Drop in MPG's

[Junster]

Cold is really hard on 02 sensors. They need to be hot to work right. If they don't send the right mixture info quick enough the ecu will go to a open loop and ignore it. The mix will be like a carb thats got a stuck choke. This would explain the EGR problems many seem to be having. Running rich clogs the EGR. Also extended "warming up" will. Do these motors have a 1 wire or a 3-4 wire 02? The one that's on the engine side of the catalytic converter. The one aft just reads how the cat is doing. OBD2 vehicles always have at least 2. 1 wire 02's depend on exhaust gas temp alone to warm them. 3 wire 02's have a heating element in them, 4 wires the extra wire is a extra ground for the sensor cause exhaust hangers are rubber and sometimes the exhaust pipe itself has a very poor ground. If it's a 1 wire and replacing it doesn't help try wrapping the exhaust with header wrap about 6-8 inches above and below the 02 sensor. One more thing, did you accidently unplug your 02 when doing the axles? If you replace a 02 disconnect the battery. They can be sensitive and they are expensive.