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Old 12-11-2018, 07:04 PM   #1
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Cummins ISC fuel backflow when engine stops

When an ISC stops running, should the high pressure fuel flow back through the filters toward the tank? Is this normal to relieve pressure and extend filter life, or does it indicate a failed check valve or other component?

Not sure if it is related, but occasionally when I run the engine to air up, then shut down for a few minutes to avoid excessive idle, it may be difficult to start a second time even though the first time had no problem whatsoever.
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Old 12-11-2018, 09:04 PM   #2
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Sounds like you suspect, check valve
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Old 12-11-2018, 11:18 PM   #3
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Go to uTube for some good info on ISC fuel systems
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Old 12-12-2018, 11:29 PM   #4
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I suspect a fuel system leak, when not pressurized it allows fuel drainback. If the leak is prior to the injector pump you will not see any fuel leakage, as the injector pump pulls fuel all the way from the tank. The lift pump only runs for ~30 seconds to pull fuel from the tank, then stopps running; which leave the entire fuel supply effort to the injector pump. This added starting effort you are experiencing may result in a shortened lifespan for the injector pump if the problem is not rectified.$$$$$
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Old 12-13-2018, 12:15 AM   #5
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"I suspect a fuel system leak, when not pressurized it allows fuel drainback"

Problem with this is, why wouldn't the fuel leak back after the coach has set before starting to air things up?



If the OP was to pull into a park and sit there for a a day or so, why would the fuel leak back within that day and then cause hard starting right from the get go?
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Old 12-13-2018, 04:58 AM   #6
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"I suspect a fuel system leak, when not pressurized it allows fuel drainback"

Problem with this is, why wouldn't the fuel leak back after the coach has set before starting to air things up?



If the OP was to pull into a park and sit there for a a day or so, why would the fuel leak back within that day and then cause hard starting right from the get go?
It first starts with the fuel in the pump and filters. The fuel could still leak back out of the long line to the tank.
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Old 12-13-2018, 11:16 AM   #7
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It first starts with the fuel in the pump and filters. The fuel could still leak back out of the long line to the tank.

So you are saying that the coach can sit there for, say a day, and fire right up.


Then after it runs for ,say 5 minutes, to air up and then gets shut down. A few minutes later it is hard to start because there is no fuel in the lines?


What happened to the fuel that is supposed to be in the filters, pump, etc?
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Old 12-14-2018, 06:11 PM   #8
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So you are saying that the coach can sit there for, say a day, and fire right up.


Then after it runs for ,say 5 minutes, to air up and then gets shut down. A few minutes later it is hard to start because there is no fuel in the lines?


What happened to the fuel that is supposed to be in the filters, pump, etc?
That's why we are here, bounce ideas, experience, and theories around until the right answer arises.
It could possibly also be the ignition switch is faulty on his 1999 MH.
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Old 12-14-2018, 06:31 PM   #9
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So you are saying that the coach can sit there for, say a day, and fire right up.


Then after it runs for ,say 5 minutes, to air up and then gets shut down. A few minutes later it is hard to start because there is no fuel in the lines?


What happened to the fuel that is supposed to be in the filters, pump, etc?
The fuel in the pump and filter was burned or returned to the tank.

An air leak in the line would prevent the fuel, in the tank, from being pulled up as fast as it was used. That would leave the filter, which is before the pump, empty.
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Old 12-15-2018, 05:18 AM   #10
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Go to uTube for some good info on ISC fuel systems


Thank you all for the input. I did have a look at the videos. On the boat, there are two fuel hoses to/from the tank for the engine and two for the generator. I have always assumed the coach was the same and when I run the electric pump after a filter change , the air gets returned to the tank.

The fellow on YouTube with the black pen and sheets of paper shows the excess fuel from the high pressure rail returning to somewhere in the low pressure side instead of the tank. It is on the internet, so it must be true. Confused.
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Old 12-15-2018, 12:28 PM   #11
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Thank you all for the input. I did have a look at the videos. On the boat, there are two fuel hoses to/from the tank for the engine and two for the generator. I have always assumed the coach was the same and when I run the electric pump after a filter change , the air gets returned to the tank.

The fellow on YouTube with the black pen and sheets of paper shows the excess fuel from the high pressure rail returning to somewhere in the low pressure side instead of the tank. It is on the internet, so it must be true. Confused.

Here is one of the comments about that video.


"Dude, your almost right! Iím a cummins diesel tech, every cummins fuel system that is not a mechanical or rotary style is common rail. The fuel is drawn from the fuel tank through a primary / suction side fuel filter at less than 25 in-Hg that includes a WIF WATER IN FUEL sensor which may not be used on some systems. Then the fuel goes through the transfer / lift pump and check valve until the predetermined cycle is completed. Then, after 30sec prime / purge air cycle the fuel goes to the fuel pump gear pump at around 30-60psi, from there the fuel goes to a engine / remote mounted secondary / 𝖯𝗋𝖾𝗌𝗌𝗎𝗋𝖾 side fuel filter. After that the fuel goes to the high pressure fuel pump head at around 120-150psi. Then, the fuel goes to the high 𝖯𝗋𝖾𝗌𝗌𝗎𝗋𝖾 common rail at 1,500-35,700psi. The HP fuel pump head drains excess fuel to a fuel return manifold at the top-left-rear of the engine block. The fuel in the HP common rail then waits to be injected via the fuel injector. Any excessive common rail pressure get vented via HPCR (High 𝖯𝗋𝖾𝗌𝗌𝗎𝗋𝖾 Common Rail) high pressure relief valve to the fuel drain manifold. Then any excess fuel / injector leakage goes out the banjo fitting (which includes a check valve) at the back of the cylinder head via the fuel injector drain to the fuel drain manifold. Then, the fuel drain manifold returns any / all fuel drains to the fuel tank below the fuel pickup level as to refrain from aerating the fuel in the tank. A HPCR 𝖯𝗋𝖾𝗌𝗌𝗎𝗋𝖾 sensor is used to monitor common rail pressure for proper engine operation. No HPCR fuel system use a fuel temp sensor. I donít mean to sound like a know-it-all but, in school I failed the HPCR comp and I diagíed that system until I remembered every detail about the HPCR FUEL SYSTEM, Iíve since become the best experienced Common rail fuel system diagnostic tech in the cummins shop I work for. Iíve also just about mastered the EGR, DOC, DPF and SCR systems as Iím the go to guy for any EGR or AFTERTREATMENT questions other techs get stumped on. Like you say Stephen keep it simple, DO YOUR RESEARCH and get out there and fix something."
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