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What causes engines to diesel/run on when you shut em off?

Discussion in 'The Hokey Ass Message Board' started by "T'RANTULA", Nov 9, 2012.

  1. "T'RANTULA"
    Joined: Aug 6, 2011
    Posts: 661

    "T'RANTULA"
    Member
    from Ohio

    Ive ran into a couple engines that would diesel/run on when you shut them off and lately my O/t chevy truck started doing this. I was thinking a vaccume leak or timeing issue would cause this. I have done a search and looked through 18 pages of nothing so please dont arrest me search police. On my truck I have changed the heads,dizzy,intake and carb and it wont stop dieseling!! Im about to get a priest to come out and bless it for me to get rid of the demons! I would just like to know what things can cause a motor to diesel. :confused:
     
  2. Deuces
    Joined: Nov 3, 2009
    Posts: 23,751

    Deuces

    Anyone have pictures of demons???... :D
    J/K... ;)
     
  3. c-10 simplex
    Joined: Aug 24, 2009
    Posts: 1,371

    c-10 simplex
    Member

    check the throttle return spring.
     
  4. "T'RANTULA"
    Joined: Aug 6, 2011
    Posts: 661

    "T'RANTULA"
    Member
    from Ohio

    THAT ISNT EVEN FUNNY!! :eek: :D
     

  5. 500 single
    Joined: Jul 8, 2006
    Posts: 119

    500 single
    Member

    In my experience, it's usually because the idle speed is set too fast.
     
  6. johnybsic
    Joined: Oct 8, 2009
    Posts: 612

    johnybsic
    Member
    from las vegas

    Normally its exessive throttle blade angle at idle (curb idle set too high). or return springs like were mentioned.

    High compression engines will do it when their hot or very warm. unless a higher octane fuel is used.
     
  7. mailerguy1
    Joined: Jul 26, 2011
    Posts: 251

    mailerguy1
    Member

    I had a chevelle that would run on whe I put regular gas in it, You might try a premium grade or an octane booster.
     
  8. "T'RANTULA"
    Joined: Aug 6, 2011
    Posts: 661

    "T'RANTULA"
    Member
    from Ohio

    Throttle spring is good and stiff, I even got a anti/diesel switch wired up to close the butterflys on the carb when you shut off the truck. It helps some but dosent stop it. I always have to shut it off in gear and it still does it at 600 rpm! :eek:
     
  9. Dillrod
    Joined: Aug 21, 2008
    Posts: 19

    Dillrod
    Member

    Kill it in gear and 9 times outa 10 it won't do it
     
  10. johnybsic
    Joined: Oct 8, 2009
    Posts: 612

    johnybsic
    Member
    from las vegas

    [QUOTE="T'RANTULA";8335894]Throttle spring is good and stiff, I even got a anti/diesel switch wired up to close the butterflys on the carb when you shut off the truck. It helps some but dosent stop it. I always have to shut it off in gear and it still does it at 600 rpm! :eek:[/QUOTE]


    What kinda motor is it.
    Also keep in mind that Timing is dead once you shut the ign off. Its all in the heat & compression, and throttle blade position (lets too much fuel in on its own).
    Carbon deposits will also attribute to this, but alotta hotrid engines are pretty clean...So to say.

    My BBC will due it when its "hot" aka has been driving and is heat soaked good. Not overheating mind you.
    at 11.25:1 compression im on the ragged edge when running 91 octane. it will stop diesling if i mix it with 100.
    You could also shut off in gear, i do.

    Yea its annoying sometimes, but unless it shuts down with temps in the 180ish range you can bet it will "burp" once or twice.
     
  11. 500 single
    Joined: Jul 8, 2006
    Posts: 119

    500 single
    Member

    Is the float level too high? Could it be spilling over and getting into the venturi that way?
     
  12. Lots of miles? Maybe there is a bunch of carbon built up in the combustion chambers creating hot spots?
     
  13. "T'RANTULA"
    Joined: Aug 6, 2011
    Posts: 661

    "T'RANTULA"
    Member
    from Ohio

    My timeing is 12 degrees before TDC with the vaccume advance plugged. With the vaccume advance unplugged it wont diesel. With the vaccume advance plugged into manifold vaccume it helps the problem. If that makes any sense, im still a rookie but im learning.
     
  14. johnybsic
    Joined: Oct 8, 2009
    Posts: 612

    johnybsic
    Member
    from las vegas

    Think of it like this.
    There is a certain amount of fuel left in the intake upon shut-down. when the motors hot, it is gonna do everything in its power to keep running. as engine RPM slows when shutting down, The cylinder pressures will spike and light the remaining fuel, unless its octane rating is high enough to resist it (too much "curb idle" or blades open will allow it to keep feeding for a second or two.)
    As mentioned carbon deposits create "Hotspots" which will trigger the detonation.
     
