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Thread: Engine spasms when Manifold Pressure reaches atmospheric.

  1. #1

    Default Engine spasms when Manifold Pressure reaches atmospheric.

    Hi all, just as the title says. I've been fighting an issue for a while now (since it was installed in September 2018) and I'm still coming up empty handed. Spoke with Holley tech numerous times and they have no idea. Basically, as the engine reaches atmospheric pressure (which is about 94 kPa here at my elevation), the engine starts to buck & spasm, and generally loses power. AFR is generally rich when this happens, and will bounce around 11:1 - 12.5:1, so it's not a lean condition. Target AFR is steady, injector pulse width is steady, lb/hr is generally steady.

    I've checked fuel pressure, looked at datalogs (the AFR & MAP bounce around a little, but nothing all that noticeable), etc. and have come up with nothing. Holley even sent me a new unit to see if there was something wrong with my original one. This is very consistent, and is basically any time I really get on the throttle. Overall, the the EFI system is phenomenal. If I keep it under 80-85 MAP, all is well. It's extremely responsive, smooth, powerful, and pulls like a freight train up to 5000 RPM. But once I press the pedal far enough to reach full cylinder pressure, it starts to rapidly spasm, and drops in power. Any ideas? This is a 300ci inline 6.
    1985 Ford Bronco. 300 (305) ci inline 6. 9.1:1 compression, P&P head with enlarged valves, high flow exhaust. 32" BFG A/T tires. 5 speed ZF5 manual transmission with 3.55 rear end. Holley 550-850 2300 Sniper EFI.

  2. #2
    Join Date
    Jan 2010
    Location
    Brisbane, Australia
    Posts
    1,335

    Default

    Ignition system misfire under load?
    What ignition system are you using?
    What plug gaps?
    Regards, Gary

  3. #3

    Default

    Probably isn't a Sniper EFI problem. Could be your ignition or perhaps your valve springs are weak? Put a carb on there and see what happens, I bet you have the same issue.

  4. #4

    Default

    To me it sounds ignition related. Have you checked your plug gaps? Try setting them a bit tighter. 035" or smaller and see if that changes anything. I had a similar issue with my engine some years back. I set the gaps to .045" and it worked great until I would put a load on the engine. After mucking around for a bit thinking it was this or that, I pulled the plugs and tightened them up to .030" and the problem went away. Check your cap, rotors & plug wires too.

  5. #5

    Default

    Check your plug wires for excessive resistance as well.

  6. #6

    Default

    Thanks for the responses everyone. Much appreciated. I've looked into the ignition system, but I'm coming up empty handed.
    I have a Performance Distributors DUI HEI ignition: https://performancedistributors.com/...1-289-302-cid/
    I'm also using their LiveWires: https://performancedistributors.com/...ivewires-ford/
    The engine was professionally rebuilt by a shop that specializes in racing engines about 3 years ago, and runs like a top.

    I initially thought it might be the ignition, so I swapped the entire system out, including the spark plug wires, for my stock Duraspark II ignition and it responded exactly the same. New spark plugs as well. The only thing they shared was the 12 gauge power wire.

    I did play around with the spark plug gap a bit. The DUI ignition brags that you can open the gap on the plugs to 0.055". I tried dropping it down to the stock 0.044". Next was 0.035", which helped a little bit, so I went to 0.025" to experiment. The small 0.025" gap probably reduced the issue by about 50%, but it's still there. I really shouldn't have to go lower than 0.025", and I also feel like it's masking something else, instead of addressing the issue. Dunno though.

    This is why it keeps pointing back to the EFI unit. Two 100% different ignition systems experienced the same issue.
    I also don't ever recall having this issue back when it was carbureted (which it was for a few years after the rebuild).
    I'd love for it to be an ignition issue, maybe something that's not playing nice with the EFI, but can't figure out what it could be.
    1985 Ford Bronco. 300 (305) ci inline 6. 9.1:1 compression, P&P head with enlarged valves, high flow exhaust. 32" BFG A/T tires. 5 speed ZF5 manual transmission with 3.55 rear end. Holley 550-850 2300 Sniper EFI.

