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Old 11-04-2019, 06:54 PM   #1
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5.9 Cummins ISB misfire off idle under light load only

O.k, Here we go. This is back story:
A few months ago we decided to pull the motorhome out & go to Vegas. It hadn't been used in about a year. I changed oil & filter. (fuel filters changed about a year prior but only about 500 miles on them). looked over everything checked tire pressures etc, good to go. About 2 hours into trip, out of Wickenburg headed towards Kingman on heavy climbs I was getting a check engine light. The light would go off as soon as you got out of the throttle at top of hill. No other symptoms at first. The farther I drove the more it came on, & I noted the power was down when it was on. Eventually it was just on all the time & it was a dog on power. Downhill, light would go off but eventually that stopped too & it was just on solid all the time. Got to Vegas, did our thing. I read on here & some other forums, found the switch for flash codes. That didn't work, no codes. Drove home, started off without light but as soon as I got into the throttle, no power & check engine light on. What a lousy drive home. Got home, ordered a scanner that will communicate with ecm, no codes stored? Are you kidding me? How stupid!

I researched the forums & found a guy with a story just like mine except he had an ISC. His remedy was fuel filters. I replaced both of mine. Test drove multiple times, no problems! awesome! Or so I thought.

Now we have a problem:

Next trip is to take the 4 year old to Disneyland for her 5th b-day. Ran solid all the way there I got 11.3mpg from Phoenix to Blythe! Go home, Getting off the freeway & come to stop @ stoplight. When the light turns green & I give a little throttle, I feel a vibration. My head & seat of my pants says u-joint. More throttle, it clears up. Hmmmm, ok, whatever. Next stoplight, same thing. Turn on the sound for my backup camera so I can hear the engine. Idle sounds normal. ANYTHING more than idle to about 1/4 or 1/3 throttle it drops a cylinder. If you go to 1/2 throttle, its fine & normal. Also seems to clear up by 1750 rpm. It drives fine on the freeway, only at light load & slow speed.

I am afraid I damaged the VP44 with the fuel filter issue but I cannot find anyone having this problem on any forum. This doesn't fit any VP44 troubleshooting guide either.

I need guidance on diag before I throw a couple grand at a rebuilt VP44.
Just in case its not in my sig, 1999 Itasca Sunflyer. Cummins 5.9 ISB. Allison MD3060.
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Old 11-04-2019, 07:07 PM   #2
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Mine on a truck it did the same thing it would run then the next time 1/2 power , what happens is it burns and electrical relay inside the pump contols out kinda acts like a throtle possion sensor .. I replaced my injector pump and added fass system with fuel press gage.. no more running the damn vp44 dry....
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Old 11-04-2019, 07:22 PM   #3
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Mine would run good till it warmed up and then i'd limp home at 1/2 throtle , like I was told the electrical control part of pump is what burns out when it doesn't get enough fuel to cool.. I bought mine new one thru www.thoroughbreddiesel.com/Diesel/Performance
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Old 11-04-2019, 07:51 PM   #4
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I have the same coach, don't hear a lot about them. curious to see what you find out
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Old 11-04-2019, 08:19 PM   #5
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Did you check the lift pump ??? If the lift pump goes out and then the vp44 burns up...

Signs of Failure

The VP44 injection pump is designed to be fed by the lift pump for proper pressure and lubrication. Unfortunately when the lift pump fails, the injection pump will continue to draw fuel through the lift pump under vacuum. Normally you can tell little or no difference in the engine operation when this happens. However, the VP44 injection pump can not handle this loss of fuel pressure.

The injection pump will eventually be unable to achieve the fuel demand from the engine ECM, causing the engine to derate and feel extremely underpowered. At this time, the injection pump will either need to be rebuilt or replaced (which is very expensive and not at all enjoyable).

Prevention Techniques

The best way to prevent this is to monitor your fuel lift pump pressure. This can be done two different ways:

1.) Install a manual gauge. 2.) Install a sending unit to run an indicator light.

