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Potential problems in changing our fuel sender

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Potential problems in changing our fuel sender

Old 03-26-2020, 08:39 PM
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Default Potential problems in changing our fuel sender

I decided I was going to change out my sending unit on my gas tank, or at least pull it and check out its condition. I am not sure if it’s the original tank or not, but I am going to assume it is. Someone told me that by removing the screws and the sender, that if there is any corrosion in the tank, doing so will release ‘pressure’ in the tank, and any compromised metal will “let go”. I have a hard time believing this because isn’t pressure in the tank constantly changing, or at the very least, it’s vented so there shouldn’t be that much pressure? Am I wrong here? My fuel gauge seems to not ever want to go above 3/4 or below 1/4. I just want to have an accurate fuel gauge for peace of mind. Can anyone confirm or rebuke any potential hazard here, other than the potential to introduce foreign objects into the tank? I plan on having it completely clean and having a new gasket in hand before I pull anything out. Thanks.
Old 03-26-2020, 09:22 PM
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Whoever told you that was sadly misinformed. If your boat was built prior to 2012 the fuel tank has zero pressure in it. Fuel systems on recreational boats are designed to suck fuel from the tank, so that hoses and other fittings would be under negative pressure, and the tank was vented to allow for that. The simple reason for that is if there is pressure in the tank and hoses and you get a leak it is a law of physics that all your fuel will be dumped into the bilge of your boat. This is not something you want to happen. If your boat is newer than that it may have the EPA tank and fuel system which only sees a pressure of about 1 psi and has a pressure relief valve built in that opens if it goes much above that. If your tank is aluminum there is probably no corrosion at all. But that slight pressure is only in the tank. The fuel pump is still placed near the engine to suck fuel from the tank to the engine for the reason stated above. As far as an accurate fuel gauge, fuel gauges on boats have always been notoriously inaccurate. If it is the type that uses a float on an arm it will never be very accurate because of the motion of the boat and the sloshing of the fuel. There are newer gauge senders that use capacitance and inductance to measure the level in the tank that are much more accurate but even those have their limits. The most accurate gauge is still a stick. I have one marked off in five gallon increments that I use to measure mine when it gets lower than 1/4 of the tank. The reason it never seems to go below 1/4, is because that is probably the lowest the arm can go, and above 3/4 it's almost horizontal and may be obstructed by a fitting that extends down into the tank or even by the gauge mechanism itself. Opening up a tank is not usually a good idea. First you have to be very careful that fumes from the tank aren't ignited by something. Make sure all electrical devices are off. In fact I would disconnected the battery. If you do this you are going to need to replace the gasket for the sender, and have the tank pressure tested to make sure you haven't introduced a leak. Any marine technician can pressure test it for you. And they only use 3 psi or less.
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Old 03-27-2020, 03:34 AM
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Originally Posted by Mrbadfish View Post
My fuel gauge seems to not ever want to go above 3/4 or below 1/4.
Its quite possible that the sender in your tank needs replacement -
and there is little to no risk in doing so - unless you are smoking while you work...

but before you remove the sender.
take a moment to check the gauge and its wiring:

- disconnect both wires at the sender -
the gauge should now read empty

- connect the two wires directly to each other -
the gauge should now read full

if either of those things does not happen,
you have a problem with the gauge, or wiring -
not the sender.
Old 03-27-2020, 05:31 AM
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That makes absolutely no sense. What do you think happens when you open the gas cap?
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Old 03-27-2020, 05:48 AM
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@ Peter—I did remove one screw before I changed my mind to wait until I had gaskets, and it came out seamlessly. There was zero corrosion on it, so I’m guessing the others are in the same shape, they appear to be from the top side. I fully intend to replace the gasket for the sender and the gaskets on the screws if I decide to move forward. I did disconnect my battery completely. Also, I love the stick idea, but my gas fill is on the side and the hose makes a pretty sharp curve to the middle of the boat (It’s a 17’ flats boat). My intention initially, is to correct either of the issues with the float that you have mentioned, whether it be shortening the arm, changing the orientation, or both.

@ FW—Sound advice sir!! And yeah I did intend on checking the gauge for shits and giggles, but I’m pretty sure it’s in good working order, just from the way it reacts with the key and the fact that it does somewhat fluctuate with the gas level to an extent.

With all this extra time on my hands from being furloughed from this virus, I will be going through the boat and tightening up any loose ends, and adding some new ones as well!

