"Add additive to an almost empty tank" why?

[wemay]

The vehicle is off so the additive isn't coasing through the fuel system.
When you put gas in after the additive, the fuel disperses the additive and dilutes it anyway. So why would this be recommended.

Or am i missing something here?

 

[Warstud]

Because the pressure from the gas helps mix the two together.

 

[mehullica]

To help wash all the additive down into the tank instead of it coating the filler neck

 

[raytseng]

yea, apply some logic though and visualize it, it doesn't make sense.

Even a couple gallons will be sufficient to wash an additive down to the tank and mix it in, no practical difference compared to driving to fumes and adding 15gallons+.

Visualize a couple gallons, it's literally 2 milk jugs.

Then driving the car + your fuel pump return/jet system will mix whatever you have in your tank.

 

[Sunnyinhollister]

Originally Posted By: raytseng
Then driving the car + your fuel pump return/jet system will mix whatever you have in your tank.


A lot of cars have returnless fuel systems now. Putting the additive in the tank first then adding fuel helps to mix the additive.

 

[jaj]

My 2014 Mustang has a saddle tank over the driveshaft that fills from the driver's side. The returnless fuel pump setup drains the passenger side first, then drains the driver's side. If you put the additive in first, it ends up on both sides. If you put it in last, it ends up on the driver's side only, with twice the concentration.

 

[raytseng]

Originally Posted By: jaj
My 2014 Mustang has a saddle tank over the driveshaft that fills from the driver's side. The returnless fuel pump setup drains the passenger side first, then drains the driver's side. If you put the additive in first, it ends up on both sides. If you put it in last, it ends up on the driver's side only, with twice the concentration.



I will agree with you, no harm, but I am not convinced that there truly is a need.

Are you sure that one side of your tank drains completely first, then a magical pump switches on? This seems like bad engineering.

This is why i mentioned a jet system. The fuel pump systems I'm familiar with don't drain one side first, but use a venturi aka jet system that continuously siphons and balances both sides. So it will mix via that mechanism.

Even if you dump with at -2gallons, that last fuel is going to be the fuel at the top, that goes across the saddle so it's not like it all the fuel or additive is going to settle just t to one side.

If your imagining the additive doesn't "mix" and just sinks to the bottom of one side; that's not right either. If you follow that logic, any cold start is going to be 100% additive that has sunk to the bottom.

If we take the flip side, and the passenger side is a "reserve" tank; a person were to always fills at half tank, would have super stale fuel in their reserve tank that would never be used. Don't think that happens.

 

[Chris142]

Its sloshed around and mixed up by the time you get into 2nd gear

 

[wemay]

Originally Posted By: Chris142
Its sloshed around and mixed up by the time you get into 2nd gear


My point exactly!

 

[Garak]

It just sloshes around faster while you're filling. It certainly will mix just fine no matter what, as already pointed out. Heck, it'll probably mix reasonably well sitting in the garage overnight.

I like doing it at the station because there's always a garbage can in close proximity to the pumps!

 

[Olas]

It doesn't make any difference.

I used to be an advocate of additive first and fuel second, but if you actually think about what's happening it doesn't matter one bit.

Let's assume it's a sticky, viscous additive that you add AFTER the fuel;

It coats the filler neck and gradually seeps down, some gets into the tank via gravity and some doesn't. Then you fill up again, and it's all washed into the tank.

The only difference is timescale, ie how long the add. Takes to reach the tank, but it will always get there and it will always get mixed?

 

[wemay]

Originally Posted By: Garak
It just sloshes around faster while you're filling. It certainly will mix just fine no matter what, as already pointed out. Heck, it'll probably mix reasonably well sitting in the garage overnight.

I like doing it at the station because there's always a garbage can in close proximity to the pumps!

I do the same thing! I rarely allow my vehicles to get bellow a 1/4 tank and add the additive at the pump station.


@ Olas,

Thanks, that's what i thought too.

 

[jaj]

Originally Posted By: raytseng
...I will agree with you, no harm, but I am not convinced that there truly is a need.

Are you sure that one side of your tank drains completely first, then a magical pump switches on? This seems like bad engineering.

This is why i mentioned a jet system. The fuel pump systems I'm familiar with don't drain one side first, but use a venturi aka jet system that continuously siphons and balances both sides. So it will mix via that mechanism.

Even if you dump with at -2gallons, that last fuel is going to be the fuel at the top, that goes across the saddle so it's not like it all the fuel or additive is going to settle just t to one side.

If your imagining the additive doesn't "mix" and just sinks to the bottom of one side; that's not right either. If you follow that logic, any cold start is going to be 100% additive that has sunk to the bottom.

If we take the flip side, and the passenger side is a "reserve" tank; a person were to always fills at half tank, would have super stale fuel in their reserve tank that would never be used. Don't think that happens.


I don't know how Ford does it, but they do. My BMW had the jet pump that used returned fuel to pump the off-side, but Ford has a returnless system so it's not obvious how they make it work. However, I know for sure that they do because the tank has two fuel gauge senders, one on each side of the saddle. You can put the dash into "diagnostic mode" and read each side separately. It really does drain the pax side first. Ford's factory service manual says it works this way, by the way. I didn't quite believe it myself, so I checked it.

A couple of years ago I was running a supercharger and doing a bunch of data logging. I wanted to be sure that when I changed octane levels that I wasn't running two half-tanks with different octane ratings. So, I tracked the two sides of the saddle to see if one actually emptied first, or if they mixed. Sure enough, there's "fuel right" and "fuel left". "fuel right" gets down to pretty close to zero before "fuel left" starts to drop. On that basis I decided I had to be at or below 1/4 tank before I filled up so I could count on having the same octane throughout.

 

[Shannow]

If I forget to add before a fill up, I'll do it at home when the car won't be run for a while rather than on the forecourt.

 

[raytseng]

Originally Posted By: Shannow
If I forget to add before a fill up, I'll do it at home when the car won't be run for a while rather than on the forecourt.


I also won't wait another tank, but instead of putting it in at home, i'd just go back to the station and do a 1 or 2 gallon fillup.

Originally Posted By: jaj


I don't know how Ford does it, but they do. My BMW had the jet pump that used returned fuel to pump the off-side, but Ford has a returnless system so it's not obvious how they make it work. However, I know for sure that they do because the tank has two fuel gauge senders, one on each side of the saddle. You can put the dash into "diagnostic mode" and read each side separately. It really does drain the pax side first. Ford's factory service manual says it works this way, by the way. I didn't quite believe it myself, so I checked it.

A couple of years ago I was running a supercharger and doing a bunch of data logging. I wanted to be sure that when I changed octane levels that I wasn't running two half-tanks with different octane ratings. So, I tracked the two sides of the saddle to see if one actually emptied first, or if they mixed. Sure enough, there's "fuel right" and "fuel left". "fuel right" gets down to pretty close to zero before "fuel left" starts to drop. On that basis I decided I had to be at or below 1/4 tank before I filled up so I could count on having the same octane throughout.

Fair enough. It is a unique system to have.

But if you were to fill to full, the 2 sides would have a liquid connection when full right? And I think that connection and the driving that happens until it gets below the saddle level would be enough of a connection to mix the fluid within say 50 sloshes back and forth.

It would seem the issue would only really be of concern if you do not fill to full and don't have the liquid connection over the top.