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Author Topic:   1993 21 WALKAROUND Fuel Tank Level Indication Never FULL
Mfior313 posted 06-02-2015 12:43 AM ET (US)   Profile for Mfior313   Send Email to Mfior313  
[I am a] long-time reader, first-time poster. In fact, the information I found on here helped me [decide to purchase what was later clarified to be a 1993 Boston Whaler 21] WALKAROUND boat. I've searched and searched this forum and others to try and find some clues, but now I felt it was time to post: I've had this boat for close to a year now, and it's been great. Only [problem] I currently have is getting the fuel tank full. [The Boston Whaler 21 WALKAROUND] has the original 92-gallon fuel tank. No matter what I've tried, I can't get [the fuel tank] to fill up to a full tank. The most [the fuel tank gauge] will read is 3/4-FULL. I have both a digital and analog fuel tank level gauge. I have a hard time thinking [the cause of the reading of only 3/4-FULL is] the gauges' fault. Maybe [the cause is] the [fuel tank level] sending unit. Will a bad sending unit show 3/4-FULL? Or, will [the reading on the fuel tank level gauges] just be EMPTY or FULL?

I've tried a bunch of tricks to fill [the fuel tank of a Boston Whaler WALKAROUND boat of unknown length to] full:

--filling on the water

--filling on the trailer with bow all the way up or down

--rocking the boat side to side to get all the fuel out of the fill hose

--blowing into the vent prior to filling

--slow filling

--filling while sealing the gas spout with a rag

--inspecting the fill hose and vent hose to make sure there are no low spots

The hoses don't have low spots but they have somewhat long stretches of flat spots before entering the tank. When the tank does hit 3/4-FULL, [fuel] will come out of the vent, so I don't think [there is a problem with] a clogged vent hose.

I tried looking under the inspection hatches so that I could test the sending unit with a multimeter, but didn't see anything that looked like a sending unit. Maybe I need to unscrew the deck to get to the sending unit.

I am running out of ideas. Anyone ever have an [problem with filling a fuel tank on a Boston Whaler boat to FULL] like this? Or, any advice?

I'm thinking maybe I just need to run [the fuel tank] dry to see how long [the engine will run] until I'm out of fuel. It seems like [the engine] will still go [run] quite a ways [long time] after running below 1/8-FULL tank and down to what reads close to EMPTY, but I've never tested my luck. Thanks for any help.--Mike

[In this thread a number of sidebar topics were introduced which are not boating topics. They have been removed. --jimh]

contender posted 06-02-2015 05:22 PM ET (US)     Profile for contender  Send Email to contender     
Sounds like the sender is bad, however the true way to tell is empty all the gas (fill your cars) then hit the gas station. If you take around 85 to 90-gallons and the gauage is 3/4, you know your problem.
jimh posted 06-02-2015 06:43 PM ET (US)     Profile for jimh  Send Email to jimh     
I don't see in your narrative any instance of where you have verified you have filled the fuel tank to capacity and yet the fuel tank level indicator says only 3/4-FULL.
jimh posted 06-03-2015 05:22 AM ET (US)     Profile for jimh  Send Email to jimh     
About 12 years ago a discussion at

reported the fuel tank level indicators were generally unreliable in several post-1990 Boston Whaler boats.

andygere posted 06-03-2015 01:00 PM ET (US)     Profile for andygere  Send Email to andygere     
When you fill the tank, can you add enough gasoline to get it to bubble out of the vent? This is usually the best indication that the tank is indeed full. At your lowest fuel gauge reading, how many gallons can you dispense into the tank? Could you dip the tank via the fuel hose (with something like an electrical fish tape or cheap tape rule) to get an approximation of the fuel level in the tank after filling? It would be helpful for you to have a diagram showing the fuel fill hose configuration for this method to be effective.
Mfior313 posted 06-03-2015 04:24 PM ET (US)     Profile for Mfior313  Send Email to Mfior313     
My boat is a 1993 Boston Whaler 21 foot Walkaround.

Spillage from the vent leads me to believe the boat is full regardless of the fuel gauge reading 3/4-FULL. Thanks for the link.

Contender, I'm leaning towards the sender being innacurate as well. I'd have to empty the tank into several gas cans in order to drain it based on my boat storage situation, but that is easy enough. The previous owner said he never had an [problems] with the fuel gauge reading, which had thrown me off. Then again, I'm not sure how often he tried to fill it to the brim.

