99 Silverado both banks running lean

Discussion in '4X4 Truck Talk and Repairs' started by Seth, Jan 4, 2009.

  1. Seth

    Seth Active Member

    Messages:
    1,807
    State:
    Owensville, MO
    I had to my truck inspected and had my friend do a quick scan to see why the check engine light was on. It's came back saying both banks were running lean. It's been on since I bought the truck a little over a month ago.

    It goes off sometimes for a little while, but comes back on. I recently replaced the fuel filter and it stayed off for almost a whole tank of gas, but then came back on when I got below 1/4 of a tank and has stayed on since then.

    It also idles kind of rough when it's cold.

    Any idea's what's going on?
     
  2. cantstopgrandma

    cantstopgrandma New Member

    Messages:
    955
    State:
    MD
    It could be the fuel pump starting to go bad, but i'd probably check your intake manifold for a leak. Chevy's plastic intake manifolds are known for leaking. Spray some carb cleaner around the intake while its running and listen for rpm changes. Too much unmetered air getting into the engine probably.
     

  3. trnsmsn

    trnsmsn New Member

    Messages:
    1,214
    State:
    Missouri Originally Now I
    My first thought would be a leaking intake manifold also, as they are a very common problem. If you don't find a problem there, take off your EGR valve and see if the pintle is off it's seat.

    HTH, Elliot
     
  4. Seth

    Seth Active Member

    Messages:
    1,807
    State:
    Owensville, MO
    Had my buddy listen for vacuum leaks and check the intake manifold and nothing was leaking. Got a PM saying to run some Sea Foam through it while the tank was really low on gas and see what that does. Gonna give it a try here shortly. Also gonna replace the plugs and wires since they need replaced anyhow I'm sure.

    Hopefully I get this figured out soon.
     
  5. dafin

    dafin New Member

    Messages:
    1,461
    State:
    Manhattan,Kan
    My Chevy truck was running lean, it was a weak fuel pump . have someone check the fuel presser
     
  6. CatfishAngler25

    CatfishAngler25 New Member

    Messages:
    70
    State:
    Prestonsburg,Kentucky
    My guess would be a bad fuel pump. Chevy is known for bad fuel pumps especially when people run it on empty. i would say that whoever was the owner before has taken their time refilling. It is best to keep your gas tank atleast to half a tank. If the problem is fuel pump make sure to keep atleast half filled. hope this helps.
     
  7. Seth

    Seth Active Member

    Messages:
    1,807
    State:
    Owensville, MO
    We are going to check the fuel pressure also. That was actually the next thing my mechanic friend said needed checked, but he still needs to go buy a gauge. He figured it was a weak fuel pump from the start. If it is, I'll just go ahead and have him replace it ASAP. I don't want to be driving around all over the place fishing and have to worry about that all the time. :cool2:
     
  8. ryang

    ryang Well-Known Member

    Messages:
    5,380
    State:
    Blacklick, Ohio
    Name:
    Gary
    Do Chevy's use O2 sensors on the exhaust. On my Ford (blowstar LOL) I have problems with those.
     
  9. psychomekanik

    psychomekanik New Member

    Messages:
    2,534
    State:
    Illinois
    your truck has 02 sensors on both banks. Since it's indicating both of them are lean, it probably is. When you check fuel pressure pinch or block off the return. {deadhead} the pump. you can also do this with the guage itself if it has a shutoff. install it in front of the fuel filter on the frame rail. check running pressure then close the valve cutting off flow to the engine. a good pump will spike up to a hundred pounds or better. If it spikes up really high the regulator would be in question. if the pressure stays the same and it's low. It's the pump. They are ,as said already, bad for vacuum leaks. And I do use a carb cleaner or better yet. brake cleaner. But, here's a few tips on using it. let the engine get hot. And use short bursts and give the cleaner time to evaporate. work slowly. If you soak the intake down you might find some residual plug wire leakage. (spark)! or if you hit the vacuum leak, it might cough and backfire a little. BAD NEWS. now you have flames. keep a fire extinguisher handy.
     
