Diesel Fuel Sender Unit

[Konan the Bavarian]

I would like to determine how much fuel is in my (2006 B37cr) poly fuel tank, and I am thinking of unscrewing the 5x screw that hold the sender unit in the top of the tank and use a dipstick to confirm the volume of fuel. I am unsure if the top flange plate of the sender unit has a gasket which can re-seal when I screw it back in. I am also concerned that I might create an issue with the screws being withdrawn & re-inserted into the plastic again. Has anyone had any experience removing the fuel sender unit?

The background to this exercise is:
the fuel tank was filled to approx. 3/4 full in January, prior to a coastal trip, when I ran the engine for about 6 hours. Since then I have used the boat regularly (minimum once a fortnight) but the motor does not get a lot of use. It is used to clear the other moored boats before the sails are set and turn the motor off. I am guessing about 30 mins x 15trips = about 8 hours, so not a lot of time.

The fuel gauge after all these months is still reading 3/4 full. I am concerned that the fuel gauge may not be working, but I have no reason to think that it does not, except that when the ignition is turned on, it hasn't indicated a level different than 3/4 for the past 9 months.

So I would like to dip the tank to confirm. Any thoughts?

Steven
Konan the Bavarian

[Lyra]

I do not know this boat and specific tank design, but if it is like in my boat I think it will be very difficult to calculate volume based on dipstick measurement. If it is an original tank then I believe the information about its volume should be available.
I find that the fuel gauge is not an accurate indicator, holding at the same place when full and declining rapidly when close to empty. I therefore base my fueling on engine hours and average consumption, which I measured over the years.
Again, not knowing the details but only according to boat size I assume your engine consumes ~2 liters/hour and that your tank is ~150 liters, so it is possible that 16 liters will not have a great effect on the fuel gauge, especially when near full.
Another point is that in any case, if your fuel consumption is low, it would be advisable to top up the tank periodically and maintain a small as possible air space.

[Symphony]

8 hours with that pattern of usage is around 10 litres, so nothing compared with a 150l tank.

You really should not use the engine like this. It should always be run enough to get thoroughly warm and periodically run at full revs for a short period. The way you are using it will mean that at the least you will join the regular posters showing off their blocked exhausts, and potentially high oil consumption from glazed bores.

[Kibo]

Fuel gauges are notoriously inaccurate and non linear over the range from Full to Empty. Mine is the same and stays at full to 3/4 for a long time, even when using the engine according to Symphony's excellent post.

My advice: Fill the tank to full. Record engine hours. Use your engine over a variety of conditions and loads/revs. After the gauge actually moves off full and you believe you have used a reasonable amount of fuel (think about 3 litres of fuel per hour). Then fill up and calculate litres/hour used.

Subsequently continue to log your engine hours and estimate fuel usage over time.

[dawntreader]

Fuel gauges are notoriously inaccurate and non linear over the range from Full to Empty. Mine is the same and stays at full to 3/4 for a long time, even when using the engine according to Symphony's excellent post.

My advice: Fill the tank to full. Record engine hours. Use your engine over a variety of conditions and loads/revs. After the gauge actually moves off full and you believe you have used a reasonable amount of fuel (think about 3 litres of fuel per hour). Then fill up and calculate litres/hour used.

Subsequently continue to log your engine hours and estimate fuel usage over time.

You can also make a drawing of your gauge. Then, if it is not on the 'Full' point, mark the positions of the needle on your drawing each time prior to filling up. Add the record of 'remaining fuel' (that is: capacity less what was put in) above the mark on the drawing and over time the drawing will give you a better idea of how much fuel you have left based on the current reading of the needle.  

[Nigel]

Before I made my own calibrated gauge it would sit for 10 days at ¾ full, then plummet.

[Kibo]

I'm going to make a gauge scale calibration too. Great idea. Thanks

Before I made my own calibrated gauge it would sit for 10 days at ¾ full, then plummet.

[Nigel]

I'm going to make a gauge scale calibration too...

I actually built a new display unit with a Nokia phone LCD. Currently working on a MK2 model, as the screen was a bit temperamental.

[tiger79]

Before I made my own calibrated gauge it would sit for 10 days at ¾ full, then plummet.

If you think about the shape of the tank, it's not surprising that the gauge is so inaccurate.  It is possible to get senders which can be calibrated, but I imagine they're not cheap.

[jonrarit]

Yup last week I found out the hard way not to trust the guage. (2006 B36c)

After 10 days cruising the Ionian the guage was down to half ....here's me thinking great still got 75 litres left.

