Fuel pump cooling, the end all be all argument.

I done went and got myself into an internet argument and I'm too stubborn to drop it.

Conversation started with a question by a guy on the NC Miata page of the Book of Faces: "So a Google search says that the NC1 fuel tank holds 12 gallons. (One search says 12.7)

So when my needle is on the empty mark, I still have 2 gallons (or 16%) of fuel left in the tank?

Is this the same in every NC or is my fuel gauge not working right?"

My response was the: "The reason that the tank is not "empty" at empty is because the fuel cools the pump during operation and the more you run it below E the hotter the pump gets and the greater the failure rate becomes. Personally I suggest that 1/4 be your new E."

To which I brought every shiny happy person to the internet.

Someone posted "I pulled a Toyota pump out and it ran 2 months before failing!" Really? Why did you pull it out then? 

Then someone called bullshlitz and said "The fuel cools it by going through it" which I never said it didn't, I just stated that there is a false empty to protect the pump from overheating, i did not not include stopping additional heat from starvation as the reason for leaving fuel in. Then I made the fourth mistake of... continuing: "So essentially every car performs the same as a Hellcat? No, that's not how it works.

I am generalizing in my statement about the fuel keeping the pump cool. Some cool by internal flow and others through submersion. Some OE units are great at it and others are not. Go ask a GM owner away there is a door cut out of his bed right above the fuel pump.

With different manufacturers come different specifications and different designs. Some of the fuel pumps are seen as a design priority and last forever. Toyotas are generally Denso pumps and seem to last forever. Ford pumps are often manufactured to a specific economic point and as such are frequently a little more prone to failure. The funny thing about that statement is that for a very long time Ford cultivated it's supplier network and the supplier network of it's subsidiaries like Aston Martin, Jaguar, Land Rover and Mazda. Why does this matter. Well because there's a pretty good chance that your OE Miata pump is no different than that of a Focus or Fusion or Mazda 3 or any other pump that was likely built to a lower quality point than other manufacturers.

IN GENERAL it is a good idea to keep your tank above 1/4 because the likelihood that you have a pump cooled primarily by external thermal interaction is actually very high." I need to not be on the lines at break. 

So all my Subject Matter Experts, can I get condensed ruling as to WHY E does not mean actual empty? 

Don't know about cooling, but my Miata fuel pump failed after being uncovered for a couple of years. 
 

But I'd be pretty confident that if E meant empty, it would infuriate a ton of customers who get stranded on the road. Much like why speedometers read a little high. 

The argument I keep is "to keep the bucket full". There is always a reserve amount for that to happen. It varies but I figure about a gal at most.

If you don't have a bucket but a "yo idiot fill the tank" light, you are closer to 2-3 gals depending on tank size.

I've ran a bucket dry ONCE. Within a month, I was replacing the fuel pump even with a 1/4 tank.

I always thought that "E doesn't mean E" as that would otherwise keep AAA and the like very busy carting cans of fuel around.

I've always thought that so you buy gas before you run out. Kinda like motorcycles and reserve.

On both my Miata and MINI, I have about 2 gallons of reserve when the idiot light comes on. The computer will give me the remaining range, too. I tested it one time, on my last MINI. Kept driving well after the light came on and watched the range drop to 3 miles when I finally made it to the gas station. I put over 12 gallons in, so I trust the accuracy.

Idk about fuel pump cooling, but if  E meant empty, aka 0 fuel, your car would've started to sputter before it hit E and would shut off due to fuel starvation at E.

I imagine there are plenty of other systems and components that are being protected. As well as the E being a warning that you're gonna be stranded if you don't fill up.

Edit:

Quick goog search linked this, and I would trust them 100%

https://totalenergies.id/en/article/tips-advice/what-does-e-my-fuel-gauge-mean#

Also this link which talks about fuel cooling the fuel pump

https://shop.advanceautoparts.com/r/car-culture/community/how-far-can-you-drive-on-e

In reply to alfadriver :

Damnit, I was hoping that you would be the one with the science. 

I know that I didn't simply make up the "fuel cools the pump" idea. I can not pin point where I heard it however. 

I don't think the reason for E not meaning completely empty is for fuel pump cooling because carbed cars with mechanical fuel pumps or inline electric pumps also had some "reserve" fuel. 2 gallons of reserve fuel would be a lot, typically it's 1 gallon or less.

As for fuel submersion being needed to cool the pump, yes an in-tank pump will run a lot hotter if it isn't submerged and that's not good for the pump's longevity. Inline pumps are made to be cooled by fuel going through them, in-tank not so much...

In reply to QuasiMofo (John Brown) :

You gotta out Facebook them. Tell them that your friend is a fuel systems engineer at an OEM that you can't make for obvious reasons and they told you that the fuel cools the pump. 