  15. bad gas or gas that has been cut with diesel or stove oil, I have seen someone gas up a diesel truck then empty the truck back into the gas tank buyer beware
     
  16. "T'RANTULA"
    Joined: Aug 6, 2011
    Posts: 661

    "T'RANTULA"
    Member
    from Ohio

    Its just a run of the mill high milage chevy 350. When I changed the heads I did notice a lot of carbon on the pistons. I dont have a temp gauge hooked up but I will do that just to see if heat is a factor!
     
  17. "T'RANTULA"
    Joined: Aug 6, 2011
    Posts: 661

    "T'RANTULA"
    Member
    from Ohio

    If I pull the heads off and clean the carbon off the pistons do you guys think that will stop the dieseling?
     
  18. johnybsic
    Joined: Oct 8, 2009
    Posts: 612

    johnybsic
    Member
    from las vegas

    [QUOTE="T'RANTULA";8335922]Its just a run of the mill high milage chevy 350. When I changed the heads I did notice a lot of carbon on the pistons. I dont have a temp gauge hooked up but I will do that just to see if heat is a factor![/QUOTE]

    Its worth a try. I had a 350 (high milage) in a motor home. It ran fine, but would try to run-on when we shut it down after pulling hills etc. it wouldn't overheat but you could tell it was "Real warm" from ambient temps.
    ever see inside of a motorhome 350, running 87 unleaded? its a bit carbon'd up. your heads could use a clean up, or seafoam treatment at the least.
    Try a higher octane fuel or a bunch of booster too. Bet it helps ;) it will give you an idication of whats going on

    It will make you feel silly when you shut down your super cool hotrod in front of people and get a "dummm dumm chaaaaaaaa" outta the carb
     
  19. johnybsic
    Joined: Oct 8, 2009
    Posts: 612

    johnybsic
    Member
    from las vegas

    [QUOTE="T'RANTULA";8335939]If I pull the heads off and clean the carbon off the pistons do you guys think that will stop the dieseling?[/QUOTE]

    yup. I think it would
    Good luck!
     
  20. "T'RANTULA"
    Joined: Aug 6, 2011
    Posts: 661

    "T'RANTULA"
    Member
    from Ohio

    Thanks!
     
  21. [QUOTE="T'RANTULA";8335939]If I pull the heads off and clean the carbon off the pistons do you guys think that will stop the dieseling?[/QUOTE]

    You could try the water down the carb while engine running.
    That removes carbon build up quite well actually.

    Recently did a head gasket job, no question which was leaking. Those pistons and chambers looked like new.
     
  22. Deuces
    Joined: Nov 3, 2009
    Posts: 23,751

    Deuces

    [QUOTE="T'RANTULA";8335908]My timeing is 12 degrees before TDC with the vaccume advance plugged. With the vaccume advance unplugged it wont diesel. With the vaccume advance plugged into manifold vaccume it helps the problem. If that makes any sense, im still a rookie but im learning.[/QUOTE]
    !2 degrees BTC is way to high with cheap gas... Try 8 degrees BTC.... ;)
     
  23. "T'RANTULA"
    Joined: Aug 6, 2011
    Posts: 661

    "T'RANTULA"
    Member
    from Ohio

    Ill give that a try, thanks deuces! :)
     
  24. Deuces
    Joined: Nov 3, 2009
    Posts: 23,751

    Deuces

    [QUOTE="T'RANTULA";8335987]Ill give that a try, thanks deuces! :)[/QUOTE]
    Not a problem!....;)
     
  25. inline 292
    Joined: Aug 25, 2006
    Posts: 295

    inline 292
    Member

    [QUOTE="T'RANTULA";8335908]My timeing is 12 degrees before TDC with the vaccume advance plugged. With the vaccume advance unplugged it wont diesel. With the vaccume advance plugged into manifold vaccume it helps the problem. If that makes any sense, im still a rookie but im learning.[/QUOTE]

    More advance means a cooler combustion chamber. Thats exactly what you need, full manifold vacuum when idleing. Soon as you open the throttle, that goes away.
     
  26. Fugly Too
    Joined: Feb 26, 2012
    Posts: 257

    Fugly Too
    Member

    "ask and ye shall receive!..."
     

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  27. Fenders
    Joined: Sep 8, 2007
    Posts: 3,921

    Fenders
    Member

    If you have an electric fan, the back EMF as the fan spins to a stop can supply voltage to the engine even after you shut the key off, unless you have it wired to prevent that (a diode, for example).
     
  28. "T'RANTULA"
    Joined: Aug 6, 2011
    Posts: 661

    "T'RANTULA"
    Member
    from Ohio

    I do have an electric fan but I only use it when im towing. I got a switch to turn it off and on but I havent had problems until recently.
     
  29. the shadow
    Joined: Mar 5, 2005
    Posts: 1,105

    the shadow
    Member

    the air pressure is low in the drivers side front tire!
     
    gripperrat54 likes this.

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