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89 Burban idles rough with ETC wire connected

Discussion in 'Chevy Suburban Forum (GMC Yukon XL)' started by Big6ft6, May 13, 2010.

  1. Big6ft6

    Big6ft6 Rockstar 100 Posts

    Did some work that required removing the dizzy. After that truck suddenly started idling really low. At first I thought it was good, cuz truck used to idle kind of high. But then at the new low idle truck seemed to idle rough. I figured something happend to the 220k miles dizzy in the process of taking it out and putting back in, so I ordered a new one.

    Installed the new dizzy and set the timing to zero BTDC with the EST wire disconnected. Engine purred like a kitten and thought I had solved the problem.

    Then I shut her off, hooked up the EST wire, started her up and she idled real low and rough just like with the old dizzy. When I hooked up the EST the timing at idle moves from 0 BTDC to 16 BTDC.

    I disconnected the EST wire and advanced the timing to 16 BTDC by turning the distributor to see if it would cause the same poor idle as the EST plugged in....and it does. So it seems like it is the advanced idle timing that makes my engine unhappy. I thought most engines liked advanced timing at idle...and 16 is pretty tame..why would that be causing such poor idle in my engine?

    Where should idle timing be with EST connected? Is 16 BTDC right?
    Last edited: May 14, 2010
  2. Big6ft6

    Big6ft6 Rockstar 100 Posts

    I think I have an idea! Of course I'm at work...so now I can hardly concentrate...I can't wait to go home and see if this is it.

    I was just reading on some other posts and someone mentioned their "adjustable timing light" and it hit me like a ton of bricks...I completely forgot that I have an adjustable timing light! I haven't used the timing light in forever and I bet (or I'm hoping) that the knob on the back of the timing gun was bumped and so the light is firing at some advanced timing already. So I'm not really set at 0 BTDC....I'm probably set at some far out advance already...so when I hook up the ETC wire...the engine can hardly add any advance to the timing without going too far advance and the knock sensor retards it back, basically messing everything up and making the engine out of time.

    I'd put some money (o.k. only maybe $10) that when I get home the dial on the back of the timing light is not at 0.

    Please lord!! Let this be the problem............
    Last edited: May 14, 2010
  3. Big6ft6

    Big6ft6 Rockstar 100 Posts

    coudn't stand it....so I went home over lunch to look at my timing light. Sure enough, the timing light was dialed in at 40 BTDC.

    Poor little engine....no wonder she didn't want to idle at "16" degrees BTDC.....16+40=56 BTDC.

    Side benefit to figuring this out...the spontaneous exhaust leak I thought had gotten worse unusually quickly....it was just detonation.:rofl:

    And for the record if people wonder this in the future. With EST wire disconnected 0 BTDC...then when I connect the ETC wire, idle timing is very stead at 12 BTDC with engine at operting temp.
    Last edited: May 14, 2010
  4. Naptime

    Naptime Rockstar 100 Posts

    Good work.

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