Diesel spurts after filling fuel tank

B

baz

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Fluid Motion Model
C-24 C
Yesterday I filled my R-25's fuel tank. Put some 44 gallons in. I was very careful pumping the last 5 gallons of fuel (trigger just barely pulled) to avoid any likely back flow in the filler neck and/or the overflow port. I was successful and not a drop of diesel came out when reaching the 44 gallon mark.

However, when I started the engine I noticed some diesel fuel had managed to spurt out of the overflow port. I've experienced this before so was ready and had fuel spill rags layered over that area of the transom to catch the spurts of fuel.

Question(s)...

1. Is there a technique for avoiding this fuel spurt after filling tank and starting the engine ?
2. Would having filled the tank with say just 40 gallons rather than the 44 gallons make a difference ?

The 44 gallons was my calculated amount to fill the 75 gallon tank, minus 1 gallon.
 
I wonder if the fuel return line is partially pressurizing the almost full tank when the engine is started, causing it to spurt fuel? Or, m/b the fuel tank vent line is partially restricted, causing pressure buildup.

I had all sorts of problems with my R21EC spurting fuel, including the partially restricted tank vent line. Fortunately, with Andrew's and Jeff's support I was able to finally sort things out. However, I never tried filling, or coming close to filling, the tank after that.

I'm not sure if the R25 fuel tanks has internal baffles. They can be very helpful in reducing the movement of fuel which can result in pressure buildup, especially if the venting system cannot handle the buildup.

gene
 
If you do not need to... don't fill it past 1/2 That way you are not moving all the extra weight around all the time. Between water and fuel is a lot of extra weight you nay not need. Just my two cents..
 
Grandpa Mark":2dnwp21o said:
If you do not need to... don't fill it past 1/2 That way you are not moving all the extra weight around all the time. Between water and fuel is a lot of extra weight you nay not need. Just my two cents..
Click to expand...

Yes -- maybe that's a good strategy. I may opt for 3/4 tank fills in future.

The fuel spurt seems to be a one-time event after filling my tank and starting the engine. Thus, so long as I remember to keep some fuel absorbent rags attached to the vent port area after a fill up it would not be a prolonged issue with running the engine.
 
Fuel can expand as it warms and filling a tank at say 65 degrees will expand the fuel as it warms to 75-80 or even 90 degrees (depending on the weather, and how the fueling station's tanks are located).

The diesel fuel expansion co-efficient is 0.00046 per degree Fahrenheit. What that means is, for every degree increase in temperature your fuel volume will go up that amount. The math works like this, say you have a temperature rise from 60 degrees f to 90 f over the course of a day and a tank with 75 gallons of diesel in it. Multiply the coefficient by the number of degrees temp rise (30 x 0.00046=0.0138) then multiply that answer by the total number of gallons in your tank, to calculate the number of gallons increase in the tank when the diesel warmed up to 90 degrees (0.0138 x 75=1.03 gal.)

You could have grown between 1/2 to 1 gallon! maybe more.
 
I tend to fill the tank almost to the top.. I have not noticed any overflow from the vent when first starting after a fill...

Now, the engine does not pressurize the tank, per se...
The fuel pump regulator has a return line back to the tank so that excess fuel flow is returned and thus there will be some rolling of the fuel... This return flow is due to using the fuel as coolant for the pump... If the pump just pushed into a dead load with minimum flow through the pump it would quickly overheat (and the fuel pressure would vary widely)... By providing a bypassing type of pressure regulator on the output side of the pump it maintains constant pressure on the fuel rail, with the bypassed fuel running back to the tank which results in a constant flow through the pump that acts as coolant...
Slightly underfilling the tank will allow room for the 'rolling' fuel from the return line to circulate... If you can see the fuel up to the top in the filler neck it is also up ithe top in the vent line so that there is only an inch clearance to overflowing... It will not take much of a surge in the tank as the engine starts for the fuel to slosh over... If that full it will slosh over during a ramp launch also..

You can double check for an obstructed vent - partly run the tank down, then rig an air hose to the vent opening (take the filler cap OFF) and blow back through the vent line to see if there is an obstruction... You can tell by listening at the filler neck...
 
Thanks Denny-O for your thorough explanation of how our diesel fuel system operates, this is what makes the Tugnuts sight so valuable. If after you've made sure your vent is clear and or you still find you have an overflow problem there is a product on the market called "No-Spill" that a boater simply attaches to the gelcoat temporarily, where the vent is located, to catch any overflow fuel during the fueling and starting process. Check this link out for more information.

http://www.tgmmarine.com/nospills/pages/products.html

Jim F
 
I installed a whistle on the vent line about one foot below the vent. Works like a champ. As you fill you hear the whistle as soon as you hear it starting to fade or stop, release your fill handle. I have not had a spill since.
 
Starting this season I began getting a healthy dose of fuel burping back out of the fill....about 30 seconds after I finished filling.  Not a drop came out of the vent however.

The vent seemed to be clear. (Kenny came out to verify) but the problem persisted. And then when I topped off my tank before a long trip a couple of weeks ago, I came back to the boat the day of the trip to find the diesel had completely filled what remaining void I'd left and was pushing past all the fittings on the top of the tanks. ( and leaking into the bilge) Again, not a drop from the vent where you'd think excess pressure/air/fuel would escape.

I've since filled up but left the tank 10% below full, with no further problems except the need to tighten and seal all the fittings again. I did a lot of looking online and burping fuel from the fill and vent seem to be fairly common and often there is no apparent reason once a clogged vent is eliminated...the recommended course of action is not to fill up all the way.
 
I like Mike's whistle... Putting that on my do-it list - lessee, that is #23,477
There is no need to have fuel clear up the pipe to the fill cap as it adds less than a gallon to the total - and causes trouble for some...
But, there is a need for knowing that you have a full tank before going off on a long run, so the whistle is probably the solution..
 
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