2010 LGT misfire at idle and approx. 4.5 psi PLEASE HELP!

Hello all, new member here! I must say how nice it is to have such an active community for these cars!

So, right to the meat of her:

I'll start by saying that the car is stock with the exception of an invidia q300 catback and a lightweight flywheel. It's also still on stock map.

I bought the car in february of last year with 100k on it, it now sits at 126k.

It was in need of a clutch when purchased so I snagged a stage 2 kit with lightweight flywheel early last year. The car ran excellent until about 2k miles ago when on the way home from vacation with the wife it began to misfire at higher boost (8.5ish psi), and also picked up a slight stumble at idle. This concerned me as i know how finicky turbo engines can be and so the car was parked for the last 2 months, only being driven if absolutely necessary. As an aside, the car has received regular 3500k mile oil changes with rotella t6 that included oil and air fliters at the same time. The misfire is only ever p0303 and p0304. When it first appeared it was accompanied by a code for the bank 2 OCV, exhaust. 2 weeks ago I had time on the weekend and decided to replace the clutch, at the same time closely following qauzimotos misfire guide on nasioc. Ill put those findings further down.

Also worth noting: When i purchased the car I gave it a solid once over, replacing all drainable fluids as well as seafoaming the engine just after the intercooler.

In regards to quazis guide, here are my findings:

1. During clutch replacement plugs were also replaced with oem ngk iridiums gapped at .028. Old plugs appeared to be factory and were a bit burnt.

2. Also replaced fuel filter during clutch job, and have run one bottle of seafoam, and one BIG bottle of lucas fuel injector cleaner over 2 full tanks.

Have not cleaned fuel injectors but did swap driver to passenger sides.

3. I have not checked valve clearance but compression was solid with a low number of 155 on cylinder 2 and a high of 173 on cylinder 4.

4. MAF cleaned thoroughly from both sides with CRC maf cleaner.

5. While doing the clutch i chose to drop the trans rather than pulling the engine and as a result needed to separate the exhaust, but all nuts at the downpipe and the flange between the factory cat and the invidia catback had turned to dust. As a result I dropped the manifold from the head and removed the entire exhaust in one piece. Upon reassembly every component was inspected for dry rot or tears/holes, and I removed all lines from the front of the engine in order to do a couple of the other things on qauzis list. All gaskets/ o-rings were replaced with new oem.

6. Also during the clutch swap was the installation of an HKS circle earth kit, replacing all factory grounds/ground straps. During this replacement I found that the strap from the head to the underside of the passenger frame rail was broken. I had really hoped this was the culprit but alas, it was not.

7. Cam timing was checked per service manual during clutch job. Well within spec, sensor pick ups and reluctor wheels cleaned as well.

8. Pulled PCV after engine had warmed and was greeted by the faintest wisp of blow-by. Certainly not enough to cause this issue.

9. Car was seafoamed when purchased, I will do again if another member thinks it wise.

10. Car does have a lightweight flywheel however the condition doesn't seem worsened by it. It will on occasion smooth right out and hold factory idle, misfire free.

11. During clutch swap coil packs were swapped from drivers to passenger side, no codes on cyl. 1&2, codes persist on 3&4.

12. Teed into fuel return line (after rail&regulator) and found 37 psi at idle, 42 psi with vac line off of regulator. A quick blast with the guage under the wiper and part throttle through 1st and 2nd showed a max of 54 psi at 4ish psi boost.

13. Front and rear O2 sensors were replaced with new oem units during clutch swap. Thank god for torches lol

14. A visual inspection of the crank gear confirmed its in pristine shape. The timing belt itself looked almost new with zero visible cracking on either side and strong tension.

15. Not applicable as I only throw codes for cyls. 3&4.

A couple more notes: The faulty OCV was confirmed via FreeSSM and cam angle viewing. However, I do not own a Tactrix cable and my buddy with one is busy with work for the forseeable future. That being said, the OCV was replaced with an off brand amazon part, and the code has not returned since installation. I think this cheap part may be the culprit, if anyone else has input on this all advice is welcome. As you guys can see i've come at this thing 15 different ways and id rather not just start throwing parts at it. I own most tools necessary for troubleshooting/ general work so if theres something i may have missed checking let me know!