Andygere, the most fuel I have gotten in the tank at it's lowest gauge reading (which is right at the empty level) is about 48-50 gallons. Again, I do believe I had noted that in my original post, which was then edited out.

My [problem] is starting to sound more like a faulty sending unit than a giant air bubble in the tank.

Jeff posted 06-03-2015 04:36 PM ET (US)     Profile for Jeff  Send Email to Jeff     
Mfior313--Have you gotten a bunch of people, or extra dead weight to sink the stern a fair bit while fueling on the water and see if you still have this problem?

Also, if after you get the spill-out from the vent, have you tried greatly reducing the flow of fuel into the tank from the nozzle to see if it will then continue to fill? My Grady White is like that and so is our Boston Whaler 23 WalkAround Whaler Drive (big bother to your hull) at times. The handle will pop and fuel will shoot out of the vent. This is usually at or just above 3/4 full. After that I just have to lightly squeeze the handle and fill the last 1/4 of the tank very slowly.

jimh posted 06-04-2015 11:05 AM ET (US)     Profile for jimh  Send Email to jimh     
You have a problem with the fuel tank level. I don't see any conclusive indication that you have filled the tank to capacity. As recommended, the best way to deduce how much fuel is in the tank is to begin with the tank as empty as possible, then fill it as full as possible. You should be able to add a volume of fuel to the tank that is close to the rated capacity.

The presence of fuel or aerated fuel coming from the vent is not a guarantee that the tank has been filled to capacity. The vent is usually located in the forward portion of the tank. If the trim on the boat has the tank tilted toward the bow, fuel will rise to the level of the vent in the forward part of the tank before the tank is completely full. It might be helpful to use a bubble level to see how the fuel tank is oriented. To get the most fuel into the tank, the tank usually needs to be oriented so its forward engine is higher than the aft end.

jimh posted 06-04-2015 11:06 AM ET (US)     Profile for jimh  Send Email to jimh     
Thanks for clarifying the length of your Boston Whaler WALKAROUND boat. It is highly recommended that you mention basic information like the length of your boat in your initial posting.
Mfior313 posted 06-04-2015 11:59 AM ET (US)     Profile for Mfior313  Send Email to Mfior313     
As like I mentioned in a previous post; when the fuel gauge on my 1993 Boston Whaler [21] WALKAROUND reads EMPTY, I have only been able to add up to 50 gallons of fuel in the tank before it bubbles out of the vent. What I didn't mention is that I can also see fuel down in the fill hose, which again, leads me to believe it is full (minus any potential air pocket that I can't get out of the fuel tank). The fueling is done with the bow raised higher than the aft end. I've only lowered the bow during fuel filling to experiment with fueling while the bow and aft end were near level.
jimh posted 06-04-2015 01:21 PM ET (US)     Profile for jimh  Send Email to jimh     
ASIDE: This thread is now the highest scoring search result on the search terms: walkaround, fuel, tank, level, sender, full, add. See level+sender+full+add

I have to observe an apparent conflict in the theory that has been proposed (by me and others) that adding fuel to the tank is being limited by the tank having a fore-and-aft trim to the forward, and fuel in the tank is accumulating first in the forward part of the tank, where it blocks the vent line, with other outcomes of that assumption of where the fuel is tending to accumulate.

When this condition is reported, we also have a report that the fuel tank level is indicated as only 3/4-FULL. We must consider the location of the sender. If the sender is in the forward part of the tank, and if the trim on the boat has caused the fuel to accumulate in the forward part of the boat, then it would be reasonable to assume that sender should be responding to a full tank of fuel at its location, and should show FULL as the indication.

On the other hand, if the trim on the boat is the same as above, and if the sensor is not located at the forward portion of the tank and instead is toward the rear, the level of fuel at the rear of the tank should be lower. A sender located there would likely show a level of less than FULL.

Can we have more details about the location of the following components of the fuel system of a 1993 Boston Whaler 21 WALKAROUND boat in the fore-and-aft directions:

--the tank vent

--the tank filler inlet

--the tank level sender

--the fuel pickup

Knowing where these components are located may help explain why certain conditions exists with adding fuel and why the tank indicator seems to be inaccurate.

