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Engine stumbling/bogging when bangshifting manual trans

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  • #16
    Originally posted by 1969L78 View Post
    What would cause the vacuum reference system to induce this kind behavior in the fuel sump pressure regulator ?
    Well, imagine what happens in intake manifold at the moment when you shift manual transmission, there is fraction of a second moment when you lift the throttle before clutch is totally disengaged, so throttle blades are completely shut and your engine is spinning, say 6000rpm, therefor there is huge vacuum spike as you can see on my videos above. So that vacuum spike adjusts fuel pressure to drop and when you have already shifted gears and hit the throttle pedal again there is not enough fuel pressure for engine to run. That's my theory and everyone is free to think otherwise, but today I did some screen recording and fuel pressure was rock solid through gear changes and car works very well at WOT acceleration, so I'm happy now.

    Of course nobody with automatic trans won't have similar issues because you can shift at WOT, and even some people granny shifting manual trans won't notice this either because everything happens in slower phase..

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    • #17
      This is interesting. I finally got my car on the road for some run time. I have the issues that are identified on this post. When hitting it hard and fast, I have the associated cutting out when shifting. 1st to 2nd or 2nd to 3rd. It either stumbles and resumes normal operation or I am coasting to a stop on the side of the road. It doesn't make me happy when I am on an entrance ramp and need to accelerate and I am slowly rolling to a stop. I am running the fuel sump system and currently have PWM set to on and have the vacuum reference hose attached. I think I will give the hose removal a shot to see if that is what I am having issues with also. BBDart, did you leave the PWM set to on with your setup?

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      • #18
        Originally posted by ddsmith View Post
        This is interesting. I finally got my car on the road for some run time. I have the issues that are identified on this post. When hitting it hard and fast, I have the associated cutting out when shifting. 1st to 2nd or 2nd to 3rd. It either stumbles and resumes normal operation or I am coasting to a stop on the side of the road. It doesn't make me happy when I am on an entrance ramp and need to accelerate and I am slowly rolling to a stop. I am running the fuel sump system and currently have PWM set to on and have the vacuum reference hose attached. I think I will give the hose removal a shot to see if that is what I am having issues with also. BBDart, did you leave the PWM set to on with your setup?
        ddsmith Sorry haven't been following this forum lately, checked the PWM and it was ON. I have had this problem happen couple times this spring also so problem didn't go away completely.. Have you got your problem fixed yet?

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        • #19
          No, with the vacuum reference hose removed I still had this condition . So I have put the hose back on. I did get a recording app so I can see the parameters when it happens. The bad part is that I have yet to catch it acting up. I would really like to see it, so I can fix it. When I find out, I will be sure to update you. The car is in the garage finishing up some interior work and then I will take it out for some spirited driving.

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          • #20
            So I have caught this enough on my recordings to identify that it is associated with the PWM of the fuel pump. I turned PWM off today with the vacuum reference hose attached to the regulator and pounded it pretty good without the corresponding drop in fuel pressure that was showing up with it on. Essentially, with it on, fuel pressure drops to 0 psig when the throttle is reopened following the shift. I think it is responding to the throttle lift on the gear shift and then when you hit the throttle 100 % the pump speed isn't high enough to keep up with the required fuel flow. With PWM off the lowest my pressure dropped from the set pressure of 43 psig was about 33 or 34 psig before quickly recovering to set pressure. I am going to leave it off. The way I look at it, with a 454 eating gas, I'm not going to have to worry about it sitting in the sump too long.

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            • #21
              PWM I believe turns OFF ( Ground output from ECU stops pulses, and become constant ) at some throttle position/Vacuum level ( sees a load )/RPM.

              What kind of fuel pump are you feeding the sump with?.....I'm thinking it cant keep up....or it's internal regulator ( not the sumps ) just can't handle quick transitions of fuel volume.
              In my neighborhood, if you open your hood to check your oil, and the vehicle still runs after you close it.......Then your labeled the Neighborhood Mechanic.
              https://forums.edelbrock.com/filedat...?photoid=42773

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              • #22
                The mechanical fuel pump that is correct for a 454. If you believe that it is a supply issue then it shouldn't matter whether PWM is turned on or off. However, when I have it on, the fuel pressure drops to 0 following the shift with a full open throttle. With PWM off, that doesn't happen. This is with shifts above 5000 rpm. If I drive like a Grandma, then it doesn't show it's ugly head. Who does that? So I will leave it off and report back after a bunch more time behind the seat to see it is totally gone.

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                • #23
                  DDSmith, thanks for the status report....
                  Don't get too caught up in the numbers.
                  Just give the Engine what it wants &
                  Let the numbers be what they are.

                  Jim McFarland....

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                  • #24
                    After much testing I can say this issue is officially solved for me. If you are running an Edelbrock fuel sump and a manual transmission and you experience a bog/stall after a high rpm shift point, try turning the PWM off on the tablet. This permanently fixed my problem. One thing to consider is that Edelbrock put it there to prevent overheating the fuel in the sump. It is nice to know what is causing your problem so that you can determine the best solution.

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