Also, the battery was reconnected while the ecu was disconnected from the BCM, in order to release the parking brake while removing the driveshaft. I believed this to be the cause of these numerous communication codes im also reading. Sometimes i can drive 45 miles without them coming on, other times they come on when i start the car in the morning for the first time. I've done a battery disconnect overnight and that made no differnce.

Here are all codes currently stored:

VDC Codes:

1- U0110 Lost comms with drive motor control module "a"

2- U0107 Lost comms with throttle actuator "a" control module

3- C10CA I understand this is related to the electronic p.brake recall for 5th gen legacys and outbacks?

4- C0057 ECM Communication Circuit

Im also getting code U1221 for the body control module, im not sure what this is other than a communications error?

And also a code 1A for drivers airbag failure. This has come and gone since purchased.

I've checked every fuse and relay with a multimeter and 12volt variable power supply to ensure relay function. I'm dead stumped on this thing, is there a chance these codes are related to my misfire issue? I must note that with exception of no cruise or tcs due to the misfire absolutely every other part of the car remains fully functional with or without codes. The misfires come sometimes one code sometimes both at once. When the other codes comes its always all 6 at once. Any help would be greatly appreciated, there are some really bright minds on here!

While this issue is FAR out of my league, I too have a 2010 LGT that had a misfire problem (cylinder 4 only). The ignition coils for cylinders 1-2 had been replaced within the past 2 years, before I bought it, but after inspection of the ignition coils for cylinders 3-4 I found that both showed delamination of the metal ?core?, cylinder 4's ignition coil was much worse than 3.

I replaced the spark plugs with NGKs and both cylinder 3-4 ignition coils, along with the timing belt, pulleys, and tensioner (had 106k mi on the odometer, it was due). I found that the drivers-side intake cam was off its alignment mark by one tooth and one of the little timing belt guides was out of alignment, slowly shaving the rubber off of the old timing belt. Since then, the engine has not misfired (knock on wood!)

I doubt any of this is helpful, but maybe inspect the condition of the coil packs. I assume that the timing belt was serviced and is in good shape, timing marks are aligned, etc. It seems that you might have multiple issues that make it difficult to diagnose each individual one. I hope for the best!

Sounds like you have 2 different issues going on.

First off here is a copy of the FSM - That should help you with your Comm DTC's - let me know if you gave any trouble getting it.

https://drive.google.com/drive/folders/1j7ol6XrohM_Vj1rvL5Xz7ryNt6p-7587?usp=sharing

Your compression seems good, so its not likely a mechanical problem

Cylinders 3 & 4 are on opposite banks, so its unlikely your replaced OCV is causing the issue. The cams don't share one OCV. I don't recall if freessm has a misfire counter. If it does pull it up and see if one or the other cylinder is really the miss. It's possible the ECM is picking up something that isn't there miss fire detection isn't always perfect.

Start with your fuel trims and see if they are off - maybe a boost or intake leak - very common on these cars. See what your MAF is reading at idle - warmed up should be about 2.6 g/s or so. Go through the DTC chart for p0303 + P0304 and see what comes of it.

Sounds like you have 2 different issues going on.

 

First off here is a copy of the FSM - That should help you with your Comm DTC's - let me know if you gave any trouble getting it.

 

https://drive.google.com/drive/folders/1j7ol6XrohM_Vj1rvL5Xz7ryNt6p-7587?usp=sharing

 

Your compression seems good, so its not likely a mechanical problem

 

Cylinders 3 & 4 are on opposite banks, so its unlikely your replaced OCV is causing the issue. The cams don't share one OCV. I don't recall if freessm has a misfire counter. If it does pull it up and see if one or the other cylinder is really the miss. It's possible the ECM is picking up something that isn't there miss fire detection isn't always perfect.

