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Thread: Mid-Range Misfire Problem

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    Default Mid-Range Misfire Problem

    Hey guys, I'm absolutely banging my head against a wall with a Sniper issue and hoping you all can help. This is a 2BBL unit installed on a freshly rebuilt Land Cruiser 2F engine. The truck starts, idles, and runs fine at light load but as soon as I go to heavy throttle (50% TPS & up) the truck starts to misfire & breakup. If you tip out of the throttle it will clear up and start to pull again. If you suffer through it the engine clears up once it hits 3000 RPM or so. It seems less sensitive in the lower gears, but definitely exhibits the problem in 3rd and 4th. Sometimes it's worse than others but it's never completely gone.

    I've been troubleshooting this for weeks and have done the following:
    Checked, rechecked, and rechecked fuel delivery. Always a solid 58 psi.
    Checked timing repeatedly, changed out the distributor and all ignition components.
    Varied plug gap from .030" - .055" with no effect.
    Switched the throttle body unit from another truck to this one and reran the wizard, still had the problem.
    Swapped the handheld between the two trucks, reran the Wizard, still had the problem.
    Wrapped the wiring in aluminum foil as much as possible to shield from RF interference, no change
    Changed out the WBO2 sensor, no change

    I've run a lot of datalogs and spent over an hour on the phone with Holley. I sent them my config file and datalog and they were completely stumped. They pointed me back to ignition problems or something else in the main engine.

    Attached is a datalog file that has a series of pulls. I start from a roll and go to WOT in 3rd gear letting the truck do it's thing. Then I shift to 4th and repeat. After letting off in 4th I let the truck slow down then do a pull in 2nd followed by another in third. In the pulls you can see the Learn percentages are sky high and it's not correcting AFR well.

    In talking with Holley and my own looking at datalog files no one can figure out why the system is going to such high Learn percentages and running rich. During driving it seems like as I tip out and manifold vacuum comes up the system gets happy again and is able to resume good fuel control. The WBO2 signal seems noisy to me, but it's isolated from the ignition by the engine block so I doubt it's getting interference. The truck is definitely running rich when it's misfiring, I've already changed out one set of plugs and the tailpipe has a lot of soot accumulation.

    Any help you guys can offer is much appreciated. I'm about 30 seconds away from throwing this thing out the window and putting a carb back on the engine.
    Last edited by ddl67; 10-16-2019 at 03:28 PM.

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