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Air in fuel lines??

Discussion in 'Ask All Sailors' started by Coddiwomple, Dec 7, 2017. Add this thread to a FAQ

  1. Coddiwomple


    Joined Jul 27, 2016
    30 posts, 0 likes
    Hunter 40 Legend
    ca Penetanguishine ontario
    So changed raycor bowl, fuel lines. Bled lines cracked injectors and bled them. Checked about three quarters of the intake line as it’s a solid 1/2 tube and I can’t get it all the way out of the tank because of height issues. Still getting a variation in rpm. New filters on. Ran for
    Ten hours the other day a few little variations. Ran for 7 hours yesterday first 6 we good then bumped speed up to 2200 rpm and a huge variation drop in rpm. That’s when I cracked the injectors to bleed them. Today we are travelling at 1800 rpm and I’m getting slight variations in rpm. Should I just hire someone at this point. I don’t want to do a crossing of the Atlantic to the Bahamas if this is still an issue.

  2. Ken Cross

    Ken Cross

    Joined Oct 24, 2010
    1,808 posts, 252 likes
    Hunter 30
    US Everett, WA
    Sorry to hear about this. Our engine always smoothed out in a minute or two after changing filters and bleeding the engine.
    Did it run well before the filter change? (I assume yes) Was the filter clean when you replaced it? What engine model?
    I'm wondering how your injectors are. Hopefully, someone else will chime in here.


  3. Ron20324


    Joined Jan 22, 2008
    6,379 posts, 634 likes
    Beneteau 323
    US Annapolis MD
    What filter element size?

  4. kloudie1


    Joined Nov 6, 2006
    8,039 posts, 602 likes
    Hunter 34
    US Mandeville Louisiana
    Sounds like a very small air leak somewhere between the pump inlet banjo fitting and the tank fitting.. Maybe even the valve packing at the tank shutoff.. but could be a loose fitting, a small hose crack, or a banjo washer .. look for any sign of very slight weepage from the pump back to the tank.

  5. jssailem


    Joined Oct 22, 2014
    6,597 posts, 2,410 likes
    CAL 35 Cruiser
    US Salem, Moored Port Everett WA
    Claude is on the track to a possible solution.
    Diesels are straight forward engines.
    You need Air, through the air intake,
    Fuel, through the fuel lines - and only fuel, and
    Compression, in the heart of the engine. ​
    The rest of the stuff Oil, Coolant, Exhaust systems, are there to take away the by products of the experience.

    So if your having RPM problems the usual first place to look is to the Air or Fuel systems.
    1. Be sure you are only getting fuel the lines. The fuel system is designed to give the combustion chamber a specific amount of fuel. If air is leaking into the system you have a problem.
    2. Air Check that your air filter or system is providing a clean air source to the engine heart.
    Check your oil level. Too much oil is a problem. Not enough is a problem. Diesel engines are like Goldilocks "Just Right" is what works.
    Then go out and run your engine. Check your cooling system be sure the level is in the correct range and that the temps are in the correct range as you open up and run the engine.

    Diesels hate to be babied. They are made to run at near WOT (wide open throttle - 85-90% of WOT)

    Oh And here is what a dirty filter looks like.
    This is the 30 Micron filter from my Racor 500. The vacuum gauge was indicating that fuel was not getting to the engine. The engine had been running smoothly for over 6 hours. Suddenly RPM's changed, surged. Shut engine down. Changed filter. Started back up. Ran 4 more hours into port. Sat down and enjoyed a glass of Willamette Valley Vineyard Pinot with friends.

