Just how bad is it to always run the fuel tank down to near empty on these cars?

[romnation]

Most of my driving is in the city and I only get about 18mpg out of my 135i. As such I can only get maybe 230 or 240 miles out of the absurdly small gas tank. Gas is also extremely overpriced in my city for premium, so I prefer to make a drive out to the burbs and get it, while also saving some money on groceries or whatever at the same time. Currently I consistently run the gas tank down to 20 or 30 miles distance to empty. A couple times I've actually had it go to "----" but I really try to avoid that. (---- is equivalent to a half gallon being left in the tank)

[kprocivic]

most newer cars fuel pumps are inside the tank. gas keeps them from overheating potentially. low gas low cooling. low cooling premature failure.

[ShocknAwe]

As long as you keep the stock fuel bucketed type of LPFP, it's not a big deal.

[chris_flies]

I ran to 0 miles with no issues...that was f***in' hairy!

[Zombie_Head]

Originally Posted by chris_flies

I ran to 0 miles with no issues...that was f***in' hairy!

I was about to do this driving back home after i picked up my 125i.

But i was unfamiliar with how much reserve i had, only the OBC stated 80 kms left. I wanted to empty the tank since i just knew the previous owner ran 95 fuel (91 i believe for you Americans) judging by the tire brand he fitted.

I bit the bullet at 60 kms range indicated, filled her up with Shell V-Power Racing 100 (96 for you).

[iminhell1]

Every answer will be an absolute guess.


Maybe it's better to figure your cost per mile instead of mile per gallon. You may find that the few cents higher local to you is actually cheaper.
Also if range is a concern maybe you should try to plan trips better. A route with the most stops or slow downs will use more fuel than one that may be at a slower speed but has fewer stops/slow downs.

- Cost Per Mile
- Route planning

[tock172]

I've killed at least one (albeit old) fuel pump running the tank too low repeatedly in another BMW. Ever since being stranded on a blind curve in a mountainous area because of a failing pump, I refuel all of my cars at the halfway point. I've also painfully stopped myself from topping them off as well, to avoid a potential charcoal canister or evaporator failure.

[mackeroni]

Originally Posted by tock172

I've killed at least one (albeit old) fuel pump running the tank too low repeatedly in another BMW. Ever since being stranded on a blind curve in a mountainous area because of a failing pump, I refuel all of my cars at the halfway point. I've also painfully stopped myself from topping them off as well, to avoid a potential charcoal canister or evaporator failure.

Are you really stopping every 150 miles to put seven gallons in your tank? That would mean I would have to stop five times enroute to my folks home just 650 miles away instead of just the two stops that I get away with today. Ain't nobody got time for that!

I am in complete agreement on avoiding the overfill, by the way.

Peace... Mack

[JimVonBaden]

This is practically an urban legend. The level of the tank doesn't matter. The fuel running through the pump cools it. Not the fuel around the pump.

[tock172]

Originally Posted by mackeroni

Are you really stopping every 150 miles to put seven gallons in your tank? That would mean I would have to stop five times enroute to my folks home just 650 miles away instead of just the two stops that I get away with today. Ain't nobody got time for that!

I am in complete agreement on avoiding the overfill, by the way.

Peace... Mack

I don't employ the same practice on long road trips, but I will plan out a route that involves me filling the tank at or above the 1/4 mark, which is right where the light comes on in the 135i. Too soon for the light if you ask me.

Also, rarely have I driven 150 miles before I reach half a tank. My fuel economy is admittedly subpar thanks to my lead foot.

My 135i is the pleasure vehicle. Both my Civic and E30 have larger fuel tanks and I drive both of those more often. I'm lways at the gas station anyway.

Originally Posted by JimVonBaden

This is practically an urban legend. The level of the tank doesn't matter. The fuel running through the pump cools it. Not the fuel around the pump.

Right, and what happens when that flow is interrupted? I am not a mechanic, but most of the wrenches that have serviced my vehicles over the years have offered a consistent consensus that it's better to not run the tank low. What do they have to gain from mitigating a potential fuel pump job?

[gjm120]

How does filling up to max leads to problems with the charcoal canister and evaporator? I always top off when stopping for fuel.

[tock172]

Originally Posted by gjm120

How does filling up to max leads to problems with the charcoal canister and evaporator? I always top off when stopping for fuel.

