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Thread: 1999 Silverado "Banks too Lean"

  1. #1
    Junior Member
    Join Date
    Feb 2008

    Angry 1999 Silverado "Banks too Lean"

    I have a 1999 Silverado and the service light keeps coming on. The truck has 137,000 miles on it I had changed the spark plugs when all this started. I have change the mass air flow sensor, new fuel filter all new oxygen sensors and have checked for any vac leaks using soap and water and sprayed carb cleaner around the intake. The truck runs great and idles good and the gas mileage was poor before I had changed the spark plugs and at that time the gas mileage went up and it still is running good. Its just the service engine light will no go off. Codes received are P0171 and P0174. Both banks too leanů any ideas?

  2. #2
    Senior Member 72'CaminoKid's Avatar
    Join Date
    Jun 2005
    i want to say maybe something with hte cats but not positive
    99 suburban xlt
    96 silverado
    91 wrangler (my baby )
    08 Honday shadow 750 (my mistress)

  3. #3
    Platinum Member
    mudseeker's Avatar
    Join Date
    Feb 2006
    i would check for exhaust leaks before your o2 sensor. also the cat like camino said. what brand of o2 sensors did you buy? most non-oem sensors are junk right out of the box.
    <----just your average 5.3

  4. #4
    Senior Member
    Join Date
    Jan 2008
    how would one check a vacuum leak with soap and water?

    i beleive the first set of o2 sensors regulate fuel injectors with air intake.

    2nd set check to see if cats are working. i think.....
    1996 Z71 white shortbox tinted
    350 30 over
    cats and X pipe
    Alumhds ported 210 cc runners 62 cc chambers 2" int 1.550" ex
    crane lift; 580/580 duration; 216/222
    4L60E w/shiftkit

  5. #5
    Senior Member NCroadracer's Avatar
    Join Date
    Apr 2007
    Most likely it will be your intake seals leaking vacuum... Spraying the intake has little to no effect on it, unless you can watch the Ho2s readings with a scan tool. they usually leak at the bottom of the ports where you can't really spray anything.

  6. #6
    Junior Member
    Join Date
    Feb 2008

    Banks lean

    Have you checked the fuel pressure yet? That would be where I would start a weak fuel pump causes this very problem and sets the same codes.

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