ECU Flash

On ECU flash; under the fuel injectors, it is labeled as A, B, C, D. (picture attached)

But my question is which fuel injector is which?

I know what spark plug goes to what cylinder but in relation to ECU Flash; which fuel injector goes to which cylinder?

Reference: 2004 Subaru Impreza WRX STi (USDM)

Does the factory workshop manual or wiring diagrams have any clues?

If I were to guess, I would expect it to firing order A to D. Perhaps you can make a test and monitor the injector pulse widths with an oscilloscope to verify.

This is a great idea; and a good reason to start building my equipment list for EFI Tuning.

In regards to a tester to check for injector pulse widths? What recommendations can you provide so that I can buy the right one.

The oscilloscope should be relatively cheap to buy, as well as a multi-meter. So I'm looking for a sort of device that I can connect to the fuel injector plugs (actually any connection plug on an ECU wiring harness in the engine bay that I can test in real time for current/voltage? Would I simply use a multi-meter with alligator clip extensions?

An oscilloscope capable of measuring 12v-15v signals at 1Mhz would be adequate. If you want professional tools, consider a 2-4 channel Picoscope get one of the automotive kits with necessary adapters and cables. The kits include back-probing pins that you can insert into the wire-side of the connector and make contact while the connector is still attached. I like buying from https://autonerdz.com/shop/ they have a great support forum.

For the test, I suggest making a 10% change to the Injector A table at say 2000 RPM, then holding the engine at 2000 RPM (might need a helper), and measuring the pulse width of each injector. You will see which one is about 10% wider (a little less due to injector dead times). Return Inj A to 0% and change Inj B to add 10%.

This makes me think of a way to test without a scope. Remove 25% fuel from one injector in the idle range. The engine will run rough due to the likely misfire. Remove one injector connector briefly, and if it gets much worse, it's not that injector. The injector with the lean condition will be easy to identify as there will probably be no change.

Brilliant! And thank you David, for the thorough and thoughtful response. I'm going to try this out this week!

Cheers mate!