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  1. #1
    Join Date
    Nov 2016
    Location
    Glasgow
    Posts
    357

    Default The dreaded diesel bug!

    My boat has sadly had very little use this winter as I have been working away a lot. I had hoped to get out this weekend, but events and weather have worked against me (possibly in a good way though!).

    I decided I would get a bit of maintenance done, and brought out a new primary fuel filter (the CAV type) and before dismantling, noticed a dark coloured area sitting at the bottom of the bowl. On taking it all apart and draining down, it's of an oily consistency and drops to the bottom of any fuel. I'm assuming that this is the dreaded diesel bug?

    I have given the tank an "initial dose" of Starbrite bio fuel treatment and have changed the filter, bled the system and got her running, etc.... The tank is of the type that uses part of the hull and then fibreglass bulkheads to make an internal tank. It has no obvious drain at the bottom that would allow me to try to get any sludge out. It does have a small hatch it one of the top corners that can be removed. Would it be of any worth to try to get a tube of my Pela vacuum pump on to it and send to the bottom of the tank?

    I'm concerned that the first bit of bouncy water I head out on, the crud gets drawn up and blocks the filter, and having changed the filter on a rocking marina berth, have realised there is probably no way it's getting done at sea...

    Thoughts from the assembled panel please...?

  2. #2
    Join Date
    Aug 2004
    Location
    Home East Lancashire boat Spain
    Posts
    4,547

    Default Re: The dreaded diesel bug!

    I would certainly investigate the hatch to see if you can get any sort of pipe or tube right to the bottom of the tank and then see what you can suck out.
    If the engine is running ok you just need to see if more gunge is getting into the bowl though, as you say, it may run clear until you start to roll about a bit.

  3. #3
    Join Date
    May 2001
    Location
    UK East Coast
    Posts
    35,507

    Default Re: The dreaded diesel bug!

    If the CAV filter is correctly plumbed, the fuel in the bowl has already passed through the filter, which should remove bits of bug residue.

  4. #4
    Join Date
    Dec 2003
    Posts
    5,332

    Default Re: The dreaded diesel bug!

    Treatment will depend on diagnosis and the fuel type. If it is low sulphur diesel which includes biofuel, then the the contamination could be a combination of organic detritus/sludge (brown/black moussy sludge), repolymerisation (oily sludge at bottom of thank), or possibly asphaltenes.

    I’d dip as suggested to find out what is at the bottom of the tank and if it’s bad send a sample off for proper analysis. If not change filters and monitor closely. If filter fitted wrong way around (as per pvb’s eagle-eyed observation) first disconnect from engine and sort. Assuming engine has a secondary filter, this may be the last line of defence for expensive injectors! Common rail engines are even more at risk.

    Then the cleaning, sterilising, polishing, stabilisation, treatment can be properly specified, and a suitable maintenance and fuel-specification regime can be adopted going forward. Otherwise the problem can unfortunately return and ultimately take longer to sort.
    Last edited by dom; 24-03-19 at 15:32.

  5. #5
    Join Date
    Nov 2016
    Location
    Glasgow
    Posts
    357

    Default Re: The dreaded diesel bug!

    Quote Originally Posted by pvb View Post
    If the CAV filter is correctly plumbed, the fuel in the bowl has already passed through the filter, which should remove bits of bug residue.
    It's correctly plumbed according to the arrows on the filter head anyway...

    So would that suggest that the bug has developed in the fuel in the bowl while it's been lying idle (engine hasn't been run in about 3 months)?

  6. #6
    Join Date
    Nov 2016
    Location
    Glasgow
    Posts
    357

    Default Re: The dreaded diesel bug!

    Quote Originally Posted by dom View Post
    Treatment will depend on diagnosis and the fuel type. If it is low sulphur diesel which includes biofuel, then the the contamination could be a combination of organic detritus/sludge (brown/black moussy sludge), repolymerisation (oily sludge at bottom of thank), or possibly asphaltenes.

    I’d dip as suggested to find out what is at the bottom of the tank and if it’s bad send a sample off for proper analysis. If not change filters and monitor closely. If filter fitted wrong way around (as per pvb’s eagle-eyed observation) first disconnect from engine and sort. Assuming engine has a secondary filter, this may be the last line of defence for expensive injectors! Common rail engines are even more at risk.

