:brainfart
Let me see if I can explain this better. Looking at the pics while I was editing the size made me think that I may not have explained it correctly. And rereading the post again. The math always goes fast in my head, so putting it down takes a bit. I never showed my work in class, just looked at it and knew. So bear with me.
And as I was leaving the shop yesterday, I had a thought to grab my small scanner when I was taking stuff out but ignored it. If I had it here at home, I could illustrate what I mean using a NA car that is MAP run with video. Seeing is believing.
The ECM calculates fuel need based on engine speed, engine temperature, ambient temperature, barometer readings, engine load, throttle position, AF sensor and MAP. It calculates a mass air flow using the engine speed, IAT and MAP data in correlation with the barometer measurement. The calculations for fuel change as any of the data from a sensor changes. Change the air temperature, and flow is recalculated. Change baro pressure and again it changes. Change the manifold pressure, guess what, new flow calculation.
The MAP sensor data is at issue. The MAP sensor voltage curve is preprogrammed in the fuel/ignition map of the ECM. It can't be changed unless you know where to find the lines of data to change the ECM's math. Since it can not change the voltage characteristic in the lines of formula, it takes it as what it sees. As in the pictures posted. The voltage on the volt meter is what the computer uses to determine the pressure on the intake and there is no changing it. WE tried. Can't find it. So if the MAP sensor is failing, it will send improper voltage back to the ECM. In this case, it would be a voltage lower than what it should be. As seen in the pics, a .1V change effects pressure calculations greatly, about .5 psi. I know its not the BARO because quick easy math says the Barometer is correct. 4.5+10.3=14.8. Close enough. That eliminates the ECM, as far as BARO is concerned.
The low MAP value suggest low pressure. Low pressure indicates the engine is pulling in more air, or a higher mass of air. The base fuel map is designed to run at a specific level of injector latency to provide the proper amount of fuel to idle the engine at the speed it holds as a target rpm. The throttle position is closed, engine temperature is leveled and the AF sensor functions but the MAP is telling the ECM that pressure is low and increase fuel to match the air flow. The AF sensor then puts its opinion in the mix and the ECM is pulling fuel to get back to stoich.
You see, the whole time the engine is actually running at a pressure equal to close to 5.5-6 psi, but the ECM is calculating the air flow at a lower pressure. The MAF reading is a calculated measurement and the measurement shown in the post is too high for a 2.5 at idle. The ECM adds fuel based on the calculated flow and adds fuel causing a rich condition. The unburned fuel lowers the AFR and the computer has to change its fuel again. This would create a fluctuating learning curve for fuel needs.
If the MAP is indeed sending the correct information, that would indicate a restriction in intake of air, a clogged air filter or PCV for instance. But that was already checked and corrected. So since we know its not a restriction, that leaves the data as incorrect which causes a rich combustion and crazy fuel trims that ultimately damaged the CAT.
I hope that makes since because I reread and edited this many times to make sure I could get the point across without taking up a lot of space with varying math calculations that would only make things worse.
Now I have a headache from over thinking how I was going to explain it.
On the flipside, if there is something wrong with the ECM calculation, say something that is causing a higher resistance to current flow and lowers the voltage the ECM is seeing, that would mean something wrong with the harness or the ECM itself. On the other post, "I Screwed The Pooch", I already made mention that the ECM may be bad.
http://www.subaruoutback.org/forums/66-problems-maintenance/52953-i-screwed-pooch-7.html#post514979