The lift pump should provide 10-15 psi at idle, and no less than 5 psi at wide open throttle (WOT). Cummins supplies a sensor kit that can be purchased, which will turn and indicator light on if the pressure drops below 3 psi. This warns you that you have lost fuel pressure and can have the problem repaired before your ruin your VP44 injection pump.

Fuel lift pumps for the Cummins diesel have been an issue since originally installed with the release of the 24 valve ISB. Cummins has changed designs several times in search of creating a more reliable pump, but it is still safest to know what your pressure is at all times.
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Old 11-04-2019, 08:58 PM   #6
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Sorry I posted so many times , I'm old and think slow / I should have said check the fuel pressure on the lift pump first !! If you own a 5.9 between 1999-2002 you have the vp44 injection pump .. You need to monitor the pressure from your lift pump.
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Old 11-06-2019, 11:16 AM   #7
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I'm going to try to get out today to take a measurement of the lift pump pressure today.

What do you think the chances are its an injector problem?
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Old 11-06-2019, 04:51 PM   #8
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If the lift pumps out , then maybe you got lucky !! I hope so
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Old 11-06-2019, 05:05 PM   #9
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I had similar symptoms on my 5.9. It turned out the lift pump had developed a leak. Im not sure how much it was functioning. We changed out the lift pump with the FASS pump system. I was hoping this would cure the engine light issue but alas it did not. We bit the bullet and replaced the vp44, and 6000 miles later, no problems. I also bought a code reader. I bought the scangaugeD. The code it was giving is not a Cummins code. I found the code on this forum. It was a low fuel pressure code. This lead us to the lift pump. Apparently you can call scangauge and they will tell you what the code means as well. Changing the vp44 is not a small job, nor is it cheap. Unfortunately, these engines are famous for faulty lift pumps wrecking the main injection pump.
We ended up removing the rad and CAC to get better access to the rear of the engine. You have to remove the gear that runs the pump through the crank vent cover. Perhaps it can be done another way, but this is what my mechanic friend recommended. Its a good chance to inspect and clean your rad and CAC while you have them out. I also changed my water pump, and serpentine belt and tensioner while it was so easily accessible. Good luck on your repair. On one other note, if the rad fluid is down at all, it can trigger the engine into limp mode, and light the engine light as well. Just a shot in the dark. The fact that going up hills makes it happen might indicate your rad fluid is low. The sensor is on the forward side of the reservoir.
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Old 11-08-2019, 06:10 AM   #10
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Quote:
Originally Posted by winniman View Post
I . Changing the vp44 is not a small job, nor is it cheap. Unfortunately, these engines are famous for faulty lift pumps wrecking the main injection pump.
We ended up removing the rad and CAC to get better access to the rear of the engine. You have to remove the gear that runs the pump through the crank vent cover. Perhaps it can be done another way, but this is what my mechanic friend recommended. Its a good chance to inspect and clean your rad and CAC while you have them out. I also changed my water pump, and serpentine belt and tensioner while it was so easily accessible. r.
No need to remove all that to replace the v44, belt, tension er or water pump if you have a bed over the engine that lifts up for access.
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Old 11-08-2019, 09:19 AM   #11
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No need to remove all that to replace the v44, belt, tension er or water pump if you have a bed over the engine that lifts up for access.
It probably can be done without removing the rad, but as stated, if the rad hasn't had a proper cleaning and inspection, the pump is much easier with the rad out. I also sand blasted and repainted the entire area while I had easy access. I can access mine under the bed. Sitting on top of the motor for hours isn't much fun either.
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Old 11-09-2019, 06:21 AM   #12
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It probably can be done without removing the rad, but as stated, if the rad hasn't had a proper cleaning and inspection, the pump is much easier with the rad out. I also sand blasted and repainted the entire area while I had easy access. I can access mine under the bed. Sitting on top of the motor for hours isn't much fun either.
I would agree. Removed my Radiator too when it overheated and had to replace it. I also took off the timing cover to inspect the interior and replace the crank seal. Also did much of what you did but did not replace the water pump.
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