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  1. #1

    Default Catalytic Converters

    I just had my Cat replaced with a Magnaflow (not a high flow cat). I'm currently getting 12.5 mpg. Would a new cat (old needed replacing, honeycomb was in the muffler) up my mpg's?

  2. #2


    Did your MPG go up?

  3. #3


    just filled up yesterday. Just asking to see.

  4. #4
    Sr. Apprentice
    Join Date
    Jun 2013
    1391 N Jefferson St., Anaheim CA 92807


    The catalytic converter in a vehicle’s engine plays a crucial role in maintaining its efficiency. Essentially, the converter’s function is to burn off excess hydrocarbons in the exhaust system, which makes the engine run clean.If your car’s catalytic converter might have a problem, checked it as soon as possible.

  5. #5


    You might see an increase, it really depends on how bad your old cat was.

  6. #6


    Quote Originally Posted by tbplus10 View Post
    You might see an increase, it really depends on how bad your old cat was.
    Chunks of the honeycomb fell out of the muffler.... pretty bad imo lol!

  7. #7


    Your truck doesnt have an after cat sensor if I remember correctly, so the only possibility is if it increases the backpressure slightly and improves low rpm torque. If it does improve you wont see much.
    Thats real low mileage, from the picture it doesnt look like you have real big tires, and Im guessing you dont haul trailets or huge loads all the time.
    Do you have a heavy right foot?

  8. #8
    Jr. Engineer Jamm3r's Avatar
    Join Date
    Dec 2012
    Minneapolis area


    No, however, there's a pretty good chance that you have a leaking fuel pressure regulator which contributed to the early failure of the cat.

    You have the 5.7, right? Fixing a leaking fuel pressure regulator will improve the MPG, by a lot. They leak into the intake manifold where you can't see it, then the truck runs rich and burns up the cat.
    Minneapolis area - 1997 K2500 regular cab long bed + 8.5' Western Unimount plow + modified transmission + 2nd battery + modified camper charge circuit + 1971 Cayo camper -and- 2004 4x4 Suburban 2500 8.1 + Maxbrake controller + 2nd battery + modified trailer charge circuit + Reese receiver, pulls 30' Airstream trailer

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  10. #9


    Thats along the lines of what I was thinking Jamm3r, something more than the cat causing low mpg, and as you noted causing an earl and for a cat what could be considered a catastrophic failure. A rich condition would do this.
    It would be interesting to know if prior to the cat burning out it had gotten hot enough to glow.
    I had an early 90's k3500 dually that looked like it had neon lights under it from a rich condition, after a few mile it would glow enough to reflect off the pavement. I didnt catch on until after the third person commented I had left my neon lights under the truck on, it was also giving me clues like poor mpg and heavy fuel smell in the engine oil.

  11. #10


    My brother-in-laws 1996 Chevy 1500 2wd with the 5.7L Vortec gets around 18 mpg or so...he also has a heavy foot

    2009 Chevrolet Silverado 1500 LT Z71 Crew Cab "All-Star Edition"

    Line-X'd Bed
    Line-X'd Custom Brush Guard(Heavy Duty)
    5.3L Flex Fuel
    275/65R18 Toyo Open Country AT2's
    2" Rough Country Leveling Kit(want to get the 2.5 RC kit)
    EGR In-Channel Vent Visors
    Exterior PLUS Package
    Interior PLUS Package
    K&N Drop-In Air Filter
    Cat-Back Dual Exhaust
    Black Bowties
    Diablo Sport Predator(AFM Disabled)
    Towing Mirrors
    Next Planned Mods
    Blacked out Tail Lights
    Aftermarket Rims

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