Mfior313 posted 06-04-2015 01:33 PM ET (US)     Profile for Mfior313  Send Email to Mfior313     
Jeff--yes, I have tried fueling up with a few people sitting on the back of the boat while fueling at the dock. And I have tried the slow trickle method of fueling once the fuel spits out of the vent. The last time I tried that, I could see fuel backing up the fill hose. This was done on the trailer using 5-gallon cans, with a slow pour rate, with the bow raised as high as possible. But it might be worth another shot fueling at the dock with the slow trickle method. I'll just need to get something to catch the fuel that spills out of the vent.
Mfior313 posted 06-04-2015 03:34 PM ET (US)     Profile for Mfior313  Send Email to Mfior313     
Jimh--the fuel pickup is located at the aft end of the tank. On the tank, the filler inlet and vent outlet are located at the forward end of the tank, which both exit the tank at 90 degrees toward the port side.
The deck fill is located on the port-side gunwale, a couple of feet aft where the fill tube enters the tank. The actual vent is located on the hull just aft of the deck fill.

As for the location of the sending unit, I cannot see it from the forward or aft inspection ports for the tank. I'm assuming it has to be somewhere in the middle. I haven't had a chance to pull the deck and take a look, but that's next on my list of things to do.

If I assume the sending unit is not at fault, and is located somewhere in the middle of the fuel tank, perhaps the boat/tank has to be perfectly level in order for the tank to reach full capacity and get a full reading from the fuel gauge?

Like I mentioned, I think I've tried all the tricks thati could gather here and elsewhere to get the tank filled to max capacity (again, assuming it's not a faulty sending unit). With tuna season upon us here in southern California I want to make sure I get I can get the full range that a 92 gallon tank can get me, without the worry of running out of fuel

White Bear posted 06-04-2015 07:46 PM ET (US)     Profile for White Bear  Send Email to White Bear     
Two thoughts on the problem:

--You mentioned both a digital and an analog fuel gauge: are you running both off the same sender? If so, that is probably the problem as sender units generally do not support two separate gauge circuits, and if such is attempted will give inaccurate results.

Why don't you remove the sender unit from the tank and manipulate the float arm from full to empty and back while watching the fuel gauge. If that movement doesn't result in full movement on the gauge, the problem is in the sending unit. If you do get full deflection of the gauge, there is probably a mechanical restraint preventing the float from attaining its full range of travel. This will require a bending of the float arm to correct.

Mfior313 posted 06-04-2015 11:28 PM ET (US)     Profile for Mfior313  Send Email to Mfior313     
White bear--that is good to know about the two fuel gauges. I am guessing the analog gauge came from the original owner. Then when the second owner swapped motors, he hooked up the Yamaha multifunction gauge and left the analog intact. I can only assume that they are serviced by one sender. But I won't know for sure until I pull the deck and take a look. In the meantime I can try disconnecting the analog gauge and see if that helps.
Thanks for your input.
jimh posted 06-05-2015 12:03 AM ET (US)     Profile for jimh  Send Email to jimh     
The fuel tank level sender is probably a resistive device, a rheostat whose resistance is proportional to fuel tank level. The simplest of these are just a mechanical arm with a pivot. The far end of the arm has a float. The arm also moves a resistor.

The way the resistance is measured can vary from gauge to gauge. In the simplest form, the gauge supplies current to flow through the resistor. As the resistance changes, the current flow changes, and this reflects in the indicator. As WHITE BEAR mentions, you cannot connect two gauges of that type to one sender, as they will each be trying to send some current through the resistor, and this will probably cause them to interfere with each other.

But some instruments are designed to be used in conjunction with a conventional gauge, and those instruments just sense the voltage across the resistor, not the current flowing through it. An instrument like that can work in conjunction with another gauge.

Unless the two gauges were chosen and wired with this possible problem in mind, they could be interfering with each other. A simple test is to disconnect one of them and see what happens to the reading.

jimh posted 06-05-2015 12:08 AM ET (US)     Profile for jimh  Send Email to jimh     
The typical resistive sender for a boat fuel tank has these characteristics:

At EMPTY tank, a resistance of 240-Ohms

At FULL tank, a resistance of 33-Ohms

You might disconnect all the wiring from the sender and measure its resistance with a good, reliable Ohmmeter. See if you get resistance reading around those values when the tank is full and empty.

Check to see if more than one instrument is wired to the sender. That may be a no-no if one of them is not a special type gauge that can use voltage measurement instead of current measurement.

