I had a "lopey" idle for about a week after I installed mine. My best guess was that it took 6 or so starts & runs to update the LTFT (and whatever other adjustments the ECU "remembers"). It's been rock-steady since.
You should have simply removed the vacuum line to the FPR, NOT plugged it.
I have tracked virtually every tank of gas since I bought my rig. Comparing the 6mo before the MAF and the 6mo after:
Best Tank - 15.2:15.5
Average - 11.0:12.9
Median - 12.9:13.4
I tend to experiment in different areas over time (both before and after), but no major changes that would explain the MPG improvements (other than the MAF).
In looking a the actual data since the MAF installation (12.5, 11.8, 14.6, 14.1, 13.5, 13.9, 13.4, 7.0, 13.7, 13.0, 12.8, 11.2, 15.5, 13.5, 12.5), it looks like the first two tanks had a little lower mileage. Could have been me playing with WOT air flows, though, rather than something intrinsic.
Do you notice difference when you're driving down the freeway?
Hants (KD0CXL) 97 LX450 Slee Armor, 285 BFG/AT, OME 2.5", Altitude Compensator, Kimberley Kamper
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