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Thread: 7.8 isuzi duramax has engine oil in fuel tank

  1. #1
    Join Date
    Feb 2016
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    Default 7.8 isuzi duramax has engine oil in fuel tank

    I have a 2004 Isuzu 7.8 inline losing engine oil and not knowing why. I was changing the fuel filters on this truck and found the fuel was black in the filters. So I feel that this is where my engine oil is going. I've asked other people that I know and they all seem very surprised at this and have never heard of this. Hoping someone has ran into this before and can point me in the correct direction to correct this issue
    Last edited by flht1550; 02-10-2016 at 08:34.

  2. #2
    Join Date
    Mar 2016
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    Buffalo NY
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    I have seen this before in many newer style diesels where engine oil pressure is a requirement fire the injectors. Not saying it's the problem, but I would take a look at your injectors and or pump. Sounds to me like you oil is being sent back to the tank via the return line

  3. #3
    Join Date
    Apr 2001
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    CA
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    Quote Originally Posted by Scortsnorbil View Post
    I have seen this before in many newer style diesels where engine oil pressure is a requirement fire the injectors. Not saying it's the problem, but I would take a look at your injectors and or pump. Sounds to me like you oil is being sent back to the tank via the return line
    This describes an HEUI fuel system (Hydraulic-Electric Unit Injector), which is common on the Powerstroke and T444E engines. Duramax engines don't use this. The 7.8L uses HPCR (High Pressure Common Rail), similar to the 6.6L.

    I suspect the black fuel and oil loss are not likely related. There are few engine oil/fuel interfaces in Duramax engines, and almost always result in fuel contaminated engine oil when the barriers are breached, due to the differential pressures. The fuel return circuit doesn't interface with pressurized engine oil. Black fuel/filter screams external contamination, biological growth/death, or vandalism. I suggest an immediate biocide treatment, as a precautionary measure. Higher than normal oil consumption is normally related to excessive engine wear (traditional causes), closed-loop crankcase vent systems (later models), or turbo spindle seal failure. At engine idle, turbo oil leakage favors the intake, and under load it favors the exhaust. A simple plumbing examination can confirm or refute this. Laboratory sample testing of the fluids will determine which fluids may be mixing, and what external contamination may be present.
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