Thanks for the responses.
Old 03-27-2020, 05:49 AM
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Originally Posted by vandlor View Post
That makes absolutely no sense. What do you think happens when you open the gas cap?
Right. I thought maybe I was missing something here lol.....
Old 03-27-2020, 06:26 AM
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Originally Posted by Mrbadfish View Post
I decided I was going to change out my sending unit on my gas tank, or at least pull it and check out its condition. I am not sure if it’s the original tank or not, but I am going to assume it is. Someone on THT told me that by removing the screws and the sender, that if there is any corrosion in the tank, doing so will release ‘pressure’ in the tank, and any compromised metal will “let go”. I have a hard time believing this because isn’t pressure in the tank constantly changing, or at the very least, it’s vented so there shouldn’t be that much pressure? Am I wrong here? My fuel gauge seems to not ever want to go above 3/4 or below 1/4. I just want to have an accurate fuel gauge for peace of mind. Can anyone confirm or rebuke any potential hazard here, other than the potential to introduce foreign objects into the tank? I plan on having it completely clean and having a new gasket in hand before I pull anything out. Thanks.
Fixed it for you.
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Old 03-27-2020, 06:35 AM
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I highly recommend the reed type column senders over the arm/float style, unless you have an odd tank configuration. They seem to have less bouncing and read more consistently.
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Old 03-27-2020, 07:15 AM
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Originally Posted by autobaun70 View Post
I highly recommend the reed type column senders over the arm/float style, unless you have an odd tank configuration. They seem to have less bouncing and read more consistently.

These sending units are the way to go. To order the correct one, you need to remove your sender and measure how deep your tank is (6 inches, 8 inches..?) and order the right length sender. Worth the effort.

When you install the sending unit, it only goes one way. There are five screws that hold it in place, but they are not a symmetrical 112 degrees apart. Not an issue if you are aware of the symmetry, or lack of, in the hole pattern
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Old 03-27-2020, 07:24 AM
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MadFish,

Before you pull / remove the sender make a reference mark on both the sending unit flange and tank. This will reduce a follow up inquiry why my sender leaks. The orientation of those screws vary from 68 to 72 and 80 degrees apart.
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Old 03-27-2020, 07:46 AM
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Originally Posted by cws View Post
These sending units are the way to go. To order the correct one, you need to remove your sender and measure how deep your tank is (6 inches, 8 inches..?) and order the right length sender. Worth the effort.

When you install the sending unit, it only goes one way. There are five screws that hold it in place, but they are not a symmetrical 112 degrees apart. Not an issue if you are aware of the symmetry, or lack of, in the hole pattern
Yep. In some cases you may be able to find the correct size from the manufacture of the boat or tank. On my Key West I was easily able to determine what make/model tank was in it from the factory, and the part # for the sender was easy to look up from there.
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Old 03-27-2020, 12:13 PM
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Originally Posted by alloyboy View Post
Fixed it for you.

LOL actually not!!!!
Old 03-27-2020, 12:16 PM
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Originally Posted by Absolute View Post
MadFish,

Before you pull / remove the sender make a reference mark on both the sending unit flange and tank. This will reduce a follow up inquiry why my sender leaks. The orientation of those screws vary from 68 to 72 and 80 degrees apart.

If I replace the sending unit, will this do any good? Or just if I’m inspecting it? Is the orientation similar?
Old 03-27-2020, 12:51 PM
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Originally Posted by Mrbadfish View Post
If I replace the sending unit, will this do any good? Or just if I’m inspecting it? Is the orientation similar?
Fuel Sending Units (FSU) are virtually never accurate. Most just read the percentage of fuel that remains. The orientation of the screw hole and the FSU are very critical. IF the little screws do not go in freely then you are out of proper alignment. What I like to do is set the gasket on the tank so all of the holes line up with those in the tank. Then make reference to the small notch in the gasket and align the gasket with the bottom side of the FSU flange. Just don't get in a hurry...….
Old 03-27-2020, 12:53 PM
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Originally Posted by Absolute View Post
Fuel Sending Units (FSU) are virtually never accurate. Most just read the percentage of fuel that remains. The orientation of the screw hole and the FSU are very critical. IF the little screws do not go in freely then you are out of proper alignment. What I like to do is set the gasket on the tank so all of the holes line up with those in the tank. Then make reference to the small notch in the gasket and align the gasket with the bottom side of the FSU flange. Just don't get in a hurry...….

Ahhh ok right on. I’m pretty careful and methodical when it comes to things like this. Thanks!
Old 03-27-2020, 12:56 PM
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Originally Posted by Mrbadfish View Post
Ahhh ok right on. I’m pretty careful and methodical when it comes to things like this. Thanks!
Feel free to drop me a PM and I can walk you through it.
Old 03-27-2020, 12:58 PM
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Originally Posted by Absolute View Post
Feel free to drop me a PM and I can walk you through it.
Awesome, You da man! Do you foresee there being any problems that may come from doing this? I guess more importantly, do you see this causing any problems that were not already there?
Old 03-27-2020, 01:05 PM
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Originally Posted by Mrbadfish View Post
I guess more importantly, do you see this causing any problems that were not already there?
Now this guy is a genius.. I wish I asked myself that question more.. cause before my project is done, I unfortunately get the answer!

Old 03-27-2020, 01:15 PM
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Originally Posted by whitey436 View Post
Now this guy is a genius.. I wish I asked myself that question more.. cause before my project is done, I unfortunately get the answer!
LOL I've learned the hard way! It's taken me 38 years to figure this out......
Old 03-27-2020, 01:52 PM
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Before you do this do this.Run a new wire directly from the sender to the back of the gauge and a new ground. See if it reads.You'll know if its a bad wire. Ask me how it know.

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