I did for years as well, but it's actually not a good idea as it can lead to excess fuel in the evap lines and then the charcoal canister itself. They're only designed to take so much over their service life and every time you click the pump on for more fuel when it's already stopped, you're increasing the likelihood that they components will have to deal with excess fuel. This was never a concern of mine until last year when a friend had a costly repair relating to one of the components in his Z4 M.

[JimVonBaden]

Originally Posted by tock172

Right, and what happens when that flow is interrupted? I am not a mechanic, but most of the wrenches that have serviced my vehicles over the years have offered a consistent consensus that it's better to not run the tank low. What do they have to gain from mitigating a potential fuel pump job?

They are not gaining anything, just passing on old wives tales. Long ago there was some validity to it, but it is no longer an issue. Like 3000 mile oil changes, it is just not necessary anymore.

[gjm120]

Originally Posted by tock172

I did for years as well, but it's actually not a good idea as it can lead to excess fuel in the evap lines and then the charcoal canister itself. They're only designed to take so much over their service life and every time you click the pump on for more fuel when it's already stopped, you're increasing the likelihood that they components will have to deal with excess fuel. This was never a concern of mine until last year when a friend had a costly repair relating to one of the components in his Z4 M.

I'll change my habits for sure. Thanks. I had no idea.

[JimVonBaden]

Took it for a drive and discovered the joy of "Sport Mode". It really wakes the NA up!

[zx10guy]

Jim is correct about the fuel running through the pump as the main method of cooling it. That said when you run the fuel level down in the tank, there is a point where on cornering, braking, or acceleration where the pump can run dry. It all depends on if there are any baffles in the tank and how the baffles are configured. The fuel pump will have a pickup which looks like a sock. When you run the tank down, the fuel is sloshing around in the tank and if the fuel level is pretty low can cause fuel starvation depending on what the car is doing.

As for the evap system, overfilling is bad because the evap system is only designed to scrub gas vapors which are naturally generated by gas. That's why the entire fuel system is sealed and the DME will monitor for this. When there is vacuum leak fault, typically the first recommendation is to ensure the gas cap is closed tightly or to inspect the gas cap for any wear requiring replacement. With that said, on the purge cycle initiated by the DME if the gas tank is overfilled, the vacuum will draw liquid gas instead of vapor through the evap vacuum line. The charcoal canister is only designed to scrub gas vapor and if liquid gas is drawn into the canister, this will cause failure in the system.

With the above said, I'm probably playing with fire. Because of all the stop and go traffic I have to deal with, I try to minimize the number of trips I do to the gas station. I typically do 2 extra clicks after the initial click off when filling up. So far hasn't caused any issues....been doing it for 4 years now.

[chris_flies]

Originally Posted by JimVonBaden

Took it for a drive and discovered the joy of "Sport Mode". It really wakes the NA up!

For me, Sport Mode is known as shift faster, release the clutch more aggressively, and move my right foot quicker...

[tock172]

Greatly the appreciate the explanation zx10guy. I actually just overfilled my 135i pretty bad recently. No adverse affects other than being annoyed with myself, but I still don't like it.

[Gangplank]

When the low fuel light comes on is adjustable via the Carly app.

I use the low fuel light and Odometer. Light on, less than 30 miles I stop. I've run it pretty low a few times due to poor timing.

[a4nut]

My neighbor owns an Indy BMW shop and always fills up shortly after he gets below 1/4 tank. He says he's seen countless fuel pumps that have died with metal shavings in them due to customers running them low on fuel. Which makes sense if the fuel is the cooling method, and when you consistently run it low, you will consistently put more stress and heat on it due to poor cooling.

[Tommm]

I have a relative in Michigan who was an engineer at the big three in the warranty department. He suggested I fill the tank when it hits half full because of the cooling issue mentioned above. Based on warranty work he has seen it is not a wives tale. .

I usually do it around a quarter because I drive too many miles each day, and I don't want to fill it up every other day. Twice a week is enough.

[chris_flies]

I normally fill up around the 1/4 mark, because that seems to always be where it's at when it's convenient to refill...

I've only run it lower when I had that one desperate situation I mentioned above, and on one HPDE run (no starvation issues at <1/8th).