Check the negative circuit wiring. Some senders have the negative circuit isolated and some have it bonded to the metal tank. The best instruments use a dedicated negative circuit lead from the sender back to the instrument, and don't rely on a ground connector or a negative common bus connection for that circuit.

Hoosier posted 06-05-2015 08:14 AM ET (US)     Profile for Hoosier  Send Email to Hoosier     
If your 21 WA has a storage locker in the deck between the helm seats pull out the liner and the fuel sender should be under it. That's where it is on my 23 WA. If it is, pull it and see if you can get a vertical float model to replace the lever arm one. The boats have very different tank setups but it's easy to check. The sender has to be accessible, without pulling the cockpit floor, since it's something that would need to be serviceable.
jimh posted 06-05-2015 08:37 AM ET (US)     Profile for jimh  Send Email to jimh     
Options for electrical tank level senders are discussed in a thread in SMALL BOAT ELECTRICAL. See

Fuel Tank Level Sender: Electrical

I'd look closely at the option of a WEMA sender. See

If the fuel tank has the common five-bolt circular pattern for the sender, the model SSS or SSL should be a good fit.

If you plan to move to more modern instrumentation using NMEA-2000, see my article

NMEA-2000 Instrumentation: Fluid Tank Level Monitoring

dfmcintyre posted 06-05-2015 07:43 PM ET (US)     Profile for dfmcintyre  Send Email to dfmcintyre     
I know this question may sound odd, but do you have a plastic or metal tank?

Regards - Don

Jeff posted 06-07-2015 10:32 AM ET (US)     Profile for Jeff  Send Email to Jeff     
Ahh, Don is on to something. It might actually have a plastic tank.
jimh posted 06-07-2015 02:17 PM ET (US)     Profile for jimh  Send Email to jimh     
What if the tank is plastic? How would the fuel tank level gauge be affected?
acseatsri posted 06-07-2015 08:04 PM ET (US)     Profile for acseatsri  Send Email to acseatsri     
It's an aluminum tank. The fill hose is 8' long and is on a gentle slope, but mine always filled without any [difficulty] at the gas dock. I couldn't get the last 5-10 gallons into it at a gas station. I can't comment on the fuel gage as I ran a fuel flow meter and a very questionable Lowrance EP-85r that was supposed to keep track of fuel used but was always off almost 20 percent.
RevengeFamily posted 06-08-2015 06:33 AM ET (US)     Profile for RevengeFamily  Send Email to RevengeFamily     
Mfior313 states he has a 92 gallon fuel tank. I understand his request for information is based upon the fact that his fuel gauge never reads above 3/4 full, but my 1990 22 Revenge WT only has a 77 gallon fuel tank. Are you positive of the tanks capacity of 92 US gallons?

Regardless of capacity, it sounds to me like a few [problms] are giving him trouble.

I would start by removing the deck above the tank. Locate the sender and remove it for inspection.

Perform the [resistance measurement] test jimh spoke of, and move forward from there. I believe the sending unit may need to be calibrated or may be interfered with on its way up to the "full" position.


dfmcintyre posted 06-08-2015 03:54 PM ET (US)     Profile for dfmcintyre  Send Email to dfmcintyre     
Jim--under certain conditions a plastic tank contributed to a significant amount of water migrating past the poured in foam surrounding the tank due to the mold release agent used in the production of the tank not allowing the foam to seal to the tank.

Normally, not a big problem, except in winter freeze zones, where the boat was stored outside. The tanks were buckling, right underneath where the fuel sender arm would travel. It would cause the arm to stop, indicating about a quarter of a tank of fuel left, but the engine would run out of fuel.

Filling the (now empty) tank would result in about 10 gallons short (due to the buckling of the bottom of the tank, you could not fill it to the labeled capacity.

Owner and service manager would be puzzled; what happened to the 10 gallons?

Drove Whaler a little buggy for a while.

Regards - Don

jimh posted 06-10-2015 12:14 AM ET (US)     Profile for jimh  Send Email to jimh     
Don--thanks for the explanation of how some plastic fuel tanks can become deformed in a way that interferes with the movement of the fuel tank level indicator arm.
jimh posted 06-10-2015 12:16 AM ET (US)     Profile for jimh  Send Email to jimh     
Fuel tanks do not have "issues." Fuel tanks have defects or problems. The existence of these defects may cause some humans, for example, the owner of a boat with a defective fuel tank, to elevate these problems for him to be issues for him. But the tank does not have "issues." The owner of the tank has them.
Mfior313 posted 06-10-2015 09:58 AM ET (US)     Profile for Mfior313  Send Email to Mfior313     
Just to confirm, the tank is a 92-gallon aluminum tank. The aft inspection port opens up to reveal the manufacturer, Florida Marine Tanks. This weekend I tried fueling-up at the dock, again with the slowest possible trickle after the fuel started spurting out of the vent. That didn't help. Any tiny amount trickled in would cause fuel spillage from the vent.

I think the next step, which is the easiest, is to determine if both fuel gauges are running off the same sender, and if so, to unhook one of the two fuel gauges. If that doesn't solve the problem, it's time to pull the deck to access the sender unit. I'm just trying to make sure I am exhausting as many options as possible prior to removing the deck.

gqwhaler posted 06-10-2015 10:57 AM ET (US)     Profile for gqwhaler  Send Email to gqwhaler     
Mfior313--check this post out:

I can confirm you must have a positive bow angle to fully fill fuel tank. if you do not fill with the bow up a bit you will be about 15 gallons short of full. As for your guage, I personally never trust them except for general referance. I fill full before a trip and use a fuel flow meter for gallons used. If you have any other questions on the 21 walkaround I would be happy to help you. I have owned mine over ten years and I know every inch of the 21 walkaround. Cheers

Hoosier posted 06-10-2015 11:30 AM ET (US)     Profile for Hoosier  Send Email to Hoosier     
Just a thought, but you say you are using the overflow/vent as your indicator of tank fill. Could the problem be that the vent hose is in the wrong place on the tank? I know it's not likely but a previous owner could have screwed things up.
Mfior313 posted 07-13-2015 06:27 PM ET (US)     Profile for Mfior313  Send Email to Mfior313     
I finally have an update on this topic.

I followed White Bears suggestion of disconnecting one of the fuel gauges, and was able to test it out this weekend.

After fueling up (again with both fuel gauges reading 3/4 full), I disconnected the analog fuel gauge and the fuel level on digital multifunction gauge read full. After running the boat ~75 miles, the gauge read half-full.

When I filled up at the fuel dock, I got ~50 gallons of into the tank. That confirmed, in my mind, that the gauge is now working properly... and that my fuel economy is about as bad as I expected.

Thanks to everyone here for their input. I never would have thought the fix could be that simple/free.

RevengeFamily posted 07-15-2015 05:48 AM ET (US)     Profile for RevengeFamily  Send Email to RevengeFamily     
And that's why "continuousWave" is so awesome...

White Bear shares his experience and we all learn something...

Good to hear you were able to solve your fuel gauge dilemma...

What size and brand of engine are you running on your 1993 21 Walkaround?

Mfior313 posted 07-15-2015 05:05 PM ET (US)     Profile for Mfior313  Send Email to Mfior313     

I have a 225 Yamaha OX66 on it. Would like to some day put a 200 4 stroke on the back for better fuel efficiency, but my current motor just reached 500-hours so I don't see that happening any time soon.

jimh posted 07-16-2015 09:36 PM ET (US)     Profile for jimh  Send Email to jimh     
Generally you cannot connect two fuel tank level electrically-operated gauges to one fuel tank level electrical sender. This is (or should be) clearly stated in the installation instructions of any of the gauges. Only a few specialized electronic sensors can be used in parallel with a conventional electrical sender.
Hoosier posted 07-17-2015 09:21 AM ET (US)     Profile for Hoosier  Send Email to Hoosier     
The dial fuel level indicator in the floor of my '78 Outrage V20 is operated by a magnet in the fuel level sender. The dial capsule is removable and I was able to get a capsule that has both an indicator in it and the resister to operate a remote gauge. The magnet turns the rheostat inside the capsule and the remote gauge is a potentiometer with a dial calibrated for tank level. The indicator in the capsule works without power but the remote gauge needs power to complete its circuit. It's nice to be able to read my fuel level while fueling without having to turn the power on.
jimh posted 07-17-2015 09:33 AM ET (US)     Profile for jimh  Send Email to jimh     
I suspect that David is using the Rochester gauge that incorporates both a visual dial pointer and an electrical rheostat. For more information on this device, see

The sender is called a "direct read with remote transmitter" or a "Twinsite" dial.

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