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Thread: Ignition timing

  1. #1
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    Ignition timing

    Hello. I have a 1989 GMC Suburban 1500, 350, TBI, 4X4. Nothing special, all stock. I know the base timing is 0. Does anyone know what the timing is supposed to be after hooking the computer back up?
    Mine is around 15 degrees BTDC. Is this normal?

  2. #2
    Senior Moderator chevychase's Avatar
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    Should be 0 like you said... did you pull timing bypass before starting or after?
    Also you maybe able to adjust it to 0 or get it close without messing with bypass.
    Do you have the sticker under the hood with info on timing?
    Is it running okay.. no predetonation ect. If not google it plenty of info.

  3. #3
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    0 degrees TDC is what the timeing should be. There is a wire you have to disconnect before setting the time, then after hook it back up, shut off and clear codes and then next time you start it up it will be ready to go, the ECM will do the rest.

  4. #4
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    I have set my base timing to 0 degrees with the wire disconnected. When I reconnect the wire it goes to 15 degrees. I was just wondering if this was normal and if anyone knew what the computer controlled timing was at idle.

    The idle on my truck seems a bit high. It won't pass smog. It seems to run ok but hesitates and wants to die when I decelerate. The smog guy said it is dumping too much fuel. Hydrocarbons are too high which is unburned fuel. Doesn't seem to detonate.

    I have changed the ignition module in the distributor, the coolant temp sensor has the correct voltage, changed both throttle body injectors, plugs, wires, cap rotor, new fuel pump, rebuilt the fuel regulator in the throttle body, O2 sensor, knock sensor, low idle sensor, throttle position sensor. I am looking into the manifold absolute pressure sensor. I also need to do a compression test.

    I know it is over fueling I just can't figure out why. I am google and yahoo-ed out and in Las Vegas a mechanic is out of the question. Any ideas?

  5. #5
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    I have a 94 and it did the same thing plugged that wire back in and at idle its at 15 btdc but rev it up a little and it jumps up to 0 tdc. I also have a issue with mine dumping too much so much it fouls the plugs with just a couple minutes of idle, I have checked everything.

  6. #6
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    I replaced the MAP sensor and really pissed off the truck. Now it won't start and kicked out a 33 code which says MAP sensor, engine misfire, rough idle. I figure I'm on the right track but don't know where the circuit goes from there. It seems like it cut the fuel now. How do I troubleshoot the fuel circuit or what have I overlooked?

  7. #7
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    MAP sensor

    Quote Originally Posted by 89sub View Post
    I replaced the MAP sensor and really pissed off the truck. Now it won't start and kicked out a 33 code which says MAP sensor, engine misfire, rough idle. I figure I'm on the right track but don't know where the circuit goes from there. It seems like it cut the fuel now. How do I troubleshoot the fuel circuit or what have I overlooked?
    Sometimes the map sensor vacuum hose comes off where it's connected on the back of the throttle body

  8. #8
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    Quote Originally Posted by 89sub View Post
    I have set my base timing to 0 degrees with the wire disconnected. When I reconnect the wire it goes to 15 degrees. I was just wondering if this was normal and if anyone knew what the computer controlled timing was at idle.

    The idle on my truck seems a bit high. It won't pass smog. It seems to run ok but hesitates and wants to die when I decelerate. The smog guy said it is dumping too much fuel. Hydrocarbons are too high which is unburned fuel. Doesn't seem to detonate.

    I have changed the ignition module in the distributor, the coolant temp sensor has the correct voltage, changed both throttle body injectors, plugs, wires, cap rotor, new fuel pump, rebuilt the fuel regulator in the throttle body, O2 sensor, knock sensor, low idle sensor, throttle position sensor. I am looking into the manifold absolute pressure sensor. I also need to do a compression test.

    I know it is over fueling I just can't figure out why. I am google and yahoo-ed out and in Las Vegas a mechanic is out of the question. Any ideas?

    This happened to my brother's '91 GMC, I did all the same things you did, (even tried a new fuel pressure regulator) had a buddy look at it and kept asking just about everybody I talked to.
    It finally turned out to be a vacuum leak in the charcoal canister located in the front left of the engine compartment. I pulled off both vacuum hoses, plugged them and it ran like a new truck. Hope this helps if you haven't already gotten it fixed.
    Last edited by nodak; 10-17-2012 at 12:17 AM. Reason: left out a word

  9. #9
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    hope it's fixed by now it was a year ago july!
    4 wheels move the body
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    Snow is a lot like religion and a lot like sex....you never know when you are going to get it!....or how much you will get

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