Evap Fuel Vapor Compensation

mrf582 · **Posted:** Thu Apr 04, 2013 7:04 pm

mowgli29 wrote:

The problem: random, creeping, "Fueling Final Base" lean-out during closed loop operation (cruising). FFB will start around 14.7, where it should be, then gradually lean-out into the 15 or 16:1 range, or even higher! Fuel trims then add fuel to correct for the lean FFB.

The theory: when the EVAP system is purging the fuel vapors into the intake, it leans out the FFB to compensate. Since our EVAP systems are non-functional, leaning out the FFB, when EVAP should be purging, actually causes our AFR to lean out, which is ultimately corrected by fuel trims.

Ryanr has the same problem. Neither of us have a functioning EVAP system. We have searched and tested, and this is about the last possible cause of our issue.

I found this post from JSarv talking about, seemingly, the same issue....yet there was no conclusion or resolution, and JSarv is no longer a Subaru owner.
http://forums.nasioc.com/forums/showthread.php?t=1847151

I also found these two posts on Evo forums about EVAP compensations:
http://forums.evolutionm.net/7636049-post1.html
http://www.evoxforums.com/forums/showthread.php?t=49938

So, are there EVAP-related fueling compensations that have yet to be defined? I have a feeling there are. Seems to me, EVERY OBD-II, EVAP equipped car should have them... I'd love to be able to disable them!! I'm available as a test-mule if anyone wants to try and sort this out!

Anyone seen anything like this on the 32-bit ECUs?
Any way to disable the FFB leaning out if the car has no EVAP system?

td-d · **Posted:** Thu Apr 04, 2013 7:59 pm

I started trying to find these for Andy, but work's gotten in the way... started working back from the CPC related parameter.

td-d · **Posted:** Fri Apr 19, 2013 2:02 pm

What I've found so far is a CPC based multiplier that adds at most up to 1gs to the calculated MAFgs - it is basically CPC duty cycle multiplied by 1 - so at 100% CPC duty cycle, you would get a maximum of 1gs added.

Not sure if this is part of the problem - I haven't heard back from Andy yet. At a range of 15g/s to 19g/s, 1g/s could be quite significant, however.

If you want to try and zero the multiplier, I'll dig it out for you.

mrf582 · **Posted:** Wed May 01, 2013 7:21 pm

I see, so at the most it'll subtract 1g/s form the measured MAF value and use that for fuel calculations instead?

MFB · **Posted:** Sat Feb 15, 2014 2:27 am

Looking for the needle in the haystack ! Any feedback by zeroing the maf cpc adder?

dadler · **Joined:** Fri Feb 28, 2014 6:34 am **Posts:** 2

Bumping an old thread, same question.

If found this in the Cobb tuning guide:

Quote:

Disabling EVAP/CPC Based Fuel Trims

Like most other road-going vehicles, modern turbo Subarus are equipped with emissions systems
intended to address evaporative fuel emissions from the fuel tank over time. The factory ECU contains
an active correction to commanded fueling based on when this system is operating, presumably due to
the assumption that the evaporative emissions that are being recycled into the engine are fuel vapor that
will richen the AFR if compensation is not made. This correction has been found to cause an
undesirable leaner-than-stoich AFR target in closed loop. If the purge system has been altered or
removed for a motorsports application, you may disable the fueling target correction by setting the
“Evap-Related Fuel Adder Compensation” table to 0.0. A value of 1.0 will retain the AFR target
correction

The table in AccessTuner Race has the following description:

Quote:

Evap-Related Fuel Adder Compensation

This is the compensation to the evap-related fuel adder. This table's multiplier is applied to the evap fuel adder which is then applied to commanded fueling final. A value of 1.00 in this table retains the factory evap fuel correction. A value of 0 in this table eliminates the correction entirely.

I don't find this table (well, value) in any of the RomRaider defs ('14 STi).

This is separate from the CPC Valve multiplier. There is a separate table in AccessTuner Race for this:

Quote:

MAF Adder (CPC Valve Multiplier)

This value is a multiplier which determines the level of an airflow adder that is applied to current airflow. This adder is related to the CPC valve duty ratio and is typically less than 1.0 g/s.

The CPC Value Multiplier is set to 1.0 in an OTS map.

I have "Evap-Related Fuel Adder Compensation" set to 0.0, and CPC Valve Multiplier set to 1.0, but I am not confident that this will prevent the evap system mucking with fuel/MAF via compensation. I still have the evap hardware installed, but the orientation of the expansion canister in the engine bay is different--which seems to affect the quantity of vapor allowed into the manifold when the CPC valve is open.

This part of the Subaru ecm seems to be poorly understood.

RomRaider

Evap Fuel Vapor Compensation

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