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What to look for if PO changed ECM program?


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What would be some tells that the previous owner changed the program in the ECM? I have black smoke come out the exhaust when I accelerate, it's enough that it leaves residue on the rear bumper. I've had the truck for about 6 weeks. Since I put a new thermostat in it seems as though the mpgs have been declining. Prior to the change of the thermostat, it would take forever to warm up, now it works like it should. Also I got consistant 18 MPG, hand calculated, now 16 seems to be the best. I would think that since the engine is warming up completely, the burn would be better therfor providing better mpgs.Thoughts?

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