In reply to Stampie :

So you're saying it's a conspiracy by Big Fuel?

In reply to QuasiMofo (John Brown) :

Outside of my sme. 

In reply to alfadriver :

I was hoping SME-adjacent

Stampie said:

In reply to QuasiMofo (John Brown) :

You gotta out Facebook them. Tell them that your friend is a fuel systems engineer at an OEM that you can't make for obvious reasons and they told you that the fuel cools the pump. 

That's why I will never be King and you will always be! 

I'm going to speculate that it's the fuel flowing through it.  I can think of two examples of why that might be the case.

1) Many vehicles have an electric pump mounted to the frame

2) My 96 Impala had an in-tank pump, but it was only about half-way down.  There was a small tube down to the filter/sock, so it would have been partially uncovered any time you were below about half a tank.

sounds like we found Quasi's three point belt

GameboyRMH said:

I don't think the reason for E not meaning completely empty is for fuel pump cooling because carbed cars with mechanical fuel pumps or inline electric pumps also had some "reserve" fuel. 2 gallons of reserve fuel would be a lot, typically it's 1 gallon or less.

As for fuel submersion being needed to cool the pump, yes an in-tank pump will run a lot hotter if it isn't submerged and that's not good for the pump's longevity. Inline pumps are made to be cooled by fuel going through them, in-tank not so much...

I've always figured it was 10%.

I've had 20 gallon tanks with about 2 gallons in them 

I think the variance on E relative to empty fuel is more up to imprecise gauges and the variation not mattering tremendously. As for cooling, I think your list should be cooling via submersion or cooling via airflow, I wouldn't expect fuel flowing through the pump to cool it, especially on an old return style system that pumped fuel to the rail first.

I have never pulled a pump that is in tank that would be substantially submerged bellow 1/3 tank.  I'm not doubting that fuel cooling of the pump is a thing, however I personally suspect damage is more due to low suction head increasing cavitation.   Cavitation can damage both the impellers and the motor bearings.  Air entrainment probably doesn't help but generally I would thing the pumps would handle non-condensible fluids until they literally couldn't hold prime.  Cavitation would be when fuel on the suction side of the impeller vaporizes, then once it it on the pressure side it condenses back to liquid.  Pumps don't like that.  Impeller pumps aren't a fan, positive displacement pumps REALLY don't like it.  I can't imagine return less puls width pumps like it either. 

Low suction head pressure increases the likelihood of vaporization in the pump inlet.  Low fuel levels would decrease the head pressure.   The sump is probably partially designed around preventing low suction head pressure and driving past E would drain the sump beyond design.  It's definitely possible to run all the way to actual empty but I really doubt the pump enjoys it.  But not likely for cooling.  

That's my thoughts on it but I am not a fuel pump expert.  

In reply to nocones :

Well with you being the first person I've heard use the word "cavitation" outside of a Tom Clancy novel, I'd say you sure *seem* like an expert

I run my minivan tank down to the light often. A side effect of driving a berk-ton and on the same roads over and over (and over and over...) again is I know where I stop for gas and food.  It's rare when I don't run the tank down to the light.  When I fill the tank it usually takes around 17 gallons, give or take.  I've been doing this for 112K miles.  I did run it down to no-start-empty once by mistake about 70K miles ago. I suppose Chrysler expects some occasional stupidity from minivan owners and built in some robustness. 

I have heard the fuel cooling theory before. Definitely seems plausible and may have been the case in the early days of in-tank fuel pumps.  I'm not so sure these days. 

QuasiMofo (John Brown) said:

In reply to alfadriver :

I was hoping SME-adjacent

Fair, but too detailed to be part of my adjacent knowledge. I know that a car can "leak" something like 50mg of hc over 3 days when it's hot.  And know what kind of pressure that is used for supply. But not the detail of cooling or lifespan details for the pump. 

I got nothing.  I usually fill up around half a tank because I never know when the phone is going to ring and I have to drive 200 miles on a minutes notice.   Also, Nocones throwing down strong pump knowledge and P3PPY with a savage Hunt for Red October reference.  

I thought it was just accepted that running 90s GM stuff down to empty could have a bad effect on the fuel pump due to lack of cooling.  My neighbor ran his '96 Caddy dry and the fuel pump quit and that just reinforced what I thought.  Coincidence?  Now I wonder if I had just bought an urban legend?  I run my Mazdas down to fumes on a regular basis with no negative effect.  So far...

Related story - we left Chuckwalla in my Mazda3 and after getting on the on ramp noticed that the remaining fuel display was zeroed.  Not sure when it went to zero, might have been on the track a while back.  I think we put another forty miles on in the desert before we hit an open station so there's at least a gallon and a half or so.  Probably more.