading vs. time compared the throttle input vs. time. Does this look significant to anyone? Any ideas on where to look? I’m confused why the CEL is on, but the codes are “history” status.

 

[D60]

If you unplug the MAF what happens? Even so I'd suspect the MAF to affect both banks, but boxers are weird about where they choose to "see" errors

That thing is really trying to pull fuel, so if it's not a sensor error I'd also wonder about a leaking injector or maybe a purge valve stuck open, the latter being easier to diag

 

[JoeDirt41]

If you unplug the MAF what happens? Even so I'd suspect the MAF to affect both banks, but boxers are weird about where they choose to "see" errors

That thing is really trying to pull fuel, so if it's not a sensor error I'd also wonder about a leaking injector or maybe a purge valve stuck open, the latter being easier to diag

I’ll try the MAF tomorrow. Subaru only designates one bank.
I’ll also look and see if my scan tool has a purge valve status/commanded/actual live data function, but if that was the issue, I’d expect a code for it.

 

[TacoFergie]

I'm guessing you've already checked the simple stuff, but here's a quick list
- check air filter, air box and air box snorkel for blockage like a nest.
- clean MAF?
- when were the spark plugs last changed and what brand?
- potentially bad gas?

Also, what brand 02 sensor was used when replaced last? OEM, Denso or NTK are the only sensors to use on Subaru's or any Japanese cars.

I think cleaning the MAF and checking the air filter and intake before the MAF would be where I'd start. You'll have to do an idle relearn after that I'm sure. It might be worth cleaning the throttle body while you're at it before an idle relearn.

 

[97prizm]

If you unplug the MAF what happens? Even so I'd suspect the MAF to affect both banks, but boxers are weird about where they choose to "see" errors

That thing is really trying to pull fuel, so if it's not a sensor error I'd also wonder about a leaking injector or maybe a purge valve stuck open, the latter being easier to diag

I kind of thought injectors as a possibility. My cavalier would run and misfire at idle but smooth out driving down the highway.

 

[SC Maintenance]

Any diagnostics experts or Subaru specialists here? My mom has a 2015 Subaru Outback 2.5i Premium with 157k miles has been experiencing issues. About a month ago, a P0172 showed up along with the CEL light. No drivability issues. She previously experienced the same thing about 2 years before and it was a bad upstream O2 sensor. Based on history, and fuel trims that were all over the place, I replaced the O2 sensor, and immediately the fuel trims normalized and within maybe 100 miles, the CEL went out. Problem fixed.

But no. Now, about 1000 miles later, the CEL came back on. Now the car is hesitating and almost feels like it is going to stall when taking off from a stoplight, and it seems like the longer you’ve been sitting at the light or the harder you accelerate, the worse it is. I plugged my scan tool in, and there are 2 codes, both “History” status- a P0172, and a P1603, engine stall history.
View attachment 320290
I looked at the fuel trims, and they are all over the place. Short term is on the high side of acceptable, but long term is way off.
View attachment 320289
I did a thorough visual inspection and I found no loose, chafing, or broken hoses or wires. At this point I’m suspecting the MAF sensor may be going lazy, but I can’t tell. Here’s the live data graphs showing the MAP sensor reading vs. time compared the throttle input vs. time. Does this look significant to anyone? Any ideas on where to look? I’m confused why the CEL is on, but the codes are “history” status.
View attachment 320288

I know nothing of Boxer engines but 0.013 lb/s x 454 grams/lb = 6 ish grams/s. If this is at idle a normal engine is around 1 gram per liter. So this seems very high for a 2.5 liter. Thinking its getting more air than it is would explain the rich condition. A dirty MAF will usually read low (lean condition). I would consider firing the parts canon at the the MAF sensor if it was mine - unless someone tells me Boxers are different.

 

[D60]

If it were bad gas I'd expect misfire codes or maybe lean, but not rich.

I went through an odyssey on an '11 FB25 with bad gas. He had misfire codes and a hiccup at idle. Initially it was nearly impossible to start, but got to where it would start with minimal difficulty.

That said, I guess there are multiple ways and degrees of "bad"

 

[SC Maintenance]

If it were bad gas I'd expect misfire codes or maybe lean, but not rich.

I went through an odyssey on an '11 FB25 with bad gas. He had misfire codes and a hiccup at idle. Initially it was nearly impossible to start, but got to where it would start with minimal difficulty.

That said, I guess there are multiple ways and degrees of "bad"

I agree - I can't see bad gas causing a lean or rich condition to this level with no other symptoms / codes.

 

[Number_35]

My first thought with such things is a crack in the bottom of the corrugated hose running from the MAF to the throttle body, but that should cause a lean condition rather than a rich one. (The air admitted through the crack bypasses the MAF, which signals the CPU that less fuel is required than the actual amount of air entering required.)

Nevertheless, paradoxical stuff like this does happen, so I would check the hose anyway.

 

[TacoFergie]

If it were bad gas I'd expect misfire codes or maybe lean, but not rich.