    JamesG161 and kloudie1 like this.
  6. capta


    Joined Jun 4, 2009
    2,675 posts, 795 likes
    Pearson 530
    na Admiralty Bay, Bequia SVG
    For two years we thought our generator had bad rings. Other issues caused us to change a fuel line between the electric fuel pump and the injection pump (a pressurized line) and all of a sudden we had the full 8kw available again. Air can get into a diesel's fuel system anywhere. Don't waste money hiring someone else to find your leak, just be very thorough and check every joint you can and change every line if necessary. The old soap trick won't work on air getting into a system, so it really is a one by one sort of investigation.
    One other place to look is in the Racor 500 filter housing before the filter, if you have one. If your tank is old and possibly dirty, the intake line of the Racor 500 filter housing can plug up and there's no way to see it or know without disassembly.

  7. NYSail


    Joined Jan 6, 2006
    1,787 posts, 93 likes
    Beneteau 423
    US Mt. Sinai, NY
    When you changed the filter did you change the gasket as well? And if so did you wipe a bit of fuel over it? Sometimes those cap gaskets can get twisted in the groove and that would cause an issue. Also, when you changed the filter did you fully prime the bowl and let the fuel fill the fuel lines? I patiently pour fuel into the bowl and watch it slowly settle.... and continue to top off the bowl until it stops going down. And what about the secondary filter? Did you change as well and even if you didn't did you bleed that housing? If you weren't having this trouble prior to you changing the filter it is something that resulted from that...... Be patient and good luck!

  8. Benny17441


    Joined May 24, 2004
    5,566 posts, 357 likes
    CC 30
    US South Florida
    Like Kloudie suggest the leak would likely be between the tank and the lift pump. When the engine is stopped air will leak in but when the engine is running fuel will leak out so check for any weepage in that section. If the engine is hard to start after sitting for a few days it would point to an air leak.

  9. Mark Maulden

    Mark Maulden

    Joined Jan 25, 2011
    1,694 posts, 149 likes
    S2 11.0A
    US Anacortes, WA
    Feed an outboard tank directly to the fuel pump and run engine. If it runs, then feed the outboard tank into the primary filter and run the engine. You get the idea...The fastest way may be to just replace the whole fuel line making sure everything in the primary filter is good. As stated previous, make sure all gaskets are correct and clean out the gasket channels. Also, make sure the pickup tube has no pinhole leaks. Another way is disconnect fuel line from pump and plug the hose. Go to the other end and pressurize with a bicycle pump....This is all stuff that a pro mechanic would do at USD $100/hr.

  10. 31seahorse


    Joined Aug 2, 2005
    848 posts, 126 likes
    Celebrity Class 19
    US Penn Yan, NY (Seneca Lake SP)
    Several years ago we had a similar condition with our Oday 272 LE. Changing filters and then air leaks through lines or gaskets might be your problem as other posters have suggested. I would consider looking "inside" the tank. Not literally of course, but the problem might be there as our problem was.

    I removed the fuel pick-up tube from the tank and found a small wire-type screen at the lower end of the tube. It was clogged with black gunk. I cleaned that and reinstalled the screen. I also had a local person "polish" the fuel in the tank. That involved filtering the fuel through several filter pads in succession. The purpose was to clean the residue in the tank. Honestly, prior to the polishing process I thought the polishing might be just smoke and mirrors. Following that process we had no further difficulty with RPM drop.

    Note: I do not know the location of the fuel tank in your boat. The fuel tank in that boat was a suitcase shaped plastic tank in the transom of the boat. In order to remove the pick-up tube I had to cut an opening in the top of the transom. I did not like to do that, but it was unavoidable. I covered the hole with a piece of Starboard that was not objectionable.

  11. DougM


    Joined Jul 24, 2005
    1,525 posts, 143 likes
    Beneteau 323
    US Manistee, MI
    How old are the fuel lines?
    I have seen examples of lines that appear to be perfectly good, but had miniscule cracks in them which were causing the type of problem you are experiencing. Didn’t know they were porous until they were pulled out and were lying on the cocpit table and left a puddle of fuel. Changed the lines and all was well.

