Question: My 2000 Chevy Silverado 1500 4x4 Z71 has the 5.3-liter engine and an automatic. It has 98,600 miles on it, and I’ve added an Airaid throttle body spacer and intake tube, K&N air filter, and Flowmaster 70 series muffler with split duals. Currently I get 12-14 mpg in town and 15-16 mpg on the road, if I’m lucky. The truck used to get 19 mpg on the road and 16 in town, and I don’t know what the problem is. I’ve changed the plugs and wires and had the fuel system cleaned, with no improvement. I had a machine plugged in and it didn’t show any codes, but shows an intermittent miss on two to three cylinders at low speeds. I think I have a miss at 55-60 mph or so. How can I improve the truck’s fuel economy?

Answer: It’s never easy to determine the cause of a loss of a few mpg -- there are so many variables. But there are some things you can check. Did the change in fuel economy occur shortly after the addition of a specific performance add-on? The mystery misfire is a red flag. I’m not sure how the person who plugged in the machine (presumably a scanner) determined there was an intermittent misfire with no trouble codes. But if there is a true misfire -- at high speed, low speed, or in between -- have that diagnosed and repaired before you do anything else. Have you replaced an oxygen sensor lately? With over 98,000 miles on the clock, it may be time. Your 5.3-liter has four, one for each engine bank in front of the catalytic converters, and one for each bank after the cats. These sensors, especially the two up front, are what tell the PCM whether the exhaust gases are rich or lean (low or high oxygen content), so it can control fuel delivery accordingly. Oxygen sensors have a significant effect on fuel economy, and they can get lazy as mileage accumulates. Last, engine compression may not be what it once was. We hope that’s not the case, but if it were, a few mpg would not warrant major engine work. I suggest spending an hour of the shop labor-time rate at a Chevy dealership, and have the techs give your truck the once over. Along with the fuel economy issue, mention the misfire you’re experiencing and request a performance check of the O2 sensors.

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