Filling gas tank but gauge only reads half full

[durban131]

Hello All,

Bayliner Capri with a Mercruiser 3.0 here... Yesterday I filled up on gas at the gas station while boat was on trailor, got out on water, and notice the guage only said half full, bummer. So spent a day shorter than I would have liked but still good times.

Anyways loaded boat back on trailer, went to gas station, and refilled thinking maybe an airbubble was in the line the first time (or something like that) but sure enough it would only let me fill approximately what I believe I used that day (5-10 gallons) and still maxed out at half-full according to guage.

Either the gas tank is full and the gauge is wrong, or my tank only fills halfway? I waited for things to settle while pumping, going slowly in steps and even multiple trip to the pumps so Im convinced the problem is either with the tank or the gauge... Anyone have and insight?

 

[slothman]

Re: Filling gas tank but gauge only reads half full

Hello All,

Bayliner Capri with a Mercruiser 3.0 here... Yesterday I filled up on gas at the gas station while boat was on trailor, got out on water, and notice the guage only said half full, bummer. So spent a day shorter than I would have liked but still good times.

Anyways loaded boat back on trailer, went to gas station, and refilled thinking maybe an airbubble was in the line the first time (or something like that) but sure enough it would only let me fill approximately what I believe I used that day (5-10 gallons) and still maxed out at half-full according to guage.

Either the gas tank is full and the gauge is wrong, or my tank only fills halfway? I waited for things to settle while pumping, going slowly in steps and even multiple trip to the pumps so Im convinced the problem is either with the tank or the gauge... Anyone have and insight?

test the gauge first. Should have a pink wire on the back, jumper that to the ground and the needle should move to fuel. If the needle moves the gauge is not the problem, in which cause you need to check the sending unit in the tank.

 

[alldodge]

Re: Filling gas tank but gauge only reads half full

I'll take gas gauge failures for 200 Alex.... Sorry I'll hate my self for it later.

Your problem is in the gauge side more then likely. To verify, find the location where your fuel line comes out of the tank; this should also be where the sender is located. Do this is a well ventalated area, no smoking or electric lights near by, and the bow of the boat down low. You want the bow low because you have a full tank and pickup/sendor is usually in the back of the tank. If your tank is half full then you can probably not bother with the bow.

Remove gas line and wires from the sendor, then remove sendor from the tank. Reattach wires to the sendor out of the tank and hold it stright up, then turn the ignition to run (do not start). Gauge should read empty, flip sendor over, gauge should read full. Turn ignition off.

You probably are getting the same readings as before. What your doing is seeing if you get the same results and seeing if the float moves the entire length of travel. If the sendor moves the entire length, remove wires and find part number and manufacture of sendor. If the ohm scale is not on the sendor look the part number up and determine the range on the net. Once range is known, use an ohm meter and test sendor for correct range. If sendor does not read correctly replace sendor, if it does you need to replace gauge after making sure connects and wiring are good.

 

[Cannondale]

Re: Filling gas tank but gauge only reads half full

Or the answer could simply be the float has absorbed gas and gotten heavier or the sender isn't making contact throughout its travel....corroded, etc. Easy to check, well, if you have a spare sender gasket handy.

 

[NHGuy]

Re: Filling gas tank but gauge only reads half full

You can test the gauge by grounding the S terminal from the G on the back of the gauge. Then briefly power it up by turning the ignition to "run". If the gauge pegs to full it is working right.
You can test the sender with a volt ohmmeter. Zero to 5 ohms is full, 240 ohms is empty (plus or minus 5 ohm on either end of the range).
Boat senders use a float on an arm. They normally use a float that slides on the arm. If the float is too far in toward the pivot, the arm might not go all the way to the full point. And it definitely would not go to the real empty point. It would read empty before it should. So if the float can be slid out to the end of the arm you'll get a truer fuel level reading.
Or if it's a replacement sender and the arm is too long it could be hitting the tank before the arm rises to the full level. In that case you would snip off the extra length, as long as the arm could go down the the empty tank bottom. Otherwise you might need to turn the sender to a better angle inside the tank or use a different sender. On mine you would not be able to turn the sender to a different angle in the tank because the sender can only be installed one way. They may all be that way, I don't know that for sure. There is a pattern to the installation bolts that does not repeat.
The best way would be to identify which sender it is and find the installation instructions. But that's basically how they go.

.

 

[Home Cookin']

Re: Filling gas tank but gauge only reads half full

gas guages on boats are notoriously inaccurate. You should run it down to empty (with spare gas aboard) then watch the guage as you fill it, and know your own tank/guage.
No two are alike. I have one that reads 1/2 when it is 3/4 full and between a 1/4 and 1/2 when it is half full. Another stays pegged on F until there is a quarter tank and it goes to 1/2 then at 1/4 best get to shore! I use it more like the low gas light in a car.

Also boats don't sit the same on a trailer as in the water so you can top it off at the station, but when you launch, have less than a full tank.

 

[alldodge]

Re: Filling gas tank but gauge only reads half full

Not to refut but to add some additional info: The 240 ohm empty is used very often but not always. There are others which use 180 ohms for lever type senders. My gauges use 180 ohms full and 0 ohms empty (just the reverse) and they also are tube type

 

[durban131]

Re: Filling gas tank but gauge only reads half full

Thanks guys, you are all a great resource and wealth of information. This is my first boat and Ive just started working on basic repairs myself and this seems like a perfect little project for me to get more familiar with the inner working of my boat.

Looks like I need to test the wires on the gauge, then take a look at the sender, and possibly the even float. Looks like my Saturday is booked, more good times!!!

 

[Chris1956]

Re: Filling gas tank but gauge only reads half full

A lot of times the fuel tanks and the gauges are not compatable. For example, you can buy all shapes of tanks, but senders are pretty universal. In other words, don't expect great accuracy.

i.e I have an 18G flat tank (8" tall). When the gauge first reads empty, I still have 6 Gal. At 1/4 I have 9 gal, at 1/2 I have 12 Gal, at 3/4 I have 15 Gal, and at a full tank reaing I have 18Gal. This is my second tank of that size (first one rusted thru), and my second gauge and third or forth sender. All read the same.