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Thread: Diagnosing closed loop misfire with TunerPro

  1. #1
    Carb and Points!
    Join Date
    Apr 2024

    Diagnosing closed loop misfire with TunerPro

    I'm learning Tunerpro and I got it set up with the latest definitions for my 85 corvette. Some history on this.
    85 corvette with 134,000 miles.
    Intermittent misfire that sounds like a popcorn machine. When Vette goes into closed loop it starts to misfire. Also very strong fuel smell.

    Things that have been replaced.
    IAC, CTS, Knock sensor, O2 sensor, spark plugs, Spark plug wires. Cap and rotor, Plenum air temp sensor, all intake manifold gaskets, Valve stem seals, Valve cover gaskets, Alternator, Belt, Belt Tensioner, Gas tank, Gas tank sending unit, fuel pump, fuel filter. oil and coolant with a flush.

    On top of that its had a Smog pump delete, EGR delete as well as all the Emissions stuff removed. 2.5" exhaust from headers back with Race Pro mufflers.

    Things that have been checked. Harmonic balance is correct, checked with a piston stop. Removed Distributor and tore down all components for testing. ICM passed check at O'reilly and AutoZone. Pickup coil passed with being in the correct Ohm range. Ignition coil tested good. Set timing at 6TDC after reinstalling and no change. Car runs great up until closed loop then starts making a puffing noise like crazy. Disconnecting EST totally eliminates misfire. Runs like a dream.
    I've spent so much money trying to chase this down but I'm no where closer it seems to figuring it out. But I do know my timing is correct. Also my compression numbers across all 8 cylinders range from 160-170


    I ending up buying a new D.U.I. distributor just to be sure it wasn't that.
    Also I forgot to mention that FPR diaphram is replaced and the Bosch injectors have been rebuilt and flow tested. Leak down test shows that fuel pressure is held all night.40 PSI before key on, losing about 20psi by morning if I pressurize the rails before bed.

    I know I've fired the parts canon on this hardcore. I'm just so ready to drive this thing.

    I finally broke down and bought an ALDL cable from OBDDiag inc. My quesiton is if I upload my datalog session could someone help me make heads or tails of it?

  2. #2
    Fuel Injected! MO LS Noobie's Avatar
    Join Date
    Apr 2022
    Missouri Ozarks
    Quote Originally Posted by LarryBoyG View Post
    My quesiton is if I upload my datalog session could someone help me make heads or tails of it?
    There are multiple people here to help. I would monitor your fuel pressure, timing, and fuel trims. It might be just the PCM. Let's see what the data log shows.

  3. #3
    LT1 specialist steveo's Avatar
    Join Date
    Aug 2013
    do send a datalog but you have replaced a lot of parts.

    that is really damn frustrating

    have reviewed your list of stuff you've done and determined that it's likely that one of your replacements or measurements is incorrect or you've missed something fundamental

    a compression test is great but what if you have an intake or exhaust leak it wouldn't capture that

    it's like 'hey i have drawn a straight line which my ruler says is straight but everything else says its bent' well maybe your ruler is bent.

    the fact it goes to hell in closed loop but you have replaced the sensors indicates a possible o2 wiring/grounding issue or massive metering issue like a really big exhaust leak pre o2 sensor

  4. #4
    Fuel Injected!
    Join Date
    Jan 2021
    Deep in the heart of Texas
    EST module replaced?

  5. #5
    Fuel Injected!
    Join Date
    Jan 2021
    Deep in the heart of Texas
    Might take notice of the O2 voltage readings.

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