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Dieseling after keys off..and timing is Correct....why?

Discussion in '1973-1991 K5 Blazer | Truck | Suburban' started by twenty_below0, Nov 21, 2002.

  1. twenty_below0

    twenty_below0 1/2 ton status

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    Just as it says, I can put a hard load on the truck and it wont ping, yet I turn the motor off and it diesels. Idle is set at about 800 rpm's
    I was taught to advance the timing until it pings then back it off a little and call it good. It wont ping but diesels I cant figure it out never had this problem before, I thought the dieseling is from being to advanced? Could someone explain my problem?
     
  2. TWISTEDJACK

    TWISTEDJACK 1/2 ton status GMOTM Winner

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    If it cranks over hard it is definately too far advanced. Always set it with a timing light. I have seen them advanced as far a 25 degrees with out pinging. Most motors
    are anywhere from 8-12 degrees. You timing is most likely the cause.
     
  3. 88k5blaza

    88k5blaza Registered Member

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    with that engine timed like it is you should be fine sometimes diseling is caused by water in the gas or crappy grade fuel try using octane boost or try different fuel
     
  4. imiceman44

    imiceman44 1 ton status

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    Another thing would be carbon deposit on your valves and combustion chambers, which raises your compression ratio, so either use valve cleaner additive or higher octane, if it helps you found your problem.
    But all that only after you confirm that the timing is actually correct.
     
  5. Rootbeer

    Rootbeer Registered Member

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    I had a 85 c1500 that dieseled bad till one day it just died while I was driving and wouldnt start again. The shop said something about a missing ground part in the distributer cap, or maybe wrong year dist. cap. Dont quite remember as it was maybe 3 years ago. /forums/images/graemlins/confused.gif
    If this is your 87 I guess its carb not TBI, and your computer can adjust your timing based on knock sensor?
    good luck - Mike
     
  6. twenty_below0

    twenty_below0 1/2 ton status

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    When I built this motor I put a Fluid dampner on it and it doesn't have the marks like some do so I just did it the old school way (ping method) until I feel like putting this pretty little billet aluminum timing tab on it, then I will time it with a lite
    ( I know I should have thought of this to begin with)
    The crappy fuel is probly a good posibility, as far as the carbon, not in this case it's a new motor with about 5,000 miles on it and is set up fairly lean, I'm sure there is no carbon build up yet.For Rootbeer it's actually a 78' with an 87 front clip and top, and has an HEI dist. I will check out the ground idea that's a new one to me hope that's applicable to my set up and is as easy as that.Thanx everyone for the idea's I'm sure there is a solution keep coming with more idea's /forums/images/graemlins/thumb.gif
     
  7. Lonnie

    Lonnie 1/2 ton status

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    Keep in mind that ignition timing has NO effect on dieseling... There is no spark, therefore, no timing with the key off.

    Dieseling is a direct result of poor fuel, hot spots in the combustion chamber or too much throttle opening (not necessarially too fast of an idle speed).

    Unless you have high compression on junk gas, or it is running very hot, it is generally the throttle plate setting.

    Try this:
    See if your vacuum advance is hooked to manifold vacuum.... not ported vacuum. If hooked to ported, try switching it to the manifold (below throttle plates) & see if idle increases. If so, then reduce the idle speed via the throttle screw & it should therefore help to restrict the air into motor. This usually helps to prevent run-on.

    Also what type of carb is on the motor & does it have a fast idle soleniod (dashpot) on the linkage? These are energized to increase the idle speed when the key is on & will retract when the key is off to prevent run-on. These work well with big cammed engines that need a lot of throttle opening to idle.
    Hope this helps.
    Lonnie
     
  8. hammer

    hammer 1/2 ton status

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    choke out of adjustment
     
  9. twenty_below0

    twenty_below0 1/2 ton status

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    Lonnie
    Thx for the post it's very informational, you put it in a little different perspective. I did run higher octane ( 90 ) and it seemed to help so I Feel like that's the problem although I will try switching to manifold vacum and see what happens. The heads that are on this engine are giving me about 9:3.1 and I assumed it would be fine with this compression and 87....apparently not.It has a fairly large cam, and Q-jet though and NO pod that you referred too, maybe I should be getting one. /forums/images/graemlins/laugh.gif
     
  10. MudNurI

    MudNurI 1/2 ton status

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    I have the same problem with my 86. Until we can figure out what it is- timing is correct, been running 93 octane (highest we have here)...everything we have tried has had no effect on it....I'm lucky I have the 4 spd- I can let the clutch out in gear, at the same time I shut the key off...mastered it now, doesn't even move forward a hair!...

    good luck
    Brandy
     
  11. dyeager535

    dyeager535 1 ton status Premium Member

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    Something that caused my '83 to diesel was a throttle kicker on the Q-jet. I wasn't "in" to the engine components enough at the time, it was either an AC kicker or one of the deceleration kickers, but whatever it was, hooked up, my truck dieseled when I turned it off (stock 305 with the 9:1 CR I imagine, it *was* a 305 at least) but when I disconnected that "kicker" the problem disappeared.

    The kicker is of course on the drivers side front of the carb if present, and pushes the throttle open when "required".
     
  12. TorkDSR

    TorkDSR 1/2 ton status

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    untill you figure out the problem it would be cheaper to shut it off in gear than to run 93 /forums/images/graemlins/smirk.gif
    imho
    Ryan
     
  13. twenty_below0

    twenty_below0 1/2 ton status

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    This carb never had the "POD" from what i can tell the adjustment that everyone has mentioned to increase the idle with the A/C on, so that's not the reason nor can I remove it...but I did find a shorting wire on the back of the motor and have been running 90 octane ever since, the Dieseling has stopped. I believe it was the fuel and 9:3:1 with Shitty 87, time to go to Aluminum Heads and 10:5:1 if I'm going to run the higher octane I would rather make more ponies and compression makes ponies. I had hoped to have the cash for the roller cam at the same time but I can wait for a few months....... I guess. /forums/images/graemlins/smirk.gif
     
  14. TX Mudder

    TX Mudder 1/2 ton status

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    I have heard that feedback through the alternator can energize the ignition system and cause dieseling.

    -- Mike
     
  15. TONYP

    TONYP 1/2 ton status

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    your butterfly's are not shutting off, look into the carb and adjust the plates till they are closed 100% with you idol screw, then adjust you low idle valves(what carb are you running) that should be in the front and on the bottom. Your still sucking air into the motor when you shut it off and it will draw gas in with the air and diesel..
     
  16. 79Beast

    79Beast 1/2 ton status

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    Inside your distributor are centrifugal weights. As your engine spins faster, they overcome the springs holding them in and sling out, advancing the timing. They are probably getting hung and keeping the timing too advanced. Try a small amount of graphite on the contact area between the weights and the plate they ride on. WD 40 is also ok, but not as good. Use whatever lube you like sparingly...most of them conduct electricity.
     
  17. Lonnie

    Lonnie 1/2 ton status

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    Electrical feedback is not likely, unless you have an aftermarket ignition such as an MSD. To check for this, just unplug the 12v power lead from the distributor while the engine is idling. If it stops without running on, you found your problem.
    Lonnie Pavtis
    Lonnies Performance Automotive
     

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