 

Start with your fuel trims and see if they are off - maybe a boost or intake leak - very common on these cars. See what your MAF is reading at idle - warmed up should be about 2.6 g/s or so. Go through the DTC chart for p0303 + P0304 and see what comes of it.

Second, a gross oversight on my part in not realizing the even and odd cylinders shared heads... That complicates but also simplifies matters. The last two or three days the misfire has only been showing on three, though it never showed on 1 or 2 at any point.

I will check fuel trims asap, would the simple long and short term trims I read on my Bluetooth logger be sufficient? My rosstech vcds cable won't connect with FreeSSM.. If not are there any cheap cables that work? I'm really just trying to avoid buying an openport as it's a bit pricey at the moment.

Any known problem areas to check for as far as boost leaks? I had everything apart and back together again when I did the clutch but it's very possible I missed some vacuum lines..

I will check MAF rate and log a bit on the drive to work tomorrow morning and post results on lunch if possible.

Thanks for the pointers!

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Here's a walk through for testing for boost leak on the 5th gen.

https://legacygt.com/forums/showthread.php/easy-5th-gen-gt-boost-leak-test-255163.html?t=255163&highlight=test+boost+leak

I keep saying this on most misfire/hesitation loss threads, but boost leaks can be a large cause for issues.

Also, I'm not sure if it applies to these LGT's as well, but I do know the turbo EJ motors in imprezas are sensitive to lightweight flywheels and the ECU will throw misfire codes if they are too much lighter than stock.

Do you know the weight of your flywheel? For my STI swap in my 2.5rs I didn't go lighter than a 15.5 lb flywheel due to this. My friend had a 12lb flywheel that would throw misfires.

@stewdogg thanks for the guide! I'll work through it this Friday when I've got time on my day off!

@Aphex28 while I do agree the car falls lower than it's targeted idle with the lightweight flywheel, it's a very noticeable misfire. I'm pretty sure it is the coil pack from cyl. #4 that I swapped to #3 because now that I've done that the misfire code has for the most part been only for cyl 3. Fingers crossed, I'll be buying a coil Friday if the boost leak test is a miss.

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If you are interested. at some point someone posted this tool on the forum for finding boost leaks.

https://smile.amazon.com/gp/product/B0825WNGP3/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1

@Fingers crossed, I'll be buying a coil Friday if the boost leak test is a miss.

Rock Auto had NGKs for sale when I needed them, but it looks like those parts are de-listed. It looks like you can order them directly from NGK, so I suggest you do this versus going with one of the other brands.

That being said, my cylinder 4's ignition coil was in MUCH worse shape than cylinder 3 (cylinders 1-2 were replaced 2 years ago), so I wonder if it is related to the potential cooling issue,

https://www.flatironstuning.com/torque-solution-cylinder-4-coolant-mod-ej20-25

It really seems unlikely to me that Subaru would go through so many iterations of the EJ engines with this being an issue, but if anyone has some conclusive evidence I'll order the kit tomorrow!

If you are interested. at some point someone posted this tool on the forum for finding boost leaks.

 

https://smile.amazon.com/gp/product/B0825WNGP3/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1

That was me

@motorbreath I got home from work about 45 minutes ago and decided to pull both rear coils for at the very least, a cursory inspection and lo and behold number 3 is extremely delaminated, kinda hard for an electromagnet to hold a field when the windings are flapping in the breeze [emoji23]

As a side note, it would not surprise me that it is indeed a cooling issue causing this problem. The number 4 coil doesn't look to be too far behind, however 1 and 2 look fine. That kit from torque solution is interesting, does it just t into the heater core line? I think this problem is compounded by the proximity to the frame rails, especially in the rear. Even in 20° Michigan weather the frame/strut tower side was about as warm as the head on both sides of the engine.

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Also, a thank you to all who have commented here, I'd gone so far down the rabbit hole after swapping coils I probably wouldn't have considered them again.. Will be buying the rear two for now, while I save for okadas. Anyone have an opinion on whether they're worth the money? Not much of a difference in price from a set of oem Delphi units..

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