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You should not need another ECM. Most of us have started with 5.4 ECM vice 4.6 ECM. So I'm not sure if there is a diffrence between 4.6 ECM and 5.4 ECM. If you have a Wiring Shop Manaual you can check. I have one and will try to check tonight. As far as starting the truck, it should not have run correctly with the stock NA tune, the injectors and MAF were different. As far as engine harness goes you must have used your 4.6NA harness so the cutting was just to get the wires longer to reach new locations. Again others have done that with 5.4 NA harness with no problems. I used stock L/HD engine harness and plugged into NA chassis harness.
So I am surprised the engine ran correctly. Maybe sensors. Tune should take fuel/load maps from L and plug into NA ECM. L/HD does have 2 IAT's where NA has only one. The tuner must compensate for whichever one you are using. You can go either way.
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2003 F150 ExtCab/Step Side
Blown & Smiling Est 400rwhp/500rwftlbs
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