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Thread: Mid-Range Misfire Problem

  1. #11

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    I setup a 2D Table for Target A/F Ratio yesterday and smoothed it to help the transitions, but didn't get any significant improvement.

    Greg: Yes, seems to clear up at 2800 RPM or so. I have checked for leaks numerous times and I'm not finding anything. Wiring is separated on good relays tied directly to the battery. I have a fuel pressure gauge in the cockpit, it never moves off of 58 psi. Still stumped.

  2. #12

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    Might not be a Sniper problem. Widen your problem solving focus. Could be some sort of vacuum leak or a bad coil. Most of the "Sniper" problems reported on this board end up not being Sniper problems. People just assume the Sniper is bad since that's what they're focused on. We swap carbs back on Sniper engines all the time just to verify where the problem is. Sometimes we find something else that we were not even thinking of.

  3. #13

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    Your Learn Table has way too much fuel in it in the high load/low RPM area. When you floor it in 1st gear the ECU thinks it's way lean then it goes full rich all while the CL COMP is adding 44%. At first blush, it looks like a tuning issue. I'd start by zeroing out just the Learn Table and start over.

  4. #14

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    At the beginning of that datalog, it looks like you got WOT for 5 seconds and the RPM never changes. Is the engine bucking & sputtering at that point?
    69 Camaro
    400 SBC, ProCharger D1SC

  5. #15

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    Significant breakthrough today. I pulled the plugs again and noticed that the rear three looked more fouled than the front three. The WBO2 sensor is in the collector for the front three (there's a Y-junction to join the two "banks" behind the WBO2). That seemed too much to be mere coincidence, so I decided to try moving the WBO2 behind the Y-pipe so it's reading all six cylinders instead of the forward three.

    Test drive - significant improvement. The truck is running much better through most of the rev and speed/load range. I was still able to get a persistent misfire at high load, low speed (lugging up a hill in 4th gear), but it's probably 90% cured as compared to where I was yesterday. I put about 85 miles on it today trying to improve the Learn Table. I still need to get a datalog for the conditions where it's misfiring, but at least it's running good in most of the rev range now.

  6. #16

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    That means you have some sort of mechanical problem which is confusing the Sniper. Could be that your intake manifold is a bad design, so you have a different fuel mixture in the front cylinders. Or the exhaust manifold could have a flow issue and the WBO2 doesn't work correctly where it was first located. I'd zero out the Learn Table or just run the Wizard and start over.

  7. #17
    Join Date
    Jan 2010
    Location
    Brisbane, Australia
    Posts
    1,581

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    The intake manifold flange for the throttle body biases the fuel towards the rear, since the butterflies are facing that way for most of the time. You need to space the Sniper up. This is a common issue with inline six cylinder engines, especially when you check A/F ratios front to rear. So adding a 1/2 inch spacer might be all you need to even it out. Gary
    Regards, Gary

  8. #18

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    I think there's definitely something happening with fuel distribution. The Sniper is already sitting on a half inch spacer though which adapts from the 2300 flange to the Aisin flange. There's also the fact that it's most prevalent at WOT where the throttle blades are full open and should be helping to straighten the airflow. The fact that it's not a problem at higher speeds though seems to indicate some sort of flow induced phenomenon when manifold pressure is high and air speeds are low.

    More confusing is the fact that there are quite a few folks who have done this setup on the same engines and are not seeing this effect. Again that points to other systems, but I've checked everything over many times and can find nothing wrong.

  9. #19

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    If you're sure that other people with the same exact intake & exhaust manifold aren't having the issue, then you have another problem. Could be almost anything including vacuum leaks, machining mismatches, something wrong with your valvetrain, etc. At this point, I'd be almost 100% sure that you don't have a Sniper problem. The problem is made worse with a Sniper because of the feedback loop, which might be why you noticed it. A carb just dumps fuel into the intake and doesn't worry about what the AFR is in the exhaust.

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