Fuel/Air leaks?

[Djoub]

Sorry for the long post but I am hoping for some good info regarding fuel problems.
Boat is a 1998 Mainship 35 with a single 2008 Yanmar 370 hp bought less the a year ago. Original engine may have been gas. Engine has stopped a couple of times, once leaving me stranded on the water, second time was in slip. First time I thought it was dirty filters so all filters changed and bled system, everything worked for a couple of weeks. Second time no fuel to primary and secondary filters. Hired a mechanic and said I lost fuel to racor input, he primed everything and it ran good. A day later while looking around the engine room while it was running, I noticed air bubbles in the glass bowl at bottom. When I went to refill the racor, the fuel kept being slowly sucked down. I’m guessing it was leveling to the level of fuel in the tanks. It would only stop drawing when I shut off both supply values on the two tanks. I have a home made fuel polisher with a Gulf Coast Filter and a electric fuel pump, all by-passed for now with shutoff valves. A couple of weeks ago I moved fuel from one tank to another, don’t remember how I left the valves but the next day both tanks were back to equal. Is this normal or do I need to close tank valves when engine is not running?
So I have 3 issues:
1) Bubbles in fuel stream while engine is running
2) Fuel flowing back into tank(s) with supply valve open
3) Small fuel leak somewhere in Port side tank area that can’t be found.

Are all three problems related? I can’t find any leaking using the paper towel method but I end up with a small amount near the stringer the tank sits on. All the fittings and clamps seem dry but as I said small leak somewhere near tank.
Someone mentioned could be a bad anti-siphon barb fitting on fuel tanks? Thinking of installing a temporary squeeze bulb on a short new hose from input of the racor, to a container of fuel or straight into one of the tanks and bypassing all the fittings and working back thru the system looking for bubbles.

Any thoughts on trouble shooting or ideas? I’m starting to think I should trade it in for a canoe Thanks

 

[Lepke]

Most Racor housings I know of have check valves that should keep the fuel from back flowing. It could need cleaning or a rebuild kit. The lift pump has valves that operate as check valves and also should keep fuel between the lift pump and injector pump. If you have bubbles in the fuel while running you have an air leak. Somewhere before the bubbles. Many boats have a vacuum gauge at the Racor or between the Racor and the lift pump. It can tell you when the Racor filter is becoming dirty, if the lift pump is creating a decent suction when pulling fuel and if the needle jumps up and down when running, usually indicates a air leak. Air leaks are not usually constant. At higher speeds, and higher pump suction, fittings or lines that show no leak when sitting will allow air into the system. It's not easy to find the leaks. If you can't find the leak, add a air pressure fitting and put low air pressure into the lines and listen for the leak or look for spilled fuel. A bicycle pump is enough, 5+ psi usually shows or sounds the leak (if you have decent hearing).
Your tanks could have a equalizing line, unknown to you, that allows the fuel to flow between tanks. Fuel doesn't flow uphill, so the tanks need some connection below the fuel level to allow equalization. Hard to diagnose w/o a fuel diagram. Fuel polishing pump if properly installed should be able to run on not with the engine running without issue. The electric fuel pump also has valves that function as check valves. If it's tee'd after the Racor, the valves wouldn't allow it to interfere with the lift pump even w/o manual valves. I suspect in all your valves and bypassing, there's a hose fitting leaking at least air.
Another source for air is a bad diaphragm in a manual lift pump. Before complete failure, the pumps sometimes suck air thru the diaphragm from the crankcase.
As troubleshooting I'd replumb the electric pump as the lift pump and see if the problem goes away.
The fuel tank probably has to be pulled to find the leak. It will only get worse with time.

 

[Riverguy]

Sorry for the long post but I am hoping for some good info regarding fuel problems.
Boat is a 1998 Mainship 35 with a single 2008 Yanmar 370 hp bought less the a year ago. Original engine may have been gas. Engine has stopped a couple of times, once leaving me stranded on the water, second time was in slip. First time I thought it was dirty filters so all filters changed and bled system, everything worked for a couple of weeks. Second time no fuel to primary and secondary filters. Hired a mechanic and said I lost fuel to racor input, he primed everything and it ran good. A day later while looking around the engine room while it was running, I noticed air bubbles in the glass bowl at bottom. When I went to refill the racor, the fuel kept being slowly sucked down. I’m guessing it was leveling to the level of fuel in the tanks. It would only stop drawing when I shut off both supply values on the two tanks. I have a home made fuel polisher with a Gulf Coast Filter and a electric fuel pump, all by-passed for now with shutoff valves. A couple of weeks ago I moved fuel from one tank to another, don’t remember how I left the valves but the next day both tanks were back to equal. Is this normal or do I need to close tank valves when engine is not running?
So I have 3 issues:
1) Bubbles in fuel stream while engine is running
2) Fuel flowing back into tank(s) with supply valve open
3) Small fuel leak somewhere in Port side tank area that can’t be found.

Are all three problems related? I can’t find any leaking using the paper towel method but I end up with a small amount near the stringer the tank sits on. All the fittings and clamps seem dry but as I said small leak somewhere near tank.
Someone mentioned could be a bad anti-siphon barb fitting on fuel tanks? Thinking of installing a temporary squeeze bulb on a short new hose from input of the racor, to a container of fuel or straight into one of the tanks and bypassing all the fittings and working back thru the system looking for bubbles.

Any thoughts on trouble shooting or ideas? I’m starting to think I should trade it in for a canoe Thanks

FYI, no Mainship Trawlers were ever shipped with gasoline engines. Curious as to what made you think it was repowered? Possibly it had one of the notorious Cat motors and a previous owner repowered with Yanmar?



1) Air-bubbles: You appear to have a leaking fuel line. Your engine is dying because diesel engines cannot tolerate any air whatsoever in the fuel stream.


2) Fuel flowing back into the tank...: This is normal. Diesel engines do not use all the fuel that is pumped into the injection system. The excess fuel goes back into the tank(s).


3) Fuel leak: Gotta find it and fix it.


Cheers!

 

[FF]

IF you can keep the fuel system pressurized ,from tank to engine, pump or hand bulb, the leaks will show up on the paper towel teltales.

 

[Djoub]

Ok, thanks for the tips.
To answer a couple of questions;
I understand the fuel return concept of diesel, all references are to supply side. Air bubbles are on the input side of racor. Increase in bubbles when engine is revered above ideal. Doesn’t seem to be any issues after racor to lift pump and eventually FI’s except when the air in the racor pushes down to the output fitting then it effects engine stall.
There is no dedicated equalizing connection (there are fittings in tanks but never punched out). However, as I look at the plumbing and diagram, both tanks tie together with tee’s, so essentially if both tank valves are open they should/will equalize.
I also counted 37 hose clamps, 54 pipe fitting interfaces, and 14 valves. If there is a leak in one place it is going to be hard to find. Looking at how messed up and complex things are I’m thinking about removing everything and starting from scratch and keeping it simple with no polisher. Simple 4 way valve for Port/Stb/Both/Off same for return line.
Lots of things done wrong, B2 type hoses and some no name hoses on return lines, teflon tape everywhere, not even sure the valves are proper type for fuel application.
See attached pic, one is the polisher, the other is a return line. lots more fittings then shown.

Thoughts on remove and replace? Is that too much for a DIY or should the pro be called in?
Thanks again

 

[Comodave]

Replacing the whole system is certainly within a DIY capability. I did it on a previous boat that we replaced the fuel tank. Not too hard if you have room to work in. With a single you should have adequate room to work. Good luck.

 

[Lepke]

Before starting over you should try pressurizing with air. I've found a lot of air leaks that way when all else failed.