I went through an odyssey on an '11 FB25 with bad gas. He had misfire codes and a hiccup at idle. Initially it was nearly impossible to start, but got to where it would start with minimal difficulty.

That said, I guess there are multiple ways and degrees of "bad"

Good point, I kinda just threw that one out there due to the hesitation and nearly stalling events. However a fuel injector stuck open or faulty fuel pressure regulator could also cause all of these issues. Though I haven't seen too many Subaru's of that generation with a bad fuel pressure regulator.

 

[D60]

Good point, I kinda just threw that one out there due to the hesitation and nearly stalling events. However a fuel injector stuck open or faulty fuel pressure regulator could also cause all of these issues. Though I haven't seen too many Subaru's of that generation with a bad fuel pressure regulator.

Agreed, FPRs and injectors aren't common problems I've personally observed with URaBus.

NOW, their EVAP systems are a different matter, although I agree typically codes are set for those problems.

Did '15 still get the green make-everything-go connector under the dash? Might worth a try, doesn't cost anything other than time

 

[JoeDirt41]

I'm guessing you've already checked the simple stuff, but here's a quick list
- check air filter, air box and air box snorkel for blockage like a nest.
- clean MAF?
- when were the spark plugs last changed and what brand?
- potentially bad gas?

Also, what brand 02 sensor was used when replaced last? OEM, Denso or NTK are the only sensors to use on Subaru's or any Japanese cars.

I think cleaning the MAF and checking the air filter and intake before the MAF would be where I'd start. You'll have to do an idle relearn after that I'm sure. It might be worth cleaning the throttle body while you're at it before an idle relearn.

Checked thoroughly for any kind of obstructions in the intake and airbox. All clean. Filter is clean too. No cracks or breaks in the intake tube.

I sprayed the MAF with some cleaner but it didn’t appear to be dirty.

Plugs were replaced with original NGKs about 10k miles ago.

New O2 sensor is a made-in-Japan Denso. Original was also Denso.

I’m just at a bit of a loss. I hate to be a parts changer, but I’m about ready to pull the trigger on the MAF sensor. Just seem odd that the O2 fixed it for a while then right back to where we started.

 

[SC Maintenance]

I’m just at a bit of a loss. I hate to be a parts changer, but I’m about ready to pull the trigger on the MAF sensor. Just seem odd that the O2 fixed it for a while then right back to where we started.

It may not have fixed it - just got it back down below -25% for some reason.

Before you switch the MAF - have you tried logging the long term fuel trims as you drive. You said its worst at idle and under load - do the trims move closer to normal while driving under constant light load at say 1500+ RPM? A leaky fuel injector or something like that is usually worst at idle.

 

[JoeDirt41]

It may not have fixed it - just got it back down below -25% for some reason.

Before you switch the MAF - have you tried logging the long term fuel trims as you drive. You said its worst at idle and under load - do the trims move closer to normal while driving under constant light load at say 1500+ RPM? A leaky fuel injector or something like that is usually worst at idle.

I have not tried that, but I definitely can give that a shot and see how it looks. She had the car for most of the day today so I didn’t have a chance to look at it, but I should be able to look at it tomorrow. I’ll report back.

 

[dogememe]

Where is the new o2 sensor from? Lots of fake parts in the industry… even if you buy from a reputable parts store. I’ve caught people returning $20 eBay o2 sensors in the box of a $200 sensor they just bought… think about how many we don’t catch. Then the next person who buys it gets a fake part.

 

[JoeDirt41]

Where is the new o2 sensor from? Lots of fake parts in the industry… even if you buy from a reputable parts store. I’ve caught people returning $20 eBay o2 sensors in the box of a $200 sensor they just bought… think about how many we don’t catch. Then the next person who buys it gets a fake part.

Yeah that’s why I don’t buy parts on eBay anymore. I got this at Advance Auto. Came in a sealed box.

 

[The Critic]

Is the air/fuel ratio sensor soaked in oil from the usual cam carrier oil leak?

 

[Chris142]

What temp is the coolant temp sensor telling ? Is it close to actual temps or showing something much cooler?

 

[JoeDirt41]

Hey guys, sorry for the lack of updates, been busy with the start of school. To respond to the previous replies, the sensor is not covered in oil, and the temp sensor reading appears to be accurate.

In the past few weeks, the lights have remained on, however the driveability issues seem to have subsided. The car is not acting like it’s gonna stall when taking off from a stop anymore.

My mom has to take a trip in the car across the state this weekend and she wants it to be in good condition for the trip, so I discussed her options with her. I told her we could let it go and she could drive it as-is, or I can buy a MAF sensor and replace it and hope that it is the issue, or we can take it to the dealer and see if they are able to diagnose it.

She said for the cost of the sensor, let’s just go ahead and replace it and see what happens. I was able to get an aftermarket Denso sensor with lifetime replacement from Advance Auto for $138 after using a discount code and some SpeedPerks. Got it installed over the weekend, so we’ll see what happens.

 

[mk378]

Check IAT reading also is it correct? Make sure she's not topping the gas tank off and flooding the EVAP system.