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Thread: Getting owned by an 87 TBI

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  1. #1
    Electronic Ignition!
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    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 in...it 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; 2 Weeks Ago at 05:38 PM.

  2. #2
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    What you've described isn't the problem. Well, maybe it is but it really sounds like normal operation. See, when the vacuum drops so does the spark advance. When you first open the throttle the vacuum drops and the ecm reduces timing. Then as the rpm increases, vacuum increases, and the spark advance increases.

    How easy is it to increase engine RPM with the MAP disconnected? The engine should be running pretty rich with the main vacuum line disconnected. Have you ever confirmed that the MAP sensor is seeing a full vacuum signal at idle when connected?

  3. #3
    Electronic Ignition!
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    Yeah, sorry, I meant the problem I want to solve is why does vacuum / timing advance momentarily but repeatedly drop when I raise rpms above idle and hold it there (while in Park)? My truck does have the usual tan/black wire that I disconnected when setting timing to 0 degrees.

    With MAP circuit unplugged (I'm unplugging the connector, not removing the hose and creating a vacuum leak), yes it certainly runs rich, RPMs of course jump up, but vacuum readings look great and continue to look as expected if raising and holding the RPMs in park.

    Map sensor is plugged into correct port on back of TBI between fuel lines. I've tee'd the line between TBI and MAP and used a vacuum gauge to confirm the readings I shared earlier.

    I thought that even without an o2 sensor, my truck should run strong as far as not stumbling and holding a consistent vacuum / timing advance for above idle RPMs while in park. It will just always remain in open loop and therefore may not run as optimally as when reaching closed loop, but it should still run relatively strong assuming all other components are working as they should. Is that not correct?
    Last edited by GetItBilly; 2 Weeks Ago at 06:10 PM.

  4. #4
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    You are correct in expecting the truck to run well enough to drive with the O2 sensor unplugged. The older calibrations were not always as good at running their best without O2. Is the ECM setting a code without the sensor?

    I would expect the truck to stumble and run poorly without the MAP connected. I would not expect the rpm to increase and the truck to run well enough to continue testing.

    MAP sensors can respond very quickly. Sometimes faster than a vacuum gauge and definitely faster than the ALDL reader can show. I have an oscilliscope that I use to watch sensor signals when needed. It's possible that the MAP reading is changing due to a loose connector or loose connector pins or due to a broken or chafed wire. You could try pinching off the vacuum signal to the MAP while the engine is running to trap vacuum and force the sensor to have the same reading, then increase RPM to see if the timing bounces around. You could also turn the key on and leave the engine off and try gently moving wires while watching voltage on the MAP signal line. I often found the old scantool readings hard to use because they updated so slowly. I think that a digital voltmeter is a better tool for finding intermittent problems.

  5. #5
    Electronic Ignition!
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    Thanks for the feedback, 1project2many.

    I think my map sensor is providing the proper readings. Video above shows map sensor voltage, vacuum tee'd into MAP sensor hose, and RPM.

    -Video starts with engine idling @ ~950rpm (i know that needs addressed) w/ map sensor and vacuum hose all hooked up.
    -@ :06 I hit the throttle and hold it steady, engine starts seeking
    -@ 1:04 I unplug the map sensor, rpms go up to ~1500 and vacuum to around 22
    -after that I blip the throttle a couple times as well as increase rpms up to 3000 towards the end and hold it steady for a bit.
    Last edited by GetItBilly; 2 Weeks Ago at 08:44 AM.

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    What happens if you move the TPS without moving the throttle plate? It should cause the injectors to deliver additional fuel. Does the engine run better then? You might have to remove the TPS from the throttle body or use a spare TPS to do this.

  7. #7
    Fuel Injected! jim_in_dorris's Avatar
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    2 thoughts. 1. Is the map sensor plugged into the correct port, and more likely, unless that is an open loop tune, it will not run right without an O2 sensor. Step 1 get an O2 sensor installed
    Square body stepsides forever!!!

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