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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. [QUOTE="T'RANTULA";8336471]I had the heads off and thats how I determined the piston was up. And the the rest just as you described.[/QUOTE]

    Then you are all good with the insides.
    Just the tune now.
    If you want to use your timing light, you need to sort the marks out with a piston stop test.
    Google it or search it here "piston stop test" pretty easy to do.

    Or just time it by ear and seat of the pants dyno.
    Everything you've said are classic regarded timing.

    You may have to get your distributor recurved for optimal timing thru the rpm.

    If you are running an EGR you want a smog curve and use ported vacuum for the advance
    If not egr then you want mamiflod vacuum on your advance.
     
  2. "T'RANTULA"
    Joined: Aug 6, 2011
    Posts: 661

    "T'RANTULA"
    Member
    from Ohio

    Ill do the piston stop test tomorrow and I got rid of the EGR, would it be ok to run the truck with ported vaccume without the EGR? Youve helped me out a ton so far!! I really appreciate it! :)
     
  3. [QUOTE="T'RANTULA";8336505]Ill do the piston stop test tomorrow and I got rid of the EGR, would it be ok to run the truck with ported vaccume without the EGR? Youve helped me out a ton so far!! I really appreciate it! :)[/QUOTE]

    Yes it would be OK, but if its the distributor that worked with the egr it has a smog timing curve. It would be Soooooooo much better of an engine to drive if you get it recurved .
    GMC Bubba is the man to see or ask about the timing curve change.
     
  4. "T'RANTULA"
    Joined: Aug 6, 2011
    Posts: 661

    "T'RANTULA"
    Member
    from Ohio

    I might ask GMC Bubba, what if I used an old late 60s early 70s points distributor? When did they start using EGRs on chevy engines?
     
  5. Engine man
    Joined: Jan 30, 2011
    Posts: 3,480

    Engine man
    Member
    from Wisconsin

    [QUOTE="T'RANTULA";8336471]I had the heads off and thats how I determined the piston was up. And the the rest just as you described.[/QUOTE]

    When you changed the timing chain and sprockets you saw that the crankshaft sprocket has 1/2 as many teeth as the camshaft sprocket so the crankshaft turns 2 revolutions for one turn of the camshaft. This means that the #1 piston comes to the top of the cylinder 2 times for each turn of the camshaft. When the mark on the crankshaft sprocket is up and the one on the camshaft sprocket is down, the rotor should point to #1 on the distributor cap. When both marks are up, the rotor should point to #6 on the cap.

    Your run on problem would probably go away by using higher octane fuel. Since you have an anti-dieseling solenoid, set the idle at 400 with the wire disconnected.
     
  6. RayJarvis
    Joined: Oct 11, 2010
    Posts: 209

    RayJarvis
    Member

    deiseling after shutdown occurimg when hot can also be indicative of fuel getting to hot and pressuring over into throtal body. try cooling spacer or getting beter airflow around carb?
     
  7. 345winder
    Joined: Oct 27, 2010
    Posts: 1,059

    345winder
    BANNED

    WOW,,,that's all i can say,,,people telling this guy to pull his heads because its diseling?? .....(worth a shot they said)....wow...

    ..it's just a timing isssue,,pretty straight foward...when you have a hopped up engine a timing light isnt correct if your setting it with your timing light,,
     
  8. Commish
    Joined: Jan 9, 2010
    Posts: 379

    Commish
    Member
    from NW Ok

    If your timing marks are correct, a timing light is a fine and accurate way to set the timing, hopped up or not.
     
  9. stanlow69
    Joined: Feb 21, 2010
    Posts: 7,348

    stanlow69
    Member Emeritus

    After owning my car for 27 years, I can`t seem to break the habit of turning my car off when in gear.
     
  10. sunbeam
    Joined: Oct 22, 2010
    Posts: 6,213

    sunbeam
    Member

    Gm top engine cleaner is a good fix.
     
