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Old 06-13-2021, 08:22 AM   #1
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ISC 350 Puzzle

I am trying to determine what course of action to take based on the problem my 2001 extremely reliable ISC 350 had on our last trip. (Approximately 90,000 miles on the engine.) I had to replace the fuel filter just to get it to start and keep running after taking it out of storage. The last trip was in September 2020. The tank had about 70 of the 80 gallons in it when it went in storage here in Houston. It ran fine for 120 miles on day one. Barely made to a fuel station on day 2 due to power loss and misfiring. Topped off and problem went away for 120 miles. Day 3 same problem until I topped off again with 42 gallons. No problem since but I have only allowed the tank to drop 20 gallons before filling up again.
Could the pressure of a mostly full tank be assisting the fuel delivery system? Or did I just run a bunch of bad fuel out?
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Old 06-13-2021, 08:25 AM   #2
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By the way. The diesel generator never missed a beat during this ordeal. Thanking the Lord every day. We give Him the credit for our safe but scary journey.
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Old 06-13-2021, 08:31 AM   #3
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If there was algae in the fuel filters, you need to use a BIOCIDE such as Biobor JF.


Available at any marine store with diesel boat owners in the area.


If not algae, please tell us what you found in the filters, including air.
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Old 06-13-2021, 08:36 AM   #4
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Your 2001 ISC , will have the CAPS ( Cummins Accumulator Pressure System ) injector pump . The system uses a primer pump ( short duty cycle after key on) then the mechanical fuel pump section of the main injector pump takes over drawing fuel from the tank.
The primer pump is a known issue ; due to gasket problems , that cause some fuel leakage when the pump cycles and air to be drawn into the CAPS pump while the motor is running .
First thing to do is locate the primer pump ( some call this a lift pump ) and check for leaks and tighten the three mounting bolts .

Many posts on upgrading the system with a FASS , fuel pump and filter set up , that might be of interest .

Picture shows the area of leak and the bolts that require tightening . New gasket no longer available from Cummins but can be found online.
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Old 06-13-2021, 08:47 AM   #5
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By the way. The diesel generator never missed a beat during this ordeal.
The fuel pickup for the generator , doesn't reach to the bottom of the tank , so it may not have picked up any crud.
The Cummins fuel system , uses only a fraction of the fuel delivered to the injector pump , the rest is returned to the tank , by the time you've used a tank of fuel , all the fuel has passed through your filter(s) 5 to 7 times ; so if there is crud in the tank the filters can plug up fast.

Keeping the tank full , my be helping keep air from entering at the primer pump gaskets .
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Old 06-13-2021, 08:53 AM   #6
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Skip426 has given a great rundown on the most likely potential problems. Investigate both possibilities, lift pump and filters, ASAP, not only to resolve the issue, but if the lift pump is leaking air, that is the beginning of the death march for the main CAPS pump.
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Old 06-14-2021, 01:56 PM   #7
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I did replace the lift pump 2 years ago. It was leaking fuel at the gasket.
Is it possible for it to suck air but not leak fuel?
I will check the bolts and change the fuel filter again to see what might be in it now. I did not inspect the filter I replace in December.
The real puzzle to me is that the last 150 miles or so have not had any of the symptoms, but the tank has always had 60 of the 80 gallons in it...
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Old 06-14-2021, 04:59 PM   #8
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Quote:
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I did replace the lift pump 2 years ago. It was leaking fuel at the gasket.
Is it possible for it to suck air but not leak fuel?
I will check the bolts and change the fuel filter again to see what might be in it now. I did not inspect the filter I replace in December.
The real puzzle to me is that the last 150 miles or so have not had any of the symptoms, but the tank has always had 60 of the 80 gallons in it...
Keep in mind the lift pump is only pressurized for about 30 sec. and the rest of the time it has internal negative pressure. Given that, it is possible that fuel is not detected during that time because it would have to accumulate and eventually drip down where you notice it, but it would have time to evaporate during running so it might take a long while. If you do not lift the cover and look at the pump from the top, or otherwise look directly at the potential leak area, you may miss it.


Explore the filter first, and cut open the old one or inspect it carefully. Much more likely than the two-year old lift pump.
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Old 06-14-2021, 06:31 PM   #9
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If it can leak under pressure it can suck air. It's a flawed design
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