International fuel gauge question

[inhof009]

I have a 2003 international, and I've been wondering on my fuel gauge for some time. Specifically, what each tick means. Here's a picture of my dash, forever gauge top right:

I figure full means full and empty means RIP. However, there's is a tick mark close to empty, and I expect that to be 1/4 full, but it's skewed closer to empty. Does anyone know why this is skewed or what it marks if not 1/4?

 

[Booyah45828]

That's a good question.

The marks to the left and right of 1/2 are definitely 1/4 and 3/4 respectively.

The reason E might be so close to 1/4 is because they consider what is truly an 1/8 of a tank as empty, and don't want you to run any more below that.

Kind of the same thing with the F mark. Anything over 90% and the gauge probably can't tell the difference.

This all might have to do with the sender operation and it's location in the tank, along with the shape of the tank.

Check out this gauge here.

See how all the readings are skewed? That's because the ohm sweep of the sender doesn't correlate with the linear sweep of the fuel level. Ergo when the sender ohm reading is at halfway between full and empty, your actual fuel volume in the tank doesn't and is actually below half.

 

[inhof009]

That's a good question.

The marks to the left and right of 1/2 are definitely 1/4 and 3/4 respectively.

The reason E might be so close to 1/4 is because they consider what is truly an 1/8 of a tank as empty, and don't want you to run any more below that.

Kind of the same thing with the F mark. Anything over 90% and the gauge probably can't tell the difference.

This all might have to do with the sender operation and it's location in the tank, along with the shape of the tank.

Check out this gauge here.

See how all the readings are skewed? That's because the ohm sweep of the sender doesn't correlate with the linear sweep of the fuel level. Ergo when the sender ohm reading is at halfway between full and empty, your actual fuel volume in the tank doesn't and is actually below half.

So instead of having a computer crunch the numbers between the sender and the gauge to make the needle a linear sweep, they modify the gauge? I'm used to car fuel gauge, which tends to go in a linear fashion. That would explain why when I drive it home the needle seemed to fly through the middle and hang on both ends.

So I can count on those marks as accurate. However, should I fill up at E, or should I still avoid going below 1/4? My original thought was that 1/4 tick was the "fill up now or face consequences" point, but I can reasonably move that down to E if it's actually 1/8. Still, though, can't hurt to be safe...

 

[Booyah45828]

Computer? You're dealing with a 2003 bus. I'm pretty sure it wouldn't have had an instrument panel computer like your car does. Yeah, they design the gauge backing plate to read the appropriate level in the tank.

Fill up at 1/4 if that's what you normally do.

What I do? I don't use my bus that often, my bus has a steel tank, and steel tanks can create condensation. So with that said, I keep mine full or near full of fuel all the time, no troubles that way, and I don't have to worry about running out of fuel like people tend to do on here.

 

[inhof009]

Computer? You're dealing with a 2003 bus. I'm pretty sure it wouldn't have had an instrument panel computer like your car does. Yeah, they design the gauge backing plate to read the appropriate level in the tank.

Fill up at 1/4 if that's what you normally do.

What I do? I don't use my bus that often, my bus has a steel tank, and steel tanks can create condensation. So with that said, I keep mine full or near full of fuel all the time, no troubles that way, and I don't have to worry about running out of fuel like people tend to do on here.

Lol, I love the lack of extensive computerization on my bus.

Speaking of steel tank, I'm considering adding another gas tank down the line. If it's steel, I'll definitely keep it full. I'm going to want to add a reserve gauge for the tank as well. If I do, do you have any suggestions on how to find a gauge that has the appropriate markings for that tank's fuel status?

 

[CMORGANSKOOL]

My gauge shows 3/4 when full and I have no idea where empty is. When I took it 500 miles home I filled up on 1/4 and was ok. Wish I could figure out how to make it work right. I think the sending unit may be on the fritz but I haven't really researched it too deeply.

 

[Brad_SwiftFur]

I remember reading an old article in Popular Mechanics(?) years ago "The liar on your dashboard", the fuel gauge. Full, 3/4, half, and 1/4 are usually calibrated to the appropriate levels in your tank. So why the short travel to empty? Because many drivers want "empty" to still have a little reserve in the tank so you can make it to the next gas station. I had a 72 GMC pickup that when the gauge said "empty", it really, honestly meant EMPTY. The article said Cadillac, when they introduced a fuel gauge that read digitally in "F" or "xx" gallons remaining, began receiving a number of complaints that after the car was fueled up, the gauge didn't stay on "F" for very long before dropping to "17" gallons. Their "solution" was to reprogram the gauges to indicate "F" until it reached 16 gallons - and the complaints vanished.


So, the fuel gauges are calibrated that way intentionally - just the way the American Public likes them.

 

[inhof009]

I remember reading an old article in Popular Mechanics(?) years ago "The liar on your dashboard", the fuel gauge. Full, 3/4, half, and 1/4 are usually calibrated to the appropriate levels in your tank. So why the short travel to empty? Because many drivers want "empty" to still have a little reserve in the tank so you can make it to the next gas station. I had a 72 GMC pickup that when the gauge said "empty", it really, honestly meant EMPTY. The article said Cadillac, when they introduced a fuel gauge that read digitally in "F" or "xx" gallons remaining, began receiving a number of complaints that after the car was fueled up, the gauge didn't stay on "F" for very long before dropping to "17" gallons. Their "solution" was to reprogram the gauges to indicate "F" until it reached 16 gallons - and the complaints vanished.