  11. SanDiegoHighwayman
    Joined: Jun 26, 2012
    Posts: 951

    SanDiegoHighwayman
    Member

    :Dwhat I used to do to clean out carbon was to, with a warm engine, with one hand run the engin @ high idle, while, with the other pour water from a soda bottle thru the carb hittin all barrells one at a time -- and keepin the engine speed high enuf that it didn't stall --

    :eek:makes a considerable cloud of grey smoke as the carbon exits yer tailpipes so don't do this inside yer garage:rolleyes: always worked fer me:cool:
     
  12. 40FordGuy
    Joined: Mar 24, 2008
    Posts: 2,907

    40FordGuy
    Member

    Idle speed.... In 70's and 80's,...they installed solenoids that let the throttle close more, when the keyswitch was shut off. I believe the higher idle speeds were to get a handle on idle speed emissions.

    4TTRUK
     
  13. Engine man
    Joined: Jan 30, 2011
    Posts: 3,480

    Engine man
    Member
    from Wisconsin

    I see we never actually answered the original question.

    When you turn the key off, it turns off power to the ignition system so you no longer have spark to fire the plugs. As long as the engine is turning, it creates a vacuum that allows fuel to be pushed from the carburetor into the engine. Your mechanical fuel pump keeps the carburetor full of fuel as long as the engine is turning. With no spark to fire the mixture, the engine normally stops. Run on happens when something other than the spark plug is causing the mixture to ignite in the cylinder. This can be carbon in the cylinder that retains heat, too high of heat range spark plug, a really hot engine, or low octane fuel that will fire from compression with a little extra heat from the engine.

    Your options to prevent it are to get rid of the carbon, use a cooler spark plug, keep the engine cooler or use a higher octane fuel that won't ignite as easily.

    It's called dieseling because the engine is running without ignition like a diesel engine does. Having diesel fuel in the gas won't cause an engine to diesel. Diesel engines run at much higher compression to make the fuel fire. Some of the octane booster products are mainly diesel fuel.

    Pouring a little water down the carburetor will break up carbon. Some people use automatic transmission fluid which makes lots of really white smoke.
     
  14. As stated above , too high rpm at idle , running too hot , partial choke closed,,,,,and I just ran into this last week , my ignition switch was bad and would not shut off after running for 30 minute ride , worked fine if only driven 5 minutes
     
  15. oj
    Joined: Jul 27, 2008
    Posts: 6,454

    oj
    Member

    I didn't read the entire thread but wwhen deiseling it is turning over backwards, thats why it sounds like a vacuum cleaner. So check your vacuum fittings because all your normally vacuum ports have briefly become positive presssure ports and it'll blow off some of those vacuuum port plugs.
     
  16. "T'RANTULA"
    Joined: Aug 6, 2011
    Posts: 661

    "T'RANTULA"
    Member
    from Ohio

    Checked my timeing tab to see if it was correct and it was off by about an inch! So I just advanced the distributor until the starter dragged and backed it off a little. Runs much better and starts better but it still wants to diesel just a touch. But if I shut it off in gear it wont do it at all so I can live with it now! Thanks everyone for all your help, this has been a real learning experience. Im gonna try the water trick tomorrow and see how much carbon comes out. Wish me luck! ;)

    -Robert
     
  17. [QUOTE="T'RANTULA";8338283]Checked my timeing tab to see if it was correct and it was off by about an inch! So I just advanced the distributor until the starter dragged and backed it off a little. Runs much better and starts better but it still wants to diesel just a touch. But if I shut it off in gear it wont do it at all so I can live with it now! Thanks everyone for all your help, this has been a real learning experience. Im gonna try the water trick tomorrow and see how much carbon comes out. Wish me luck! ;)

    -Robert[/QUOTE]

    Good luck!
    Slow and steady on the water.
     
  18. Commish
    Joined: Jan 9, 2010
    Posts: 379

    Commish
    Member
    from NW Ok

    Not so, when a car diesels it is running on in the same direction that is was when you shut it off. It does not turn into a vacuum cleaner. Turn the switch back on and it will light right back off in the normal direction.
     
