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Fuel dilution cause

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Hey all, currently In Japan and couldn't find any previous topics regarding this. I sent an oil sample off just before leaving and unfortunately this is the 3rd one showing fuel dilution.


Rebuilt engine that is about 5 years old and done 18,000kms. Did 2 changes in first 1500. Next did 6000km and sampled revealing 6% dilution. Next did 3500km and tested at 4% dilution.


Removed fuel rail housing 6x nismo 740's. Pressurised outside of car and it held and no drops. Rotated Cas with injectors firing into bottles and all visually had good and even spray patterns. Took injectors to be professionally cleaned. They couldn't flow test due to being side feeds. Replaced all o-rings. Replaced Cas as it was fluctuating at all speeds and is now fine.


Just did 7500km of mostly long 150ish km journeys and had 7.7% dilution. I was expecting the Cas to be the major cause.


I'm guessing its either tuning or an injector failing with temperature or at higher speeds etc. injectors are about 15yrs old and done 100,000km.


Thoughts? Thinking about reducing cold enrichment and re-sample. Any other ideas?





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  • Oh and for extra info.


    2.5l bored to 86.5.

    Forged JE pistons

    Hastings rings

    No O2 sensor. Open loop

    Virtually zero unnecessary idling time

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  • Update.

    Removed cold enrichment above 50C
    Reduced cold settings up to 50C.

    Fitted a wideband and tuned cruise cells from 13:1 to around 15:1-15.5:1 while doing 1200 highway kms driving to work for a week.

    Latest sample has come back with high dilution again. Every other time I've sampled mid stream while draining but this time sucked through dipstick tube and may have sucked off the top section of oil as it was 5.9% from 1200km which is the worst sample so far.

    Next step is to swap out injectors and re-check

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