Accounting for Seasonal changes - Temperature, Fuel blends, etc.

Hey Guys,

So with the current calibration I have on my car (2018 Subaru STI), I did most of the work in colder temperatures during the winter and spring.

Recently I decided I should probably do some logging now that its hotter in the summer to see if things are still looking good as there are a few different IAT and ECT related compensations I wanted to make sure were working OK.

For the most part, everything looked good...but the one issue was that my AFR during a WOT run was like 0.5 to 1.0 AFR points lean of target at an IAT temp of ~85f or so, where as under the exact same type of scenario (eg: similar rpm, load, g/s, etc) with the same tune, but back when it was ~65f... The AFR was spot on.

Im not entirely sure if this difference would be attributed entirely to just temperatures & heat soak, or whether some (all?) of it could be attributed to different winter/summer fuel blends? If the issue is caused by summer temps or summer fuel blends... What would be the correct way to compensate for this? Perhaps the MAF IAT compensation table which currently seems to be all 0'd out even in the factory calibration?

Could it be that the issue is entirely something else unrelated to seasonal changes and I should be investigating elsewhere?

Any insight on how to account for this issue would be great!

If you were running SD I'd say your SD IAT comp might be off, but it sounds like you have a MAF tune.

The MAF doesn't read differently if the temp gets a bit hotter or cooler, so I don't suggest altering that table which is normally zeroed out.

A pressurized smoke test to check for leaks might turn up a pre turbo intake leak which is skewing your MAF readings.

Emissions and starting related additives in cold weather fuel have an impact on knock and often require ignition adjustment, but the stoich point doesn't change significantly with those additives.

Even if you were switching between E10 to E0 it wouldn't cause a shift in AFR as big as you're seeing, so I think it's time to compare fuel pressure readings from then vs. now. You may be dropping pressure from a worn fuel pump, corrosion, filter aging, or other issue.

If you've been using the same wideband sensor for a while, are you sure it's still accurate?

Hmmmm. Unfortunately I dont have a fuel pressure sensor on the car so no log of that currently or from before. Sounds like it might be a sensor worth having though. I do have an analog gauge attached to the FPR but obviously I cant reference that at WOT. Not sure if that would pick up on a potentially subtle fuel pressure drop like this if there is one at idle.

I'll check the car for any leaks and see what I can find.

In terms of the wideband, yeah I think it should be ok its probably less than a year old. It also seems to be aligning with what the front o2 sensor is indicating when thats in its operating range.