Solution Summary: The timing chain on Bank 1 had jumped 1 tooth on the crank sprocket. Ultimately, the giveaway was 30psi of reduced compression for Bank 1. (Bank 2 = 190psi, Bank 1 = 160 psi).
I've scoured every existing thread I could find, and I'm coming up short. I appreciate any help (or even just sympathy!). I just bought a 2009 Subaru Legacy 3.0R with 155k miles as a project to tinker with. The car wasn't well maintained, and it has CEL for Bank 1 misfires P0301, P0303, P0305. No other codes are thrown.
One advantage I have in diagnosing is that I also own 2 other 3.0R cars that run well. Swapping parts between cars to test functionality is pretty easy and economical.
- Occasional, but persistent misfires on 3 cylinders of right side (passenger) bank: P0301, P0303, P0305. The misfires at idle never really go away.
- You can feel the misfires, so I think they are real. However, there are not enough misfires to ever get anywhere close to stalling (RPMs don't even really dip down)
- Worst misfiring on cold-startup, and it's enough to throw a blinking CEL with some minor RPM fluctuation
- There are fewer misfires after warm-up and driving. Few enough to not throw a CEL, but they still exist as you can see "roughness" or misfire counts continue to go up at avg rate of ~0.5 - 1 per second per cylinder (ie. 5% misfire rate)
- The misfires (timing and quantity) appear essentially random across the three cylinders.
- During driving, there tends to be no (or far fewer) misfires unless coming on and off the throttle lightly while coasting.
- Car is very fast and runs smooth and hard on full throttle. Feels faster (likely lighter) than my other 2 cars. This makes me think I probably don't have a fuel pump issue, or a timing issue.
- The car never stumbles when you try to accelerate. Nor does it tend to stall when approaching a red light.
- Dash computer says 25.8mpg. That's way higher than my other two 3.0Rs
- Compression tested: Cylinder 1 - 195psi, 2 - 220, 3 - 200, 4 - 220, 5 - 200, 6 - (I didn't bother; too annoying) (Update: This symptom was a red flag I initially ignored. Gauge red high, but the delta for each bank was just too high even though it was '10%'
- No leakdown results as I couldn't figure out how to find TDC for each cylinder
- Short term fuel trim (STFT) tends to go to -14% at cold when misfiring is the worst. Once warm, it goes within spec (<10 for both STFT and LTFT). -14% STFT means the ECU is leaning out the fuel mixture. Why would it do this?
- Cold Vacuum is -19 inHg; Warm vacuum is -21 inHg (my other car runs -22 warm vacuum). MAP KPA reads ~30
- Vacuum drops / fluctuates by ~0.5 InHg during a bout of misfires. Unclear if it dropped simply because RPM dipped or because valve didn't close right?
- The VVT / AVCS angles for Bank 1 and Bank 2 appear to follow each other closely via FreeSSM
- There are no timing chain sounds, so its unlikely the chain jumped timing due to a failing tensioner
- The oil looked good (not metallic, not fuel smelling) when I changed it out after 5k miles
- The coolant expansion tank doesn't bubble at all, or look oily
- No oil had seemed into the spark plug holes
- No external seeping on headgaskets. No reason to suspect internal gasket leaks.
- Battery voltage and alternator charging voltages seemed normal
- Engine starts quickly. However, after sitting a week the car did crank much longer before it started.
- Fuel injectors do not appear to be leaking at the fuel rail o-rings
- No smoke coming out of the exhaust at any time
- Unclear what octane fuel the previous owner used from 62k miles to 155k miles.
* This may not mean anything, but unplugging the Bank 1 upstream O2 causes the car to stop misfiring (or stop reporting misfiring via OBDII). I cannot confirm whether this is a weird glitch, or whether the car isn't misfiring anymore!
Steps taken so far (all on just Bank 1) that didn't help:
- Previous owner tried putting in a new ECU (after other debugging, and obviously never found the root cause)
- Fresh oil change with my favorite: 5W30 Castrol Edge Synthetic with Subaru OE filter
- New Upstream/front Denso O2 sensor for Bank 1
- Swapped Downstream Bank 1 sensor from functional car
- New plugs
- Swapped coil packs from functional car
- Swapped fuel injectors from functional car
- Swapped VVT AVCS OCVs from functional car
- New ground straps to each valve cover
- Sprayed carb cleaner all around engine/manifold and RPMs never fluctuated (i.e. no vacuum leak around manifold and airbox)
- Swapped knock sensor from functional car
The classic H6 half-engine misfires! It seems like a strange coincidence for the right 3 cylinders to have a random misfiring issue, unless every cylinder had something in common. I'm having a hard time finding the common issue here.
- Are the valves on half the engine built up with carbon? Are they not sealing well? Worth trying seafoam and techron? I realize a leakdown test would have shown a minor valve seating or clearance issue better than a compression test. Is it worth buying a borescope to try and visually look for carbon buildup?
- Do the valves need adjusting (i.e. too little clearance after 150k)? Maybe they are already burnt? What are the odds all three Bank 1 cylinders developed a valve issue while Bank 2 runs fine?
- I think there is a single fuel regulator on the 09 H6, and two fuel rails had their own damper. Is the right fuel rail damper not working right? I guess I could swap the fuel rail from my other vehicle over to this one, but that seems like a real long shot.
- Am I missing something with the VVT? The advance angles / etc all seem fine, and the car drives hard on full throttle.
- Would an EGR issue affect both banks equally? Same thing with an IACV and PCV issue? Would they ever cause misfires?
- Is Bank 1 timing off by 1 tooth? The compression was about 20psi less on Bank 1 than Bank 2. That's only 10%. Maybe the intake or exhaust cam is slightly off? (Update - This turned out to be the issue!)
Any other ideas? I really don't want to spring for a $1400 JDM engine. It's just a ton of work replacing the motor or pulling the head off my existing one. But maybe I'll need to in order to make this car pass NY inspection.
Useful H6 Misfire Reference links:
H6 3.0 dead bank 1,3,5. - 1990 to Present Legacy, Impreza, Outback, Forester, Baja, WRX&WrxSTI, SVX - Ultimate Subaru Message Board
2003 Subaru Outback H6 random misfires all cylinders - 1990 to Present Legacy, Impreza, Outback, Forester, Baja, WRX&WrxSTI, SVX - Ultimate Subaru Message Board
Help needed!!06 B9T misfiring in Cylinder 2,4,6 - SB9T.com