    Then the cleaning, sterilising, polishing, stabilisation, treatment can be properly specified, and a suitable maintenance and fuel-specification regime can be adopted going forward. Otherwise the problem can unfortunately return and ultimately take longer to sort.
    Sounds like it could be an expensive process... So is whacking in something like Marine 16 to shock dose it not enough? If I go to the effort of trying to clean, sterilise, polish, etc., is putting fuel of an unknown quality (i.e. from the marina's red diesel bunker) back into the tank not just going to result in the same happening again?

    The engine has a secondary filter fitted as well, which the last time I replaced it was very clean on removal. The engine is a Bukh DV20, so old technology, not common rail.

  7. #7
    Join Date
    Dec 2003
    Posts
    5,332

    Default Re: The dreaded diesel bug!

    Quote Originally Posted by stevie69p View Post
    Sounds like it could be an expensive process... So is whacking in something like Marine 16 to shock dose it not enough? If I go to the effort of trying to clean, sterilise, polish, etc., is putting fuel of an unknown quality (i.e. from the marina's red diesel bunker) back into the tank not just going to result in the same happening again?

    The engine has a secondary filter fitted as well, which the last time I replaced it was very clean on removal. The engine is a Bukh DV20, so old technology, not common rail.
    It will arrest bacterial growth if that’s what is going on. But it won’t dry your fuel if wet, won’t retabilise it if old, and it won’t remove the sludge which will henceforth lie in wait to plug-up your system in a blow.

    As you have an inspection hatch at the top of the tank, it would certainly be useful to suck a sample from the bottom to help try and figure out what’s going on. Posting a pic on here would also be helpful.

    Also, I’d recommend swapping the CAV for something like an oversized Racor top-loading filter which can be easily inspected and changed in a jiffy if the system plugs up at an inopportune moment. If going this route, I’d also recommend reducing primary filter down to 10 Microns or whatever the secondary is. That way any plug-up will be confined to one easy to change filter, leaving the secondary effectively as a backup.

    Finally, as with other chemical reactions the presence of a catalyst or heat can speed-up the natural oxidisation process. Check for the presence of copper, zinc, and their alloys in your system. Going forward I’d specify FAME-free fuel if you possibly can.
    Last edited by dom; 24-03-19 at 16:40.

  8. #8
    Join Date
    May 2001
    Location
    UK East Coast
    Posts
    35,507

    Default Re: The dreaded diesel bug!

    Quote Originally Posted by stevie69p View Post
    The tank is of the type that uses part of the hull and then fibreglass bulkheads to make an internal tank. It has no obvious drain at the bottom that would allow me to try to get any sludge out. It does have a small hatch it one of the top corners that can be removed. Would it be of any worth to try to get a tube of my Pela vacuum pump on to it and send to the bottom of the tank?
    Yes, it would be a start, and you might discover some rubbish in the bottom of the tank. I had a similar problem many years ago. Even though I’d used conventional biocides when refuelling, after motoring for a while in a fairly rough sea, the engine faltered and stopped. The cause was a blocked primary fuel filter. Eventually, back in the marina, further investigation showed that there was a lot of gunge in the tank - unfortunately it was a keel tank, baffled, with quite a small access hole.

    I rigged up a home-made “fuel polishing” system, with an old electric water pump, some plastic hose, and a fuel filter. First, I emptied the fuel tank. Then, for the best part of a day, I repeatedly tipped 25 litres of fuel back in and immediately pumped it out, from the bottom of the tank, through the filter, and into a plastic jerry can. The action of quickly tipping 25 litres in stirred up the gunge in the bottom of the tank, and some of this lodged in the filter. I needed to change the filter element several times during the day. Then I refilled the tank with clean fuel and triple-dosed it with biocide.

  9. #9
    Join Date
    Mar 2008
    Location
    Argyll
    Posts
    6,207

    Default Re: The dreaded diesel bug!

    What I would do in these circumstances is what has already been advised, use the access plate or the fuel guage sender aperture to get a tube to the very lowest corner of the tank, extract 4 or 5 litres and decant in to a white plastic bucket which will show up any nasties, repeat until the fuel come out clean, seal the tank and replace filters if you have not done it already. Let the extracted diesel settle and pour off the clean stuff to go back into the tank. Use a tun dish (aka funnel) with a fine mesh filter. Dose with your favourite poison.
    Of course you will have a small amount of dirty diesel left which you should not be tempted to use to start your next garden bonfire.
    Last edited by Quandary; 24-03-19 at 17:11.

  10. #10
    Join Date
    Mar 2014
    Location
    Arrived. But thinking of a bit more east.
    Posts
    1,115

    Default Re: The dreaded diesel bug!

    If necessary:



    Really like this guy. Ex-carrier fighter pilot

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