  7. #7

    Default

    One other idea that someone on another site mentioned. On the inline 6, the intake hangs out over the side of the engine, as opposed to being up on top like a V8. So, the stock 1BBL carbureted setup had a heat riser that allowed a small amount of exhaust from the exhaust manifold to warm up the bottom side of the intake. This gave the carb a stable operating temperature. It also, supposedly, helped prevent fuel condensation from a cold intake floor, causing it to puddle, and have a bog off the line. It also aided in cold weather startups.

    I no longer have the stock intake/exhaust manifolds, so that provision is gone. When it was carbureted with a 4bbl, I had a plate that supplied heat with engine coolant. When I went with the Sniper EFI, I removed the heat source from the bottom of the intake since I figured it was no longer needed. Someone was mentioning that the TBI still needs the heat source:
    First big problem is assuming the EFI throttle body doesn't need manifold heat. It needs MORE heat than a carb. It does NOT "atomize" fuel better than a carb. It does not emulsify fuel at all, which a carb does, and it merely tries to squirt small streams of fuel into the air stream to be sheared. For that reason, when sheared across throttle blades, it will do a better job of "atomizing" than at full throttle, with worse air velocity and no vacuum to help with phase change, etc.

    Second thing I see is the VE map is majorly over compensating for what looks like a poorly tuned acceleration enrichment. Problem is that the acceleration enrichment can't be properly tuned if it's causing a misfire from not having manifold heat. The system is definitely chasing it's tail adding way too much fuel when it isn't needed, and in my opinion you're fighting a rich miss. Cylinder #6 plug is evident of it and is the first to miss from being overly rich and fuel dropout.
    When the engine went fuel injected in '87, they skipped TBI and went straight to MPFI, so there was no need for that anymore. I have no idea how they would have set up a TBI on an inline 6. Any thoughts on this prospect? Would just heating the underside of the intake make a difference with the Sniper?
    Last edited by 85 Phoenix; 04-24-2019 at 12:30 PM.
    1985 Ford Bronco. 300 (305) ci inline 6. 9.1:1 compression, P&P head with enlarged valves, high flow exhaust. 32" BFG A/T tires. 5 speed ZF5 manual transmission with 3.55 rear end. Holley 550-850 2300 Sniper EFI.

  8. #8

    Default

    Lastly, this looks odd to me. Here's a view of my Volumetric Efficiency Table. I've tried re-initializing it to base a few times and it always, almost immediately, relearns to this. Large View: https://www.supermotors.net/getfile/...iginal/afr.jpg

    1985 Ford Bronco. 300 (305) ci inline 6. 9.1:1 compression, P&P head with enlarged valves, high flow exhaust. 32" BFG A/T tires. 5 speed ZF5 manual transmission with 3.55 rear end. Holley 550-850 2300 Sniper EFI.

  9. #9
    Join Date
    Dec 2009
    Location
    Connecticut
    Posts
    21,460

    Default

    That's not the Learn Table, that's the Base Fuel Table.
    May God's grace bless you in the Lord Jesus Christ.
    '92 Ford Mustang GT: Dominator MPFI & DIS, A/C, Spal dual 12" fans/3-core Frostbite aluminum radiator, Pypes dual 2.5" exhaust/X-pipe/shorty headers, 100HP progressive dry direct-port NOS, 160A 3G alternator, Optima Red battery, LenTech Strip Terminator wide-ratio AOD/2500 RPM converter, FRPP 3.31 gears, Cobra Trac-Lok differential, Moser 31 spline axles, '04 Cobra 4-disc brakes, '93 Cobra booster & M/C, 5-lug Bullitt wheels & 245/45R17 tires.

  10. #10

    Default

    I know it's not the Learn Table. It's the VE table (with the current Learn values transferred in).
    1985 Ford Bronco. 300 (305) ci inline 6. 9.1:1 compression, P&P head with enlarged valves, high flow exhaust. 32" BFG A/T tires. 5 speed ZF5 manual transmission with 3.55 rear end. Holley 550-850 2300 Sniper EFI.

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