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problems filling diesel tank

richk

.
Jan 24, 2007
483
Marlow-Hunter 37 Deep Creek off the Magothy River off ChesBay
I’ve always had a problem filling my diesel tank full . The boat is a 2014 Marlow-Hunter 37 and at a certain point in the filling process the tank ‘burps’ and fuel will exit the fill hole. Recently I’ve taken to using an external 5 gallon can and a siphon, first filling the can and then siphoning into the tank. After successfully putting about 17 gallons in via siphoning, the tank burped again. I wondered if the tank was full so, rather than trust the gauge, I pulled the sensor out of the tank and manually measured the fuel level with a wooden dowel. I realize this won’t yield the number of gallons directly since the tank is not a regular shape. With this method I confirm that the tank is not full. At the point where I sounded it, the tank is 13 inches deep and there is diesel up to 8 inches deep.

I figure the tank is not properly vented. When I was removing the sensor there was ‘air’ rushing out. Does anyone know where the vent is located? Here’s a picture of the tank and the model number.
 

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Jan 19, 2010
793
Catalina 34 Casco Bay
That is caused by the fuel entering faster than the vent works. listen for the air pressure change and stop the filling. Remove the nozzle, this will allow almost instant neutralization of the pressure. Now start the top off...but slowly. You may get a few suggestions to install a larger vent hose. BUT, it all comes back to the size of the nipple atop the tank.
 

richk

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Jan 24, 2007
483
Marlow-Hunter 37 Deep Creek off the Magothy River off ChesBay
Thanks, sailme88, i appreciate it. Any idea where the nipple is?
 
Nov 6, 2006
8,914
Hunter 34 Mandeville Louisiana
Sounds like the bugs have plugged your vent outlet fitting..
 
Jun 8, 2004
187
Hunter 49 60803 Lake Erie
On my previous boat a Hunter 44 I had a similar problem. I had to run the fuel pump very very slowly to fill the tank. I kept assuming it was the vent. I cleaned it and removed the vent cover all together but neither fixed the problem. Turns out my issue was a bend in the fuel fill line as it wrapped around a corner causing restricted flow of the fuel into the fuel tank.
 
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Likes: deltaten
Jan 7, 2011
2,270
Oday 322 East Chicago, IN
There can’t be too many vents to locate....one for each water tank, one for each holding tank, and one for each fuel tank. Pretty easy to see if you look at the sides of your hull, pretty close to the rub rail.

You should probably check them all, as they are all there for a reason.

Greg
 
Jan 19, 2010
793
Catalina 34 Casco Bay
Thanks, sailme88, i appreciate it. Any idea where the nipple is?
Looking at the first of your 2 pics of the tank, I would say that the top fitting with loom around the wires is the sending unit for the gauge. The two below are a toss up. Small lines, so vent and and maybe the return line. Could also be the pump suction and return. No sign of the tank feed. That should be inch and a half hose . The vent nipple will be at the top of the tank. The vent should have it's thru hull either high on the transom or close by on the after quarter. Locate and check this . Mud wasps like to build nests in it's ports. Total occlusion will place a vacuum on the tank and not let your engine run. A partial occlusion will allow the engine to run but will severely choke off the air balance needed when fueling up.
 
Last edited:
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Likes: jssailem
Nov 6, 2006
8,914
Hunter 34 Mandeville Louisiana
Yup.. the line to the left with the valve at the ell is probably the fuel out to the pump and the other probably the fuel return to the tank from injectors.. Vent and fill are hidden somewhere else..
 
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Likes: jssailem

richk

.
Jan 24, 2007
483
Marlow-Hunter 37 Deep Creek off the Magothy River off ChesBay
Thanks everyone. Based on your input my next step will be to try to unplug the external vent; I know where that is. As far as tracing the run from the tank to the vent goes I'll wait until everything's off the top of the area...mattresses, bedding.. This is because total visual access to the top of the tank will only be possible after removing a larger composite board which is screwed in place. I suspect the vent line is at the aft end of the tank.

Thanks again.
 
May 7, 2012
841
Hunter e33 Maple Bay, BC
I notice that no other MH37 owner has jumped in to help and I am not sure if these pics will be of any assistance or not. Different year (2012), different model (e33) but maybe close enough to get you started. Have you have reached out to Marlow Hunter? Also INCA Plastics Molding maybe be of assistance.
INCA Plastics Molded Inc

e33 - Fuel Tank Air Vent.jpege33 - Fuel Tank(1).jpege33 - Fuel Tank (2).jpeg
 
Jan 7, 2011
2,270
Oday 322 East Chicago, IN

RoyS

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Jun 3, 2012
862
Hunter 33 Steamboat Wharf, Hull, MA
Vent line must be located on top of tank at the highest point. Line must not dip down after leaving tank or it will likely be blocked with fuel at the low point. It can dip down where it exits the boat hull as long as there is no way for it to retain fuel. You will know if you have a fuel trap in your vent line if you have difficulty filling your tank except extremely slowly. Had this problem once until I located the low point in the vent line and discovered that it had a pocket of fuel trapped there. Time to crawl around your bilges. Good luck.
 

senang

.
Oct 21, 2009
156
hunter 38 Monaco
On our 2005 Hunter Marine 38 the vent and fill line are next to each other on the backend of the tank just under the aft bulkhead.
 

richk

.
Jan 24, 2007
483
Marlow-Hunter 37 Deep Creek off the Magothy River off ChesBay
Thanks to the recent posters on this topic; very helpful!
 

richk

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Jan 24, 2007
483
Marlow-Hunter 37 Deep Creek off the Magothy River off ChesBay
Well, I've made some progress on sorting out the original problem. It took having the boat on the hard so I could remove all the cushions and discover an access plate for the stern end of the tank. Took the vent line off an tried to blow in it. Nothing doing. Line is blocked or kinked somewhere, right? Here's a picture.
 

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Oct 22, 2014
13,887
CAL 35 Cruiser moored EVERETT WA
You got a great start Rich.Now you just need to track the vent line down and clear the line.
 
Jan 19, 2010
793
Catalina 34 Casco Bay
If the exterior vent cap is with our screens, there is a good possibility that nesting insects have built a home in the line. I'd remove the line at the tank and slide a vacuum cleaner hose over it. Then run a wire fish or maybe start with a coat hanger down from the outside.. If you can't advance anything thru the transom cap you may need to remove the hose entirely. In any event once cleared replace the screen(s)