  10. trnsmsn

    trnsmsn New Member

    Messages:
    1,214
    State:
    Missouri Originally Now I
    If the scan tool your using has data stream (live info), check to see what the EGR % is @ idle , it should be 0%. The EGR valve is digital & will allow you to monitor it's position & movement if any.

    It being a Vortec Engine, the fuel press. regulator is under the intake manifold & usually ruptures if anything, thereby causing hard restarts & a VERY rich condition..

    The Intake manifold can warp on the inner side causing a INTERNAL vacuum leak that you won't be able to detect from outside. If your fuel trim % is high, it's probably been a long term failure...I.E. weak fuel pump :confused2:

    HTH, Elliot
     
  11. Moonlightn

    Moonlightn New Member

    Messages:
    84
    State:
    California
    This is just a thought... If the fuel pump checks out OK. I would check the MAF (mass air flow) meter. But in most cases it would cause more of a drivability concern as would the fuel pump. Just a note oiled filters such as K&N have been known to kill MAF sensor. So if it has one on it I would suspect the sensor.
    Good luck with the fix!
    P.S
    A spray bottle of water works just as well as anything to find a vacuum leak or an ignition fault and best of all it's not flammable!
     
  12. Catpaw

    Catpaw Well-Known Member

    Messages:
    9,342
    State:
    Central Cail
    Name:
    James
    funny you mentioned that mine was running lean on the right bank and it turned out the MAF was faulty .
     
  13. ateamfisherman

    ateamfisherman New Member

    Messages:
    297
    State:
    Texas
    I think the two weak banks on this one might be freddie mac and fannie mae. I really thought that is what you were talking about. Thanks Sam Davis
     
  14. Seth

    Seth Active Member

    Messages:
    1,807
    State:
    Owensville, MO
    I just have a regular air filter and it's good and clean. How do I check if my MAF is bad or is that one of those things you just have to replace to see if it's bad or not?
     
  15. BajaCoop

    BajaCoop Well-Known Member

    Messages:
    3,209
    State:
    Poplar Bluff,MO
    might just check and see if it is dirty and try cleaning the MAF too.
     
  16. vlparrish

    vlparrish New Member

    Messages:
    1,276
    State:
    Bedford, Kentucky
    Seth, from what you are saying I'd go with the weak fuel pump. If it was that way when you bought the truck most likely why the guy traded it off. Good Chevy fuel pumps are not cheap, but then again cheap chevy pumps are not usually good. It takes around 45 to 50 PSI to make that truck perform properly and I have seen cheaper fuel pumps fall short of the factory specifications. That said, a fuel pressure test would be the best way to troubleshoot your problem. So whatever it takes I would do that first. Now, back to the previous question you had about Mass Air Flow sensors. You can purchase a can of MAF sensor cleaner at most any parts store. Have your mechanic show you where the sensor is located and how to remove it for cleaning. I recommend that you give it a good cleaning whether it needs it or not, I consider it routine maintenance on the vehicles of today. A dirty MAF sensor can cause many different symptoms depending on how dirty it is and the vehicle it is on. Vern
     
  17. Seth

    Seth Active Member

    Messages:
    1,807
    State:
    Owensville, MO
    Thanks for the info vlp. I'll get my buddy to check my fuel pressure and show me how to clean the MAF. He said that he'd replace the fuel pump for me if it needs it. The new pump would runn around $300-400 I think it was he told me.
     
  18. walsvikjo

    walsvikjo New Member

    Messages:
    12
    State:
    North Dakota
    Seth

    I have had numerous 5.3L come into my stall with the same problem. Majority of the problem was a bad fuel pressure regulator. The regulator will get stuck open or partially open and allow fuel to go back into the tank and not into the injectors. If your buddy has a scan tool that can show the fuel trim status pull the vaccum line off the regulator and rev the engine watching the trims if they stay the same the regulator is bad. I do know that some auto parts stores will loan or rent tools so you may want to see if you can get fuel pressure tester. the test port is right by the fuel pressure regulator.

    JW
     
  19. drpepper

    drpepper New Member

    Messages:
    6,133
    State:
    Indiana
    lol, alot of this stuff is gonna go right over the average guys head.
    I can tell tho... BOC has some drivability techs out there- and maybe a shop owner or two.:wink: