Ive been thinking about the general advice for LTFT after AutoVE of the VE table, LTFT's should be -1 to -4%, because if positive extra fuel will be added in PE mode.
LTFT's only add fuel in PE, they do not subtract.
Which makes me think if the LTFT's are negative and the adjustment isn't made in PE mode then AFR will be rich than the commanded PE AFR anyway, by not compensating for the LTFT -% value. Which kind of defeats the reasoning behind not having positive LTFT's.
How ever, if you have positive fuel trims, fueling is corrected in CL and fuel also added and thus compensating when in PE mode which should be to the commanded PE AFR?
I also not that if the VE table is not as close to BEN = 1.0, then the predetermined (stock) spark value will not be at optimum. If you add to the VE table so that LTFT's are negative, the calculated g/cyl value is higher than the true value and thus a lower spark value is looked up.
Like wise if the VE table is lower and LTFT's are positve, the g/cyl value is lower than the true value and the spark value looked up will be higher. However, I think with dynamic fueling the PCM will go with the MAF g/cyl value and not the VE table in this circumstance.
Any thoughts?