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Help with 91 TBI Tune Up problem

Discussion in 'The Garage' started by Madbomber88, May 20, 2005.

  1. Madbomber88

    Madbomber88 1/2 ton status

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    Background info:
    Picked up my second K5 after my 90 K5 was totalled the one time I let someone else drive it. Bought a 91 K5, completely stock with only 108k original miles. Finally got around to doing a tuneup. Took the intake off and cleaned out the EGR ports, they were completely clogged with carbon.

    Put it all back together with new OE GM distributor, coil, plugs, wires, everything. Set the timing (not yet with a timing light), but got it to run smooth until...

    It will start up fine and run like a top for a minute or so. Then it will bog down and run rough, almost stalling for 7-10 seconds or so until you hear a hiss (best way to describe it) and then it runs smooth again. The cycle then continues.

    Is this just a sign of needing to do a more accurate timing job, or is something else wrong?
    Any advice is appreciated.
    -Tim
     
  2. dirtwarrior17

    dirtwarrior17 Banned

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    Hard to tell whats wrong with a motor until you get a timing light on it. I spent days trying to figure out why my motor wasn't running right then i realized i didn't unlplug the computer advance when i timed it... unlpugged it and timed it and it ran like a champ. I would say before you do anything time it at the stock setting (6 degrees in advance IIRC) then go from there.
     
  3. southernspeed

    southernspeed 1/2 ton status GMOTM Winner

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    The hiss is the idle air control valve opening. The ecm opens it a set amount when you're out of park/nuetral to stop it stalling with a load against it. Have you disconnected any wires? It takes it signal from the p/n indicater switch.
    As Dirtwarrior says, you have to disco the 'advance' wire before strobbing it. It's the tan/white (I think?, haven't got mine here with me) by the brake servo. This is like pulling the vac adv hose off a regular distributor. Disconnect your battery at a later date for a minute to clear the trouble code it'll throw.
    Another thing to consider is that after disconnecting the tbi unit it oftens needs to be driven at 30mph for a short distance on a constant throttle to allow the sensor to 'get their bearing' .
     
  4. Madbomber88

    Madbomber88 1/2 ton status

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    Thanks for the help, I didn't know I had to disconnect the electronic spark control plug, but finally reading Haynes, I see that I do. Also, there's no way to adjust idle because it is all computer controlled, correct?

    To make sure I get this right, unplug the 'advance' wire, hook up the timing light, start the engine and adjust timing to specified degree of advance (according to my sticker it is 0), then do I plug the advance wire back in while the engine is running or do I have to shut it off first?
     
  5. southernspeed

    southernspeed 1/2 ton status GMOTM Winner

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    Shut it off first. Never pull or connect things on a comp controlled vehicle with the motor running. As to adjusting tickover, You need to put the jump wire between A&B on the aldl socket. Turn ign on but don't start it. Wait 30 secs then unplug the IAC plug. It's the square plug on the right hand side of the throttle body (as you sit in truck). It sticks out and backwards a bit, not the vertical one just in front of it (TPS). Take jumper wire out, start engine and set the idle speed (you may need to drill out a covering tab over the screw). Switch off, replace plug and disconnect battery for 1 min. Run and check idle!
     

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