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I was wondering if there is someone that can help me with this truck. It has a rebuilt s&j engine (5.7l 4 bolt main) that I have been fighting to get running well. It has had all new sensors put on it except one thing to note is I received a bad engine with a cracked block installed the first go around and it was ran for a day to work and then next day back to the mechanic so it may have affected the new o2 sensor put on the good engine but I'm not sure. Well this 2nd engine runs really rough when its cold out and the colder it is, the rougher it runs. It takes like 5 to 8 revolutions before it starts and it runs good for about 2 seconds then it just hits the floor, back firing and missing like crazy. Another thing it does is once in a while the volts will drop from 14 down to 10 like the alternator stops working for 1 second then it jumps right back up. I've tried 3 different alternators and nothing seems to change it. It's like something gets turned on for 1 second thats shorted to ground and then shuts right back off but I'm not sure what it could be or if it has anything to do with the other issue. I've put new plugs and wires, rebuilt throttle body with injectors, map sensor, distributor, spark control module, knock sensor with pigtail, ignition coil, fuel pump, both coolant sensors, egr valve and solenoid, iac valve with pigtail, ecm and 3 different alternators. It really lacks power when it is warmed up and its getting bad gas mileage compared to the original engine. I used a red devil river aldl cable and tuner pro rt to pull some data off of it but I need help figuring out why the blm is so low. Is it because of my o2 sensor or what could it be? What could be causing my voltage to drop off like that? I can't get it to drop voltage with the tuner pro connected either. Any help would be greatly appreciated as I am new to this tuner pro data stuff. Thanks! truck2.xdl

I have a 1999 K2500 regular cab with the 5.7 and 4L80E transmission, 285s for tires. I got 14mpg bringing it home, which was about 250 miles. I've been driving it around and I'm getting about 9-10mpg. I have read that an exhaust, cold air intake, and new 02 sensors. I plan on replacing all of the 02 sensors so I was wandering how many upstream and downstream sensors there? And is there any other ways to improve my fuel economy? Thanks