  19. gypsy
    Joined: Apr 23, 2006
    Posts: 324

    gypsy
    Member
    from florida

    [QUOTE="T'RANTULA";8336422]On TDC when I took it apart both dots pointed up in the 12 o clock position and the rotor button pointed to number 6 on the dizzy. Thats after the timeing had jumped 3 teeth.[/QUOTE]


    In a SBC that is the correct firing position (compression stroke) for #6 cyl
     
  20. Mr4Speed
    Joined: Nov 16, 2008
    Posts: 89

    Mr4Speed
    Member

    [QUOTE="T'RANTULA";8336134]My timeing tab was originally welded to the timeing chain cover, the old cover was bent so I put a new cover on and a timeing tab that bolts onto the timeing chain cover. The timeing tab is about 3/4 of an inch away from the harmonic balancer. I never tested to see how accurate it was and I agree that was dumb of me to assume it was correct.. :eek:[/QUOTE]


    If the timing tab is 3/4" away from the balancer then the tab is probably meant for the larger 8" truck balancer.

    Get the correct tab and check your marks again.

    And did you check your marks by turning the crank both ways (clockwise and counter clockwise) against the piston stop?
     
  21. Dreddybear
    Joined: Mar 31, 2007
    Posts: 6,084

    Dreddybear
    Member

    Dude? You're joking right now, yes?




    Posted from the TJJ App for iPhone & iPad
     
  22. "T'RANTULA"
    Joined: Aug 6, 2011
    Posts: 661

    "T'RANTULA"
    Member
    from Ohio

    I poured a small amount of water down the carb and a ton of carbon came out the exhaust! I did it next to my friends white truck and covered it in carbon. He was not pleased. The truck is runnin smoother now and hasnt dieseled since I did the water trick. I think it is fixed! I gotta say you know exactly what your talkin about 31vicky. I love the HAMB! :cool:
     
  23. [QUOTE="T'RANTULA";8340606]I poured a small amount of water down the carb and a ton of carbon came out the exhaust! I did it next to my friends white truck and covered it in carbon. He was not pleased. The truck is runnin smoother now and hasnt dieseled since I did the water trick. I think it is fixed! I gotta say you know exactly what your talkin about 31vicky. I love the HAMB! :cool:[/QUOTE]

    Glad you got it fixed , glad to have had a part in helping.
    Now you help the next guy!
     
  24. oj
    Joined: Jul 27, 2008
    Posts: 6,454

    oj
    Member

    Nope, it is running backwards.
    The preignition fires as the piston is coming up - it is quite a ways down in the hole, the combustion stops the piston from advancing and pushes it back down the hole and then continues to run backeards for a few cycles. You can hear the 'hsss' as the presuure reverses and it'll blow a loose fitting vacuum cap off.
     
  25. "T'RANTULA"
    Joined: Aug 6, 2011
    Posts: 661

    "T'RANTULA"
    Member
    from Ohio

    You can count on it! ;)
     
  26. "T'RANTULA"
    Joined: Aug 6, 2011
    Posts: 661

    "T'RANTULA"
    Member
    from Ohio

    Mine dident turn backwards, you could just flip the key back on and it would fire up.
     
  27. Max Grody
    Joined: Dec 15, 2005
    Posts: 94

    Max Grody
    Member
    from Ky

    Not arguing the issue, but I really don't get it: How does incorrect timing cause dieseling? The way I understand it, the timing of a switched off ignition should be immaterial since it's no longer delivering a spark. Hence, the term "dieseling", meaning an engine that continues to fire without an electrical spark. If you're saying incorrect timing can cause an engine to run hot and a hot engines sometimes diesel, that I understand.
     
  28. deadgearhead
    Joined: Mar 14, 2009
    Posts: 315

    deadgearhead
    Member
    from Washington

    This is complete bs. The reverse pressure is from preignition occurring when a intake valve is still open. ALL engines I have witnessed with a dieseling condition still turn clockwise.
     
  29. Retarded timing causes super heated combustion chambers.
    That red hot carbon deposits causes dieseling.
    If you read the thread you'll see that both issues are here.
    Hope that helps.
     
  30. Diavolo
    Joined: Apr 1, 2009
    Posts: 824

    Diavolo
    Member

    Glad your timing is better and the water trick worked! Here is how I set my timing "by ear".

    Fire it up. let it idle. Grab the distributor and retard the timing whichever direction you have to go. Slowly turn it and retard until the idle speed just starts to slow down. For a V8, turn it back the other way about 1/16 a turn.

    Not exact, but a real quick and dirty way to get it really close to where it should be.
     

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