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Poor Idling

Discussion in 'GM Powertrain' started by cmurphy84, May 24, 2010.

  1. cmurphy84

    cmurphy84 New Member

    I need a little help troubleshooting a new issue with my truck.

    1999 Chevy Silverado 1500
    5.3L V8
    202,3xx miles

    I drove the truck last night and had no problems with it at all. This morning, I started it up and it sound like it was running odd. Meaning, my exhaust was not as loud as typical when I first start it up. Looked at the Tachometer and it was idling just a tick or two off the bottom (under 500 RPM). Then as soon as I noticed that, it jumped up to typical warm-up revs of about $750. Every couple seconds it would switch from almost dying to normal. I turned the AC off and that seemed to help, but it was still idling lower than usual.

    I hopped in and took off towards the office. At the first stop sign, it idled down and died. Popped it in neutral and it fired right up, but still idling low. I just kept it off idle the rest of the way to the office. On the highway, you would never know it was having an issue.

    My initial thought is fuel pump, but I really don't know where to start with this one. I have all the tools necessary to fix about anything, but I'm at a loss on where to start with this one. I don't want to start by just throwing money at it, if I can help it.
  2. MrShorty

    MrShorty Epic Member Staff Member 5+ Years 1000 Posts

    If my initial thought was a fuel pump issue, the first thing I'd do would be to put a fuel pressure gauge on it and see what the fuel pressure was when it was stumbling.
  3. cmurphy84

    cmurphy84 New Member

    I'll try a fuel pressure gauge if it starts stumbling again. I was out driving a couple other times today and it is not doing it anymore. :neutral:
  4. trapperdon

    trapperdon Rockstar 100 Posts

    plug in an OBD 2 code reader & see if it stored a fault code, even if its not doing
    the problem at this moment. that'll tell you which part to look at.

    if you dont own one the parts stores will let you borrow it if you plan on spending $$ there...
  5. TJbear

    TJbear Rockstar 100 Posts

    Sticky idle control valve?
  6. KirkW

    KirkW Rockstar

    I doubt it's a fuel-pump. I would imagine that a weak fuel-pump would have more trouble at high flow-rates (i.e. - highway cruise) than at low flow-rates (i.e. - idling).

    My first guess is a vacuum leak. The Idle-Air-Controller sounds like it's working, but the computer is 'hunting', cycling the IAC in and out while it tries to find a setting that keeps the engine idling. At idle speeds even a small vacuum leak is a large percentage of the total volume of air going into the engine. For instance, the PCV system supplies about 25% of the idle air-flow.

    But at highway speeds the large amount of air being gulped down by the engine renders a small vacuum leak meaningless.

    Try tracing the vacuum lines and hoses - there's not too many on that engine. Make sure none are split, cracked, loose, or missing.

    My next guess is the EGR valve. Try unplugging it at idle and see if the 'hunting' goes away. There should be no EGR input at idle. A leaking EGR valve will cause idle problems.
  7. ntbush83

    ntbush83 Rockstar 4 Years ROTM Winner 1000 Posts

    idiling problem.

    Hey I had the exact same problem with my 2003 silverado. Exact same issue. The guy at the shop checked hoses and cleaned out the intake on the throttle body and it did the trick. He also cleaned the area around the intake sensor. I haven't had a problem since.
  8. cmurphy84

    cmurphy84 New Member

    Thanks for the input guys. I've driven it quite a bit since the issue and it has cleared up so far. When I get a little free time (this weekend) I'll take a look at the vacuum lines and check the EGR valve (if it is doing it again)
  9. bowfishrp

    bowfishrp New Member

    Take choke cleaner and a toothbrush and clean the throttle body where you can reach. I have to this every 6 months to a year because of this exact same problem. Dont know who told me that but it works like a charm.

    I THINK the crud buildup causes the throttle body to not close all the way...but that is purely a guess.
  10. HippyD

    HippyD New Member

    Use some B-12 Chem Tool Cleaner the crap will just melt off. probly got carbon buildup on a sensor out there and its trying to decide if its cold or not, clean out the crap.

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