I've always heard two things about fuel tanks:

1. Don't let them get really low, because all the sediment and gunk at the bottom of the tank gets sucked into the engine.

2. The gasoline (petrol for you Euros...) also acts as a coolant for the fuel pump, and if the tank gets low then apparently the fuel pump is no longer bathing in the gasoline...

Any truth to these? Does the X-Type fuel pump actually use the gasoline in the tank as a coolant?

Thanks,

K

 

Kevin, as for question #1 (gunk issue), this is true if you have a steel gas tank. The tank will corrode on the inside due to a finite amount of water found in gasoline. The corrosion will settle to the bottom of the tank and if you continuously run the tank low, you are increasing the chance of sucking up this stuff. Granted, with the fuel pickup on the bottom of the tank, you are likely to suck it up anyways. With the newer tanks, as long as you are getting gasoline from a reputable station, this won't be an issue due to improvements in gasoline quality and all modern day cars using a plastic fuel tank.

As for question #2, yes, the gasoline in the tank is what cools the pump, but not like you think. The gasoline on the outside of the pump is simply there. It doesn't really provide any cooling per se. Does it help, of course. But, the main cooling is to the bearings on the inside which are cooled by the gasoline flowing through the pump. So, as long as you don't run the tank empty and then keep the key in the RUN position, you are fine. If the pump was cooled by the outside gasoline, you wouldn't be able to run the tank down beyond half a tank since you would start uncovering the pump with how it is mounted inside the car. Keep in mind that the fuel pump also uses the gasoline as a lubricant for the bearings too. So, if you run the tank empty and keep the pump running, then you are trashing the bearings since they are not being lubricated.

 

Thanks, Thermo -

Glad to have the real story!

K

 

I respectfully disagree

 

Brutal, I know a little bit about fuel system. Like to know why you disagree with Thermo as I agree with what he said. Temperature of the fuel in fuel tank can get very hot, as much as 125degF so cool liquid wicking away heat is not it. But it does provide lubricity to pump internals.

 

Im not dissagreeing on the lubrication. First the fuel helps to keep the pump cooler. 2nd the tanks are no longer a return system. And this keeps the fuel cooler since it doesnt go to the hot engine and then back.less parts and better fuel control with pulse width modulated pumps.(also helps them to run cooler)
ive been in many fuel tanks(obviously) and crud in the bottom is in the bottom wether full of fuel or 1/4 tank or empty
a fuel pump gets pretty damn hot when running and heat is always an enemy for a motor. Another reason pumps are in the tank is packaging. A smaller more efficient pump can be used if its sitting in fuel with a "no lift" setup. Also lessnoise since the fuel dampens the noise instead of a rigid frame mounts. ( I always know when the race pump is on )If you have an external pump, it has to be much larger and even then theyre rated by how far away they can be from the tank and the lift theyre capable of(this is true of all pump, read info on water pumps for wells)
the fuel pump i have on the race car is ext. Mounted no further than 18" from tank and as low as possible in relation to the tank to minimise the lift is has to endure.
Even on the new direct injection 5l jag that uses 2500psi. Those pumps are mounted on the engine, but require an in tank pump to prime (no lift) those pumps, funny thing is, if the front pumps fail the engine will still run on the in tank and can be driven, but if the rear pump fails the engine dies and wont start.
Another reason for intank is that(if you remember back that far) in days of carbs engines and pumps mounted on the engine and runs of fuel lines running from rear to front that were not pressurised, this led to fuel percolation in the lines and carbs. (vapor lock) by having an in tank or rear mounted tank that keeps the fuel lines pressurised vapor lock is just about totally eliminated.
This made me also remember my suburban fuel system i modifyed as well. When i switched it to an accel super ram injection setup, the inline external pump is pretty parge and has large heat sink fins to keep it cool. Accel told me if i wanted to switch to an in tank setup up would be more work but better performing and longer lasting for all the above mentioned reasons. Smaller pump, cooler running, better performing, even cheaper pump. Just more work to install. A hipo in tank walbro pump is $100 all day long, ext pumps same capabilties are alot more

 

I can show evidence to give credence to one of your notions....
For racing events, instead of using a fuel cell, I used to constantly run my first x-type with less than a quarter-tank to reduce weight.

I replaced fuel pump TWICE, which, from what i've seen is NOT common. Eventually put in a Walbro with my ECU re-tune via CP-E performance.

 

this is why I recommend not letting fuel not going below1/4 tank. I have put in more fuel pumps with less than 1/3 of tank, than more. Makes it easier for me to not have to drain fuel, or much less to drain

 

I still don't see what you disagreed with, but I don't disagree with your follow up : )

 

Just that fuel doesnt keep pump cooler(i hold them to transfere fuel with a setup i made, and they get hot quickly and fail often as well this way) and that empty or full the pickup gets trash, they have a fine mesh sock on them and of all the pumps i ve taken out, none really have much of anything on the socks thats large. Now the fuel filters upstream take out alot of what i would call finer dirt. I like to dump the filtersupside down to see just how dirty they are, and theyre pretty bad after 60k. On testing pumps that fail ive only had a couple that were seized and that "might" be bushings or vanes. Most are open circuit or short to ground which is electrical. Ive never really cared why, so never bother to disect. I meen if the engine on your plane quits your not as concerned about reasons why so much as getting it working again
well your get my point, but i guess I do agree with the running out of fuel issue on just a different reason. and thats pump cavitation with quickly damages pumps. Think of it as detonation for a pump. Even though tanks have baffles and are in plastic cans, conering, braking and acceleration can lead to air sucked into the pump momentarily causeing cavitation. The pump momentarily speeds up with no load(air offering no resistance) only to then meet Mr. Fuel again and high resistance. You would probobly never notice this driving, but the pump sure does, and its been an issue in oil pumps for decades both in stock and racing configurations

 

I've never understood the logic behind this one nor have I observed any difference in practice, and call BS on it accordingly.

 

I cangt think of a fluid that is worse as a lubricant than gas! It is a fantastic lubricant remover....
I sorta understand the initial layman's acceptance of this... (is is derived from oil they say) but sheesh, common sense says that gasoline does not lubricate.

 

Brutal, I get it now. Understood.

Sport30, even water can be lubricant if used as hydrodynamic buffer.

 

And, science says different.

Anyways, there is nothing you can do about. If you try greasing the pump, just as you said, the gasoline is just going to wash it away. And your downstream fuel filter is just going to *love* that grease.