Probable Air Leak in Fuel Line Lehman 80 HP

[Klos2019]

Love this forum. I have learned so many things reading the posts over the last year or so. I am now at the point I need feedback on a diesel problem.


Nefertari, my Whitby42 Ketch with an 80hp Lehman Diesel was underused and not well maintained when I purchased her a year ago. There have been ongoing engine issues where the engine will just die. Initial assumption was dirty fuel, filters, etc. and so I had the fuel in the two tanks polished and interior sprayed out in the process. I have pulled and fully cleaned both the primary and secondary fuel filters.


In every instance where the engine dies, I am able to bleed the air from the secondary fuel filter and the bleed screw at the injection pump and the engine starts right up and runs again.


I have tightened every connection and re-seated all fuel lines and the problem persists. Of concern is that in the last 40-50 hours of engine time, the interval that the engine dies has become shorter and shorter. Previously, it would run for 8-12 hours before dying. It is now to the point that it will usually die within less than an hour of last air bleed.


I am currently in Turks & Caicos, waiting on parts to try to fix this finally, but I wanted to see if anyone had any additional feedback or experience that might give me additional things to look at so I can resolve.


- I have NOT pulled the pickup tubes yet, but it is on my list. I have switched from Port to Starboard tank and back and the problem is consistent so I am guessing that is not the source, but want to be sure.
- I have just replaced the lift pump to rule out faulty diaphragm.
- I am replacing all of the fuel hoses from the tank selector switches to the lift pump and the return.
- I am replacing the secondary filter where the air is accumulating.


I am ASSUMING this is an air leak.



What am I missing? Any other reasons that I could be seeing this problem? Are the worsening symptoms relevant?


Appreciate any input as I work through this issue.


Thanks


Karl

[FPNC]

If you are convinced that your pickups are not the source of the air intrusion... install a low pressure electric fuel pump between the tank and the first filter. The pump should be wired to come on anytime the engine is running. This will change any source of air leaking into the system into a source of fuel leaking out of the system. Repair any leaks your see. Be aware that selector valves are bad about leaking air in and fuel out.

[phorvati]

Originally Posted by FPNC

If you are convinced that your pickups are not the source of the air intrusion... install a low pressure electric fuel pump between the tank and the first filter. The pump should be wired to come on anytime the engine is running. This will change any source of air leaking into the system into a source of fuel leaking out of the system. Repair any leaks your see. Be aware that selector valves are bad about leaking air in and fuel out.

I have lehman 90 , I've had simmilar symptoms and what got rid of it is rebuilding my primary, racor 900 series turbine. Few times my engine stalled, when I open the top of primary, fuel level has dropped. I had rebuilt the racor primary and the problem was in that top gasket and tee handle gasket, they were letting air in. Now same is happening on my northern lights genset that uses another 900 series racor which I also rebuilt. I like that suggestion of turning vacuum into pressure and looking for fuel leaks. Part of what we perceive as mystery, is just our inability to visualize and understand the problem. So we blindly follow instructions that some other human wrote in hope there will be no problems.

[Klos2019]

Thanks for the feedback and ideas. On the suggestion of American Diesel, I attached a couple of clear hoses to see if I could trace the air back to its source.


Since I was always finding air in the secondary, I put one hose on the input and one on the output. When I ran the engine with these, I saw no air. My suspicion is that there is a tiny air leak in the secondary filter assembly and it is taking a while to push down the fuel level in top of the chamber enough to allow air to flow further into the system and since there is no air going into the secondary, it jumps up on my list of suspected sources.


I have taken apart the primary and fully cleaned it and with new filters come new gaskets for t-handle and lid. I haven't gone the rebuild route yet though.

[Compass790]

Yes agree with the pressurising the fuel system. You can buy dirt cheap 3-5 psi 12v pumps for that or even use an outboard type squeeze bulb. ( can pump those up to over 10psi ) Then you can wrap all the joints with toilet paper & even small leaks will show up then.
It's amazing how many there can be! Ask me how I know

[Diesel Bill]

Originally Posted by Compass790

Yes agree with the pressurising the fuel system. You can buy dirt cheap 3-5 psi 12v pumps for that or even use an outboard type squeeze bulb. ( can pump those up to over 10psi ) Then you can wrap all the joints with toilet paper & even small leaks will show up then.
It's amazing how many there can be! Ask me how I know

Just went through this exact thing a few months ago. Same symptoms as the O.P. I put a squeeze bulb before my Ra core filter and pressurized from there to the engine mounted lift pump. A thin wall brass fitting had developed a hair line crack and was allowing air to enter the fuel suction line just before the lift pump. Found it easily with fuel leaking out while I squeezed the bulb. Ain't life grand.

[Klos2019]

OK, so the fuel system is a little convoluted. I has an electric pump on the vacuum side but it is not wired in. I have used it in the past but I also have a manual squeeze bulb for priming that is prior to the primary filter.


I pressurized the system with the squeeze bulb and while I thought I heard a slight air leak, I did not see any fuel after checking all joints and connections with tissue paper. I noticed that the pressure dropped after about a minute so did it again and I could feel air bubbles in the squeeze bulb so I am thinking the problem lies at or before the bulb. Of course, the bulb only pressurizes down line so leaks behind it will not show up this way.


I went ahead and replaced the bulb and the hose up to the primary, as well as most of the other hoses in the system. For good measure, I also replaced the secondary Racor fuel filter as I had ordered a new one already and I was concerned about the priming pump and bleed screw on the existing unit being possible sources of leaks.


I'm ready to prime the system and try it out in the morning. Thanks for all the suggestions.


Karl