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Still alive... both myself and my B9. But I graduated to a 2020 Ascent last summer and the B9 has become the daily driver for our au pair, but there just hasn't been any daily driving during the pandemic, i.e. nothing to diagnose, fix, etc.... UNTIL NOW.

I have to say, I'm pretty proud of this one.

Problem: Single cylinder misfire

DTC: P0301

Troubleshooting Sequence:
#1 - Cleaned MAF sensor & checked coil pack connector tightness on all cylinders. Still misfiring only in Cyl 1.
#2 - Swapped 1 and 2 coil packs. Still misfiring only in Cyl 1.
#3 - Checked gap and swapped 1 and 3 spark plugs. Still misfiring only in Cyl 1.
#4 - Replaced spark plugs. Still misfiring only in Cyl 1.
#5 - Tested output signal to Cyl 1 at the engine harness (behind cylinder 5) from ECM D8 in ON (no-start) condition. +9.47 VDC, OK.
#6 - Tested resistance between ECM D8 and fuel injector (FI) 1. <0.2 ohm, OK.
#7 - Checked FI 1 harness for short to ground on pin 1. Open circuit, OK.
#8 - Checked output signal to all FI's at the FI connector. All tested +9.52 (+/- 0.5) VDC, OK.
#9 - Checked resistance across all FI connections. All tested 14.2 (+/- .1) ohms, OK.
#10 - Inspected/cleaned FI's 1, 3, 5 (also replaced o-rings) & swapped fuel injectors 1 and 5. Still misfiring only in Cyl 1.
#11 - Listened to FI's 1, 3 and 5 using the "screwdriver trick" (tip on the injector, ear on the handle, listen for clicking). Normal pulse on FI's 2-6, but no pulse on FI 1.
#12 - Checked for rodent damage to wires with inspection camera. No visible damage.
#13 - Removed, inspected, cleaned and replaced Bank 1 oil flow control solenoid. Still misfiring only in Cyl 1.
#14 - Removed, inspected, cleaned and replaced Bank 1 Camshaft Position Sensor. Still misfiring only in Cyl 1.
#15 - Checked connection on Crankshaft Position Sensor and gave sensor a few "technical taps." Still misfiring only in Cyl 1.
#16 - Banged on the steering wheel a few times. Still misfiring only in Cyl 1.

At this point, I have come to the realization that 99.9% this is an internal fault somewhere on the ECM, i.e. the one result I was hoping wouldn't arise because that was going to mean a trip to the dealer to reprogram a new ECM. So I would like to give a special shout out to "Druboi" over on the Ultimate Subaru Forum for inspiring me to do the following:

#17 - Pulled the ECM and examined the board... no obvious indicators of damage, e.g. burned traces, blown caps, broken joints, etc. Took a closer look at IC608, specifically the solder joints, and lo-and-behold, one of these things are not like the other:

2357


Notice the bottom joint on the right side (just above and a little to the right of the "8")... all of the solder joints are clean, rectangular joints, but this little guy on the lower right is a bit more rotund than the others. I take the tiniest screwdriver I can (IC608 is <1 cm across) and touch all of the joints, and while all the rest were nice and solid, the joint in question has just a slight amount of give to it. So I go back to the B9, plug in the ECM (leaving it out of the case), and start the engine. I can clearly hear the misfire, so I take a little plastic tool and apply a bit of pressure over the solder joints on the right side of IC608, and voila... MISFIRE STOPS IMMEDIATELY. I let go, and the misfire returns within a few seconds. Culprit identified... shut everything down, take the ECM back inside, heat up the soldering iron with the finest tip I have to 375F and ever-so-carefully, I touch the tip to the solder joint for a split second to reflow the joint. For good measure, I reflow the next 3-4 joints to provide a little extra strength. Back to the car, plug it in, start it up.... DTC is gone, engine is humming beautifully, and my au pair is a happy camper.

DIAGNOSIS: Cold solder joint on IC608.

FIX: Reflow the joint.

EPILOGUE: Once you have confirmed no FI pulse in one - and only one - cylinder at idle and ruled out any wiring/connection issues, chances are you have an ECM fault... don't rush out to the junkyard to get a new (used) ECM, check IC 608 for any poor connections and reflow the joint(s).

253k miles and the B9 still running like a charm (save for that damn HG leak which I still haven't gotten around to).
 

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Nice job!

This is a 'common' cause of misfire on subarus in general, not just B9. Outbacks and legacys have the same rare issue. But I bet they are all cold solder joints.

Again, good work!

Tom
 
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