First, let me say that when I check gas mileage I do it over at least a thousand miles of travel, so I think what I found here is accurate.
I have an '87 F250 300 fuel-injected truck. It has been rebuilt twice, the last time 10,000 miles ago, and it runs great. There were no other changes made to the engine during this test.
In '06 I changed it from a straight shift (T-18) to an automatic (C-6). At the time, I also changed the computer to one for an automatic transmission. (I know that the C-6 has no computer controls, but the computer for the automatic truck has slightly different idling speed and also the engine doesn't die when you put the tranny into drive). My gas mileage with the auto. dropped to about 12.5 mpg.
Three months ago, I switched computers back to the one from a straight shift transmission. My gas mileage immediately changed to 16+ mpg. Thinking it was a fluke/miscalculation, I went back to the auto. computer. Gas mileage dropped to 12.5 Back to the straight shift computer, the mpg went back up.
The truck ran perfectly with both computers. Both computers are "good." But one gives significantly better mileage. So, if you're getting poor mileage and you've got a spare computer laying around . . . .
Specificly which program codes are on those ECU's?
If the ECU for the automatic was intended for a computer controlled auto it would be interesting to see what error codes it was throwing. There are several codes that will not trip the CEL. Willing to bet that without a good return signal from the tranny the ECU used some defaults that are the cause of your poor mileage with it.
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1990 F-150XLT Lariat Stillon the original engine with no squeaks or rattles at over 300k miles
4.9/M5OD/3.08
16.89@77.09
145 rwhp, 272 rwtq
2001 Lightning #127
13.18@104, no chip, no pulley, no nitrous
Confuscious say "Man who have no use for jackstand have promising career as jackstand".
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