  12. Malcolm/Trilogy


    Joined Aug 27, 2012
    7 posts, 1 likes
    Hunter Passage 42
    Ch RHKYC Hong Kong China
    I agree with Mark - check every portion of the fuel feed system- I had a similar issue on a return journey from Philippines to Hong Kong -as every time I increased revs the engine slowed and stopped. So I ran over 200 miles feeding diesel directly into the engine filter from a spare tank that I kept in the cockpit and it got me home.
    Afterwards I cleaned the filters, the pipes, the connectors, even the fuel tank twice and still could not find the issue... until I unscrewed the 90 degree right angle pipe that comes from the internal of the fuel tank and joins to the rubber hose and although it looked clear it wasn’t. Wedged in the corner of the elbow was three quarters of an ‘o’ ring which was reducing the flow of diesel - once I removed that everything was fine but it took weeks to find. As to how and why it got there, I of course have no idea, but when you say you have checked the fuel line- pretend that you haven’t and go back to the tank and start again and pass diesel through every part of it and gauge the flow - because as Mark says You will Find which bit has the blockage- good luck

  13. JimInPB


    Joined Aug 22, 2017
    1,197 posts, 344 likes
    Hunter 212 & 170
    us West Palm Beach
    The strangest place that I ever found an air leak was a cracked housing on an off-brand fuel filter. The crack was too small to see, but when I dropped the entire assembly into a bucket of water, I stopped seeing air bubbles in the sight glass on my fuel line. I then twisted the filter assembly around in the bucket until the bubbles came back to identify the exact location of the leak. I needed a 20x magnifier to see the crack, but it was big enough to suck air. I replaced that cracked unit with a genuine Racor & have had no problems since.

    Fuel filters are sometimes fussy about being tight enough. I'd check that first. I usually tighten my filters with rubber-palmed gloves.

    I'm also fond of having a sight glass in the low pressure fuel hose, as close as possible to the high pressure pump as you can get it. That way, you have a chance of seeing what is actually happening. If you have the sight glass, you can jiggle different parts of the fuel system & look for changes in the stream of bubbles.

  14. hatterasjack


    Joined Mar 22, 2017
    4 posts, 1 likes
    Hunter 37.5 Legend
    US Atlantic City
    I experienced the same problem many years ago with a 2 GM 20 that drove me crazy. Finally I drained all fuel from the tank pickup tube to the lift pump and pressurized the system with air. I found a MICROSCOPIC leak at a compression fitting!! this was done with a paint brush,water and Dawn dish detergent. Painted entire run of fuel delivery objects and then sat back and had a beer, ok maybe two..., after conversing with all interested dock mates we discovered bubbles forming where the leak was. Replaced fitting, re pressurized and tested. Never had the problem again 15 years!!

    justsomeguy likes this.
  15. MitchM


    Joined Jan 20, 2005
    688 posts, 94 likes
    Nauticat 321 pilothouse 32
    US Erie PA
    some of the old diesels used copper crush washers to form the seal. the service manuals reminded that you needed to put in a new washer or 'anneal' the old one (by heating it then letting it cool) to get a good seal...

    thanks all- this has been an interesting thread, worth copying and handing out to the next USPS engines class.

  16. Mark Maulden

    Mark Maulden

    Joined Jan 25, 2011
    1,694 posts, 149 likes
    S2 11.0A
    US Anacortes, WA
    If you have threaded brass fittings for any reason, make sure to use straight threaded fittings and not tapered. Tapered gives far less thread engagement. Also use a liquid thread sealant..

  17. JimInPB


    Joined Aug 22, 2017
    1,197 posts, 344 likes
    Hunter 212 & 170
    us West Palm Beach
    Copper, unlike many steel alloys, can be quenched in water immediately after heating to a cherry glow & still maintain it's annealed state.

    I have managed to successfully reuse both aluminum & copper crush washers a few times without annealing, as long as the service conditions were not harsh & the washer was not real old. Copper strain hardens over time, but it takes a while to happen unless the stress is significant.

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