GM Truck Club
THE PREMIER CHEVY TRUCK & SUV FORUM
Founded in 2004 ~ We're the #1 Chevy Truck & SUV Forum.
Silverado & Sierra | Tahoe & Yukon | Suburban & Yukon XL | SUV & CROSSOVER

Warm start black smoke

Discussion in 'Chevy Suburban Forum (GMC Yukon XL)' started by RussCrypto, Dec 7, 2008.

  1. RussCrypto

    RussCrypto New Member

    Joined:
    Sep 17, 2007
    Messages:
    5
    Likes Received:
    0
    I've lingered here for a while, but this is my first post. Hopefully someone will have some ideas.

    I have a '99 C2500 Burb with around 80k miles on the 7.4L. A couple of weeks ago I noticed that on warm starts it will crank and crank before it starts up. Sometimes I'll have to pump the gas pedal a couple times. When it finally does start, there is a bit of black smoke that dissipates quickly. On cold starts, it turns over like a champ. No problems what so ever.

    Any suggestions?

    Thanks in advance!
    #1
  2. dwill3015

    dwill3015 New Member

    Joined:
    Jan 2, 2007
    Messages:
    4,338
    Likes Received:
    16
    Welcome to the site! hopefully another member here with knowledge on the 7.4 will jump in and shed some light on your problem.
    #2
  3. 04sierracrewcab

    04sierracrewcab New Member

    Joined:
    Aug 15, 2008
    Messages:
    1,924
    Likes Received:
    1
    black smoke from a gasser??!!!?? :neutral:

    and by warm start you mean if you were to turn the truck off to order at a drive thru or sometihing?

    :sign0016:
    #3
  4. RussCrypto

    RussCrypto New Member

    Joined:
    Sep 17, 2007
    Messages:
    5
    Likes Received:
    0
    Well, not exactly. It's more pronounced when after the truck has been driven enough to warm up, and then turned off for more than just a couple minutes, even up to a few hours.
    #4
  5. 04sierracrewcab

    04sierracrewcab New Member

    Joined:
    Aug 15, 2008
    Messages:
    1,924
    Likes Received:
    1
    when was the last time the fuel filter was replaced?
    #5
  6. RussCrypto

    RussCrypto New Member

    Joined:
    Sep 17, 2007
    Messages:
    5
    Likes Received:
    0
    Just over a year and 15k miles ago.
    Last edited: Dec 7, 2008
    #6
  7. 04sierracrewcab

    04sierracrewcab New Member

    Joined:
    Aug 15, 2008
    Messages:
    1,924
    Likes Received:
    1
    hmm, well im not sure
    the black smoke does not come out when driving correct??
    and im not sure if it would help but may wanna change the filter anyway:neutral:

    somone else will chime in:great:
    #7
  8. RussCrypto

    RussCrypto New Member

    Joined:
    Sep 17, 2007
    Messages:
    5
    Likes Received:
    0
    Correct, once running, there is no smoke. It's almost as if a choke plate were to get stuck on a carburated engine.
    #8
  9. 04sierracrewcab

    04sierracrewcab New Member

    Joined:
    Aug 15, 2008
    Messages:
    1,924
    Likes Received:
    1
    hmm, well could the fule pump be the issue?

    does it run fine when started?
    #9
  10. RussCrypto

    RussCrypto New Member

    Joined:
    Sep 17, 2007
    Messages:
    5
    Likes Received:
    0
    It runs like a champ actually.

    Would a leaky fuel injector cause this?
    #10
  11. ct9a

    ct9a New Member

    Joined:
    Jun 2, 2008
    Messages:
    2,818
    Likes Received:
    1
    Leaky injector could cause this, but I would also think that there would be other symptoms too. Might be worth looking into though.

    I assume there's no CEL?
    #11
  12. JMoney02

    JMoney02 New Member

    Joined:
    Oct 23, 2008
    Messages:
    749
    Likes Received:
    3
    EGR/switch getting stuck on warm starts and functioning when in operating mode? Pindle stuck at start due to extreme carbon build up, but will not produce smoke when running. You have to hold the pedal
    down to get it to start when warm? How do your spark plugs look? Just check the easiest one and see how black it is and lets see what you get here...

    Jeff
    #12
  13. Big6ft6

    Big6ft6 New Member

    Joined:
    Jul 23, 2007
    Messages:
    211
    Likes Received:
    0
    One thing you might want to check is your coolant temperature sensor.

    the coolant temp sensor signal acts as a choke on fuel injected engines, when it is cold it puts more gas in the engine (like pulling the choke on your lawnmore when you first start it) and as your coolant temp warms up, it tells the computer to lean out the gas mixture.

    If the sensor is bad and is sending a cold coolant temp to computer, the computer would always be putting too much gas in. When you're starting from cold, that wouldn't be a problem, but when your engine is already warm, the air/fuel mixture would be too rich to start well and you'd need to open the throttle (pump the gas) to get more air in to allow it to start..the extra gas would come out as incomplete combustion and carbon deposits (black smoke)

    New sensor is only $11.99 and can be checked with a ohmeter

    Only thing is usually a bad CTS would cause a CEL code...but maybe it isn't bad enough to throw a code.
    Last edited: Dec 12, 2008
    #13
  14. vncj96

    vncj96 New Member

    Joined:
    Feb 4, 2008
    Messages:
    3,390
    Likes Received:
    3
    black smoke is usually the by-product of your engine running rich, I would be checking the intake and TBI
    #14
  15. MrShorty

    MrShorty Moderator

    Joined:
    Jan 12, 2006
    Messages:
    1,368
    Likes Received:
    9
    My first thought is fuel pressure regulator. What happens is the diaphragm between the pressurized fuel and the vacuum line ruptures, allowing excess fuel into the intake. If the leak is slow enough, you don't see a problem while it's running, but, once you shut off the engine, the fuel system "depressurizes" through the FPR into the intake. Once you get enough in the intake, it will act "flooded" when you try to start it. If you wait long enough, the excess fuel in the intake will dissipate and you don't have a problem.

    Easy enough to check: simply pull the vacuum line off of the FPR (As I understand it, your 7.4 FPR is easy enough to get to) after the engine's been running and see if there's any gasoline in the vacuum line. If yes, then replace FPR.

    If no, I'd probably put a fuel pressure gauge on it and see if the fuel system holds pressure.
    #15

Share This Page