Came out of Mogonisi into a BIG rolling sea and 30 knots. Only intending to motor round to Gaos (10mins).

Half a mile off the entrance the engine dies!!! Ok onto a leeshore (rocks) with no engine & following sea choices were to sail the entrance (narrow, dangerous & a one-shot or you're in trouble) or sails out ahead to sea. Went for the latter with buttocks clenched to clear the headland..... lived to tell the tail and sailed for a couple of hours over to mourtos. That gave me time to diagnose the engine problem ....... air in the fuel line. Topped up, quick bleed and all good again.  Subsequent fill up shows we were down to 1/4 tank despite the guage telling us otherwise.

live and learn

jonathan

[mikeiso1192]

Yes - it was a bit rough out there last week - we were holed up safely on Meganissi.

Does remind me of one chap caught similarly to yourself; with big rolling seas, stirred up the fuel and brought the remenants of diesel bug off the bottom of the tank with consequence of failed engine - resulted in an expensive tow in to Preveza.

Mike

[Kibo]

Nigel that's a bit beyond my capabilities but I would be interested in more info as to how you did it just in case I can swot up and manage it

I'm going to make a gauge scale calibration too...

I actually built a new display unit with a Nokia phone LCD. Currently working on a MK2 model, as the screen was a bit temperamental.

[Jackho]

I agree with all the comments re: long tapering tank and inaccuracies of sender. I have recorded all my fuel use and engine hours over the 10years from new and average works out at 1.5 lt/ hr. D1-30 engine.  On my wish list for anyone with nk2 backbone network is to install a fuel monitor at about £200. Simple to install with readout and usage on chartplotter.
For now at bottom of wish list.☹️☹️

[tiger79]

On my wish list for anyone with nk2 backbone network is to install a fuel monitor at about £200. Simple to install with readout and usage on chartplotter.
For now at bottom of wish list.☹️☹️

Does that take data from the engine, or from flow sensors?

[Jackho]

Looked at fuel sensor again and see its only for petrol engines.  Now off my wish list.

[Konan the Bavarian]

Thank you Lyra & Symphony or your advice.
Topped up the fuel tanks, and yes the fuel gauge went from 3/4 t full, so yeah - the sender/gauge is confirmed as still working.
No wind on Sunday, so we took the boat for a good motor around - thanks again for the advice/prompt about motor usage.
We will need to review how we use the boat. I enjoy sailing on/off the mooring, and sailing in close quarters to the surrounding boats to keep the sailing skills honed. This does of course reduce the amount of time spent motoring.
Cheers
Steven, Konan the Bavarian

[Nigel]

Apart from the shape of the tank, another factor that affects the linearity is the height of the sensor pivot and the shape of the arm. I attach a plot of my original float, and the modified one.

Just for fun, I also include the rather complex instructions I wrote for my MK1 electronic gauge.

[tiger79]

Apart from the shape of the tank, another factor that affects the linearity is the height of the sensor pivot and the shape of the arm. I attach a plot of my original float, and the modified one.

More recent boats use capacitative senders, which are just tubular, with no float arm.

[Bavnav]

Yes had the same problem, sender/gauge reading 3 quarters full. Replaced sender and gauge. Logged new gauge by adding 20 L. at a time and noting pos. on gauge.

[s/y Susanne]

The sender does come out with the same difficulty as a metal puzzle, and by loosening the screws only and rotating slightly one way or another you may find that the readout is a bit more linear without having to remove the sender altogether. That said logging engine hours at each fill will give you a good working average for fuel use which means you know when to top-up before any specific passage.
Stewart

[Craig]

Never seen an accurate fuel gauge.

Suggest following:-
When tank near empty start filling tank.
Record about of fuel put in tank when gauge gets to 1/4 full, 1/2 full, 3/4 full, full and also when tank actually full.

On my Bav 38 ( 2009) with a D1-30 Engine I average 1.5 to 2 litres per hour at 2100rpm.

When gauge shows 1/2 full it is actually 1/3 full. Time to refill.

I have the conversion noted in my log as a reference. Now about to predict how much fuel I can input in to around 5 to 10 litres.

As a word of warning, when tank is below 1/4 full and seas are rolly, you may suck a lot of air up the line. This was the only complaint about a Bav 44 that went around Cape Horne 2 years ago.

Craig
"Shirley Valentine"
Gold Coast
Australia