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Strange behavior, Cummins P-Pump

Discussion in '5.9L 12 valve Cummins' started by bob_442, Mar 28, 2019.

  1. bob_442

    bob_442 mad scientist

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    I've got a mid-90s 6bt from a Dodge implanted in a 1988 Chevy V30 Dually Crew Cab. I use this rig almost exclusively with a truck camper on it. The whole thing is just shy of 12K lbs with me in it. Behind the engine is an NV5600 6-speed manual transmission and behind that is an NP205 transfer case. The truck also has an exhaust brake; it is very nice for those descents. Generally, the truck does a really good job, though it is a bit stressful after a long day of driving a very top-heavy package.

    I've got a bug in the system that is dogging me that I would like to get to the bottom of. There are several symptoms that may or may not be related.

    First, the performance between the right and left fuel tanks is noticeably different. The truck performs better on the right tank. Each tank has its own electric pump inside, and the 6-hose selector valve with return lines is in place. Second, often after, or during a climb, the engine gets into a funny state whereby there is a severe loss of power. In this state, it starts running rough. When increasing throttle, it does respond, a little, but only after a delay...about a second or two. If I push in the clutch and release the throttle, it dies. If I let out the clutch, it restarts, and continues misbehaving. If instead, I wait 5 seconds before releasing the clutch, it generally comes back to life -- normal operation.

    This generally is more likely to happen on the left fuel tank. It is very, almost certain, to happen on modest climbs, or even on level ground if the tank is very full. Weird.

    OK, so I mentioned I've been fighting this for a while. I figured I had a bad pump and have a pressure problem. So I installed a fuel pressure, pyro, and a boost gauge on the a-pillar. Really glad I did that! But, I digress. There isn't a problem with the pressure. Initially, the pressure was around 29-31 PSI. I lowered it to 25-ish (there is an adjustable regulator inside the frame rail). No change. When the engine misbehaves, you cant see it on the fuel pressure gauge. It is obvious on the boost gauge of course. I figured, well, it's not the fuel feed, it must be the fuel return. So, I put the truck on the rack and pulled the lines on the selector valve. I manually pumped fuel in and out of each tank. I blew compressed air back in the return lines (with the caps off, of course). I even tried driving with the caps on loose, to eliminate tank pressure or vacuum as an issue. I replaced the selector valve.

    One more thing. I thought maybe the left tank was letting air bubbles into the system. I think I have ruled that out because when I flip the selector switch under load, there is zero delay in the exhaust note changing a bit between the tanks. If there was air getting in, I figure it would take at least a second to make it from the selector valve, thru the filter, and into the IP. And, yes, I have replaced the fuel filter.

    I drove the rig last week from Morgan Hill, Ca., south to 101/58, east to Barstow & Vegas, north thru Nevada to Gardnerville, and then finally west back to Morgan Hill. Again, I suffered with this issue.

    The IP does have some mods done to it. I think the springs have been replaced to raise the governor speed and/or increase the fuel supply. This was done by the PO. I have to be very careful not to let it get too hot. It can easily fly past 1100 degrees if I don't pay attention and downshift or slow down when climbing.

    I'm wondering if the collective wisdom out there might be able to help me shed some light on this. Thanks for reading. Hopefully at the very least it's an entertaining story. :)
     
  2. Oledirtypearl86

    Oledirtypearl86 Full Access Member

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    I was having the delayed throttle response with my Jetta and it turned out to be the IP was shot and by the time I figured it was the IP the pump was trash but your symptoms match up with my jetta
     
  3. Crawler

    Crawler Full Access Member

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    If this is a p-pump 12v then you will actually want more fuel pressure.

    Can you give a few more details about the engine please.

    Happy to help in any way that I can. What electric pumps are you running?

    Most 12v guys stick with the mechanical block mounted pump unless we go to monster electrics. Not many small electric pumps will flow the amount of fuel needed at the pressures needed.
     
  4. bob_442

    bob_442 mad scientist

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    The pumps (one per tank) are the stock electric pumps that originally fed what was almost certainly a very thirsty TBI 454.

    I turned the pressure down a bit because I was getting slightly lower readings (like 28-29 instead of 31-32) from the better-performing tank. I can easily turn the pressure back up, to whatever the pumps are capable of delivering.

    Seeing better performance with slightly lower pressure is what put the idea in my head that maybe the poorly performing tank has a return-line obstruction.

    Bob
     
  5. Crawler

    Crawler Full Access Member

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    Is this a 94-98 12v with a p-7100 injection pump?
    Or
    A pre 94 12v with a ve rotary style pump?
     
  6. bob_442

    bob_442 mad scientist

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    The former -- inline -- not rotary.

    Bob
     
  7. bob_442

    bob_442 mad scientist

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    An update on this thread. After lowering my regulated pressure before my last trip and not having great results, I did the opposite. I raised the regulated pressure a bit -- up into the high 30's. In doing this I was reaching the limits of the stock electric pump, so the pressure varies a bit; more so than before. I drove the truck with the camper on it 1700 miles through California, Nevada & Oregon over the last 8 days, and never once did the strange, undesirable behavior recur. Raising the regulated pressure limit also gave me insight into what is going on with the performance difference between the tanks. The left tank, which when selected, lowers my performance has a bit, has lower pressure than does the right tank.

    There was one other change. Now that I have an easy-to-read pyprometer mounted on the A-pillar, as opposed to one hidden way down low where the driver can't see it, I'm doing a much better job of lifting up my right foot, or downshifting when the temps exceed 1100 degrees. It is quite easy to reach that temperature, especially when using the right tank.

    I'll likely need a pump replacement, upgrade, and/or relocation at some point, but for now I think I'm happy that I've eluded the bug I've been chasing.

    Bob
     
  8. Thewespaul

    Thewespaul Supporting Vendor Supporting Member

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    I think you could really simplify the setup by installing sumps in both tanks and only running one quality fuel pump. Both my 80s chevys when I had them had issues with crap in the tanks causing similar issues to what youre sharing.
     
  9. 79jasper

    79jasper Chickenhawk

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    Though I've never ran one, I'm a fan of sumps. Especially after the problematic Ford pickups breaking off. ..

    Sent from my SM-N960U using Tapatalk
     
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  10. bob_442

    bob_442 mad scientist

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    I've been thinking of having a single (easily serviceable) external pump. One big downside is loss of redundancy. With one pump/tank, if a pump fails, it's no biggee with respect to getting home. Another upside is I can run a tank dry without worrying about a pump running dry while the other tank keeps the truck operating.

    I know the in-tank pumps are well known for just giving it up like a light switch in the field, stranding the vehicle, requiring major surgery. Can the same be expected for an external electric pump?

    Bob
     
  11. Thewespaul

    Thewespaul Supporting Vendor Supporting Member

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    No, and if you put it in an easy to reach place it’s a simple replacement, maybe 30 minutes tops. Can’t do that with in tank pumps
     
  12. Ayla

    Ayla Registered User

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  13. Ayla

    Ayla Registered User

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    Get a FASS 150 GPH fuel delivery system or an AIRDOG 150 GPH system.
     

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