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low idle

Discussion in 'Chevy Silverado Forum (GMC Sierra)' started by Cros89, Jan 7, 2009.

  1. Cros89

    Cros89 New Member

    hey guys i would like you to give me some reasons why a truck would idle loe... it idles about 500 consistently, and sometimes when i stop or put it in gear it will drop to about 400 or 300. what could be the issue? the service engine soon light is on solid. i was thinking possibly an oxygen sensor that is bad? would that do it? i dont really know all too much though...
  2. chevygurl

    chevygurl New Member

    I'm pretty sure it would lobe with a o2 sensor out. Has it just started doing this recently, or has it always been low? Your idle will always drop when you put it in gear.

    My mom's suburban (5.3l) idles the same, but it always has. Are there any other problems, like a rough idle at times too?
  3. Springthing

    Springthing New Member 1000 Posts 100 Posts

    What code comes up from the CEL?
  4. Cros89

    Cros89 New Member

    i dont remember the exact code, but it said it was running lean... ive recently replaced the fuel filter and fuel pressure regulator... and it doesnt really ever idle rough i guess, just kinda low.
  5. MrShorty

    MrShorty Moderator Staff Member 1000 Posts 100 Posts

    The first thing I'd do would be to go back and get the exact code(s). The code will tell us if it is bank one or bank two or both, so we can start the diagnosis in the right spot.

    I sometimes will clear a code and wait for it to come back to see if I consistently get the same code.

    Even though you've changed the filter and regulator, I'd probably still put a fuel pressure gauge on it and see if fuel delivery is up to spec.

    A quick check of the O2 sensor wiring might identify a fault between the sensor and the PCM.
  6. Cros89

    Cros89 New Member

    ok.... yea when i got the codes it as bank one and bank two.....
  7. MrShorty

    MrShorty Moderator Staff Member 1000 Posts 100 Posts

    1st though is that I wouldn't expect both O2 sensors to go bad at once, though it is certainly not out of consideration. I would probably start with a fuel pressure test and a thorough check for vacuum leaks. I would probably still clear codes and see if they both come back, too.

    O2 sensors can be bench tested with a voltmeter and a torch (and enough common sense not to set yourself or the bench on fire).

    Do you have access to a scanner (either purchase, rent, or borrow)? If those tests don't tell us what's wrong, it's going to be difficult to diagnose thoroughly without access to some means of "seeing" what the PCM is seeing from the engine sensors.
  8. GA SIERRA

    GA SIERRA New Member

    usually if both banks are lean it will be an intake leak or a bad mass airflow. like mentioned earlier you will need a scanner to watch your engine data while running the engine. also make sure the throttle body is clean
    Last edited: Jan 10, 2009
  9. Cros89

    Cros89 New Member

    i am planning on cleaning the throttle body soon... and yea i had my truck inspected by a mechanic after i bought it to make sure everything checked out ok, and he said that i have a possible intake leak but he said he wouldnt know for sure unless i took it back in to him.
  10. cascott325

    cascott325 New Member 1000 Posts 100 Posts

    seafoam it baby! clean that beeyotch out!

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