7747 ECM? If so did you also swap the CALPAK for a V6 CALPAK? If you didn't you are likely seeing a cyl count mismatch. Soft codes won't be stored if you turn the key off, so you need to stall the engine if you want to pull a code set while running. Disconnecting power from the coil will stall the engine. However a cyl count mismatch should come up without the engine running.