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Subaru STI - MAF scaling for new short block?

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Hi Guys,

I'm having a strange issue with a Subaru STI (2015) after re-building the engine and while using the stock maps.

Car is completely stock besides a closed deck block and forged pistons.

Compression rate should be as stock, although the head was surfaced at least once.

I'm seeing very large fuel trims during cruise (3rd gear, 3k).

I'm comparing the results to the logs I recorded before the block swap.

Logs are attached (see Air/Fuel Correction #1 (%) & Air/Fuel Learning #1 (%))

I'm looking at both the Learning and Correction

Before: the fuel correction was up to 10%

After: fuel correction is at 10-20%

What would be the best way to fix this?

I can probably re-scale the MAF sensor, but I would like to understand the reason to this change.

MAF sensor was replaced with a new one and no change was seen.

Tuning via ECUTek.


Attached Files
  • af590_0000-enc-WITH_RACEROM-AZ1N100G-AFTER-3k-cruise.csv
  • Attachments may only be downloaded by paid Gold members. Read more about becoming a Gold member here.

  • af590_0000-enc-WITH_RACEROM-AZ1N100G-BEFORE-3k-cruise.csv
  • Attachments may only be downloaded by paid Gold members. Read more about becoming a Gold member here.

Unless you changed the intake itself, you shouldn't need to recalibrate the sensor.

How old is the fuel in your tank?

Do you see any other driveability concerns?

With the nature of engine replacement a lot of things can be missed or damaged during the swap. I would ensure there are no vacuum leaks if you are seeing high fuel trims.

The intake is stock, the fuel is new and the car is being used (almost) daily, overall it drives exactly as before.

The only other thing that I noticed is that it retards timing (around 4 degree) during medium pulls. I assume that this has to do with the fuel trims, so I want to resolve it first, before moving into WOT pulls. DAM is kept on 1.0, so there is no significant knock.

Your comment about the vacuum leak reminded me that one of the vacuum connectors got broken during previous engine rebuild (at the dealers warranty, and they somehow "glued" it), I will check this area for leaks. Thanks!

Quick update - I finally did a vacuum smoke test today and there is a leak from the turbo inlet. I'll replace it with an after-market one, and hopefully this will resolve the issue.

Thanks for the help Philip!

Always boost leak test when have anything apart on these cars.

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