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Thread: Getting owned by an 87 TBI

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  1. #1
    Electronic Ignition!
    Join Date
    Oct 2018

    Getting owned by an 87 TBI

    First, thanks in advance for all the helpful information on this forum. I've got an '87 GMC V2500 w/ HD option. It stumbles and bumbles under load. I've googled the hell out of my issues and below is all the new parts and tests I've thrown at it over time. The new distributor was the last new thing I installed. I really thought it was going to fix the problem since the old one looked crusty and rusty, but no such luck. The truck needed all new ignition parts anyway, so no regrets replacing them, but the truck is still undrivable.

    Here is my main problem I'd like to solve. Looking for any ideas on where to go next, I'm at a loss at this point.

    With MAP sensor plugged in (which is brand new and verified via OBD scanner to be showing proper readings), I'm getting about 15-16in of vacuum at idle. Watching the timing mark as I increase rpms at the throttle and hold them steady, I see timing go through a cycle of dipping down into where it shouldn't be. It dips down to around 8 degrees and then goes back up past the end of the timing mark gauge where it should be...then keeps bouncing back and forth as vacuum bounces up and down, too. Unplug the MAP and vacuum at idle goes to 21 in. As I increase and hold a higher rpm, I see timing advance increase, no more dips as when MAP is plugged seems to be operating properly in terms of what you expect a healthy engine to do as far as timing and vacuum readings are concerned.

    -New MAP Sensor (MAP voltage and kpa readings taken from obd1 reader are appropriate/correct)
    -New IAC Valve (did the relearn)
    -New Distributor (set at 0 degrees w/ advance unplugged), plugs (gapped at .035) & wires.
    -Smoke tested, replaced some questionable hoses and capped off non-essential stuff
    -Compression tested when I did the plugs and all looked OK.
    -New fuel filter
    -Went over grounds as best I could
    -Fuel pressure (gauge placed between filter and TBI unit) holds steady at 11psi as the truck stumbles and bumbles around the block at various throttle positions.
    -Truck has headers and has no O2 sensor (needs a bung welded in)
    -EGR solenoid is capped between solenoid and EGR.

    Amazon has an acdelco ECM for $105 with a $90 core rebate, so that is in the mail as well. For $15, I figured why not go for it?
    Last edited by GetItBilly; 10-30-2018 at 05:38 PM.


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