So, the fuel gauges are calibrated that way intentionally - just the way the American Public likes them.

Reason for mechanic visit: fuel gauge decreases as customer drives

I was always taught to fill up at 1/4. My father's reasoning was once the fuel fell below 1/4, the pump got exposed and would overheat and shorten its life. That's probably not true anymore with more modern vehicles, but I've never been walking in the side of the road with a gas can, so I want to keep it that way!

Gotta love how the fool-proofing has impacted the rest of us slightly more knowledgeable and careful drivers.

 

[inhof009]

My gauge shows 3/4 when full and I have no idea where empty is. When I took it 500 miles home I filled up on 1/4 and was ok. Wish I could figure out how to make it work right. I think the sending unit may be on the fritz but I haven't really researched it too deeply.

Have you looked into the software for our engines? It's a bit pricey to get the adapter, but the software is free and it could help you watch your fuel consumption.

 

[Booyah45828]

You have a couple of options.

Semi trucks for instance, use saddle tanks and have a free flowing line at the bottom between the two. In this scenario, you only need your current sender with no additional pump. All you would really have to do then is to make sure the add on tank's top and bottom are at or within the current tank's top and bottom.

You could also get a second tank with it's own pump and sender. When your primary fuel tank gets low, flip the switch and pump the contents of the second tank over into it. GM did this on all of their dual tank trucks, so finding one of those tanks in the scrap yard and retrofitting it over would be easy. Some lines, wire, switch, relay and a fuel gauge and you'd be set. Just make sure not to overfill the main tank from the add on.

At the end of the day though. You have to come to grips with how much fuel you're going to use at one time. All the vehicles I own, including the bus, could drive a minimum of 4 hours before fill up. After 4 hours of driving, I honestly could use a break, use the bathroom, get a snack, whatever, ya know? So I've never felt the need to have 8+ hours of drive time in fuel on board.

If I was a long haul semi driver, that was limited to so many hours of on time per day, then the story would change, but I'm not.

 

[inhof009]

I saw a guy on here (I forget who) rigged a second tank with a pump that he set on one of those timer switches. Once his primary tank went down, he turned to 30min and that dumped his full secondary into primary.

I'm planning for a diesel coolant heater for heated floors, so I'm considering a second tank at least for the heater to about draining the primary. Whether I get a small one that's a few gallons just for the heater or get a second, bigger one and rig it to dump into the primary is still a topic of conversation.

Your mention of saddle tank is actually very intriguing. I could hook the heater to the second tank, and have the bottom of that tank be around 1/4 of the primary tank. That would prevent being high and dry as far as fuel is concerned! I'm showing a bit of ignorance here, but could you touch on why the add on tank's top doesn't have to be exactly at the primary top? I'm assuming I only need to fill the primary and it'll fill the secondary from the bottom free flow tube, letting the air out of the cap at the top, but what stops the fuel from coming out of the add on cap in this scenario? Am I completely misunderstanding this idea?

 

[o1marc]

Gauge shows quarters of a tank. Weird that your last 1/4 is smaller than the first, though that means nothing. I always try and calculate a new vehicles tank by running the first tank dry to see where it actually runs out, when it actually hits zero. Not good to do on a diesel.On my truck I used to fill it up and I would get 50miles before it came down to the F, and then 50 miles each quarter till it hits E, and then 40 miles more before it ran out. I had the fuel pump replaced a couple years ago and it's totally changed. Now I get 120 miles before it hits F, and then it rapidly goes through the 4 quarters, but still at least goes past empty before running out.

 

[Booyah45828]

When using two tanks with a free flowing system. The system usually isn't free flowing enough that you can fill one side at the pump and it fills the other. The reason that is is for slosh and overflow.

You honestly want the tank tops to be even, but that's sometimes impossible unless you're using duplicate tanks. The top of the 2nd tank can be slightly below the primary tank's top, as long as the vent is at the same height and the cap seals. It cannot be above the primary top, otherwise, upon filling, you'll be forcing fuel out the primary's vent.

If you have vented caps, the tops have to be even.

 

[inhof009]

When using two tanks with a free flowing system. The system usually isn't free flowing enough that you can fill one side at the pump and it fills the other. The reason that is is for slosh and overflow.

You honestly want the tank tops to be even, but that's sometimes impossible unless you're using duplicate tanks. The top of the 2nd tank can be slightly below the primary tank's top, as long as the vent is at the same height and the cap seals. It cannot be above the primary top, otherwise, upon filling, you'll be forcing fuel out the primary's vent.

If you have vented caps, the tops have to be even.

Okay, so I'd basically only be able to fill it to the lowest cap/vent, hence why the tops should be approximately equal. That makes sense why only the bottoms need to be connected, at the cap will vent in air as the fuel is consumed and the level drops. That's very helpful, thank you!

I assume the connection at the bottom would have to be somewhat flexible so that it doesn't shatter on the road and cause a very expensive and unfortunate problem.

 

[Booyah45828]

They're usually nylon fuel hose, protected by a piece of angle iron attached to the frame. If you're off-roading or something then this wouldn't work. But for on road vehicles it's fine.