well, I answered my own question.... the TSB 09-74-21R with the ridiculously long title of "Cleaning Procedure for Carbon Deposit Removal to Address Engine Misfire DTCs, Black Smoke from the Exhaust, Lack of Power, Knocking (Ping) and / or Rough Idle Concerns" is crazier than I thought it might be.
Sorry, no walnut shells in this one.
Instead, we use Genuine Subaru P.E.A. (Polyether Amine) Carbon Cleaner (p.n. SOA868V9166) and the “Carbon Clean Tool” applicator tools (p.n. SOA868V9430 or SOA868V9432)
Step one: remove the intake manifold and look for buildup, "Inspect inside the runners closely along with the cylinder head intake ports, plates and intake valves for any excessive
amount of carbon / soot accumulation. Clean any readily accessible components with carburetor / throttle plate cleaner prior to re-assembly." I guess excessive build up might be scraped off at this point? Maybe that's what they mean by clean with carburetor cleaner... scrape and spray. Dealer's choice on that one. After that, reassemble and road test... you need a working motor for the rest of it.
Step two: look for any DTCs and fix other stuff first so you aren't chasing a different problem, make sure the oil level is full, get the customer OK to fill the tank (it needs to be full for a later step)
Now the fun stuff:
the first part of the procedure is to log into the Subaru Select Monitor, and read a roughness counter. Now, right away, the procedure is confusing, as it says, in bold lettering, to only perform that for the 2.5 naturally aspirated motors only.... however, a later step in 4 pages says to compare "all other models" to the prior data from step 2... What they were trying to indicate here is that there is a specific limit (value) for roughness on the 2.5l motors... above that threshold perform the fix, below it don't - or - after cleaning it needs to be below that fix... the other models you "Verify the cleaning procedure results by confirming the 2 conditions below: Has the rough idle condition smoothed out? Have the Roughness Counter values become smaller than those recorded prior to performing the cleaning procedure?"
Next Step: after the first step of recording the roughness counter... if it's a value of 34 or more on 2.5l engines only, or any value for all other engines listed, pour one bottle of P.E.A into the fuel tank. Also, the fuel tank must be FULL.
Next step: you use the carbon clean tool and attach it to the purge port on the throttle body. This tool is a fancy hose and valve assembly... Now here is where it's amusing (to me at least): the flow rate is critical, and you can only achieve the critical rate by eyeballing the position of the valve at exactly 22 degrees... or wherever the flow rate is about 1 bottle of P.E.A per hour @ 2000 rpm as indicated from prior trial and error runs on other vehicles. Is 55 minutes "about" an hour where 50 is not? Maybe it's more 50 minutes is ok, but 20 is not. Dunno.
moving on...
Next Step: burn that carbon off the spark plugs... italian tune up baby!
Next Step: log back into SSM and look at the roughness data - here is where it clears up the first time you do this - you read the data, 2.5l motors the value needs to be below 34 for all cylinders, for all other motors it just needs to be smoother. How much smoother, no idea. Just smoother. If it's the first time you read the procedure and it's not a 2.5l motor, you probably skipped the first reading and will have no idea at this point if it's smoother based on the readout. The decision here would be interesting - call it done, or repeat the process as if it's not smoother?
Continuing this step: if it's not smoother, change the engine oil (but not the filter) and repeat the cleaning procedure a second time. There are four cleaning attempts total and the oil changes are only specified every 2 attempts (so before the second and the fourth attempt).
NOTE: there is a footnote there that indicates you don't repeat the step of adding more P.E.A to the fuel tank, but that isn't indicated in the main procedure steps to only add it once, so I could see more getting added if someone followed it line by line and didn't see the footnote.
Here are the claims information - gives you an idea of how long this can take:
The full procedure list.. note that one treatment is 7-7.6 hours, and four treatments would be 18.2-18.7 hours ... the intervals between cleaning attempts is 3.8 hours, then 3.5 hours, then 3.8 hours. That's because not every attempt has an oil change.
this is for setup, inspection, things that aren't the full cleaning procedure... other line items that can get tacked on.
maybe walnut shells would be easier.
