Adding additive to full tank

Joined
Oct 25, 2008
Messages
42
Location
Maryland, USA
Several times I have circumstances when I would like to put an additive into the tank when the tank was already filled [by wife for instance or not available at time of fill up]. Most additive directions say add product then fill tank. I assume this is because they want the fuel to wash additive from inlet into the tank or allow fuel flow to help distribute additive. Is there any problem with adding additive to a full tank then using a little fuel from the lawnmower can to wash additive down then moving vehicle to mix it?
 
Joined
Jul 11, 2014
Messages
3,447
Location
Paradise of Florida
No problem that way either. Or, just wait a couple of commuting days, and go to the gas station, add additive, and top off tank for that 1-2 gallons that was used.
 
Last edited:
Joined
Apr 25, 2017
Messages
8,440
Location
Ohio
It's for dilution reasons. 4 ounces of product in 1 gallon of fuel is a LOT stronger than 4 ounces into 12, 16, 20 gallons.

Some additives say to add to an almost empty tank, some say add to tank just before refueling, and others say add to a full tank.
 
Joined
Jan 22, 2011
Messages
7,909
Several times I have circumstances when I would like to put an additive into the tank when the tank was already filled [by wife for instance or not available at time of fill up]. Most additive directions say add product then fill tank. I assume this is because they want the fuel to wash additive from inlet into the tank or allow fuel flow to help distribute additive. Is there any problem with adding additive to a full tank then using a little fuel from the lawnmower can to wash additive down then moving vehicle to mix it?
Wait till you fill up at the pump next time, that way it gets mixed up well within the tank.
 
Joined
Jun 19, 2020
Messages
1,438
Location
Iowa
FWIW I’ve added additives both before fill ups and unfortunately after fill ups. Usually the fuel will slosh around enough to mix the additive in. On top of that your car may have a fuel return line adding fuel back to the tank. This should mix it up even more. My Cummins Ram has a fuel return line off the high pressure pump.

IMO if you can add it before that’s better but after will work as well.

Just my $0.02
 
Joined
Jul 14, 2020
Messages
875
Location
California
The instructions to add to a tank before filling up are simply to aid mixing of the additive with the total fuel load. If you add to a full tank it will get thoroughly mixed eventually and shouldn't take long. Go out and do a few donuts if you are worried about it, that should mix the additive with the fuel in short order. 🤪
 
Joined
Aug 20, 2003
Messages
1,641
Location
ottawa
Several times I have circumstances when I would like to put an additive into the tank when the tank was already filled [by wife for instance or not available at time of fill up]. Most additive directions say add product then fill tank. I assume this is because they want the fuel to wash additive from inlet into the tank or allow fuel flow to help distribute additive. Is there any problem with adding additive to a full tank then using a little fuel from the lawnmower can to wash additive down then moving vehicle to mix it?
It does not matter how you add it...osmosis will do the rest.
Especially if the vehicle sits overnight.
 

CleanSump

Site Donor 2022
Joined
Dec 13, 2019
Messages
1,046
Vehicle movement shakes it up, vibration mixes it, and the fuel return line further moves it around and mixes it.
 
Joined
May 6, 2005
Messages
8,899
Location
San Francisco Bay Area
I've poured an additive (usually Redline SI-1) into gasoline. I could clearly see that it wasn't quickly mixing. It requires some sort of agitation in order to properly mix. But if I shook the bottle I could see it evenly dispersed within seconds.

This is pretty common with miscible liquids, especially if they're of different weights. This demo is of potassium permanganate dropped into water, where it mostly sinks to the bottom. Even after 5 hours it's clearly mostly at the bottom.



However, in a moving vehicle, it's going to slosh the fuel around enough that it will eventually mix with an additive. However, ideally one would pour it in first and have the agitation from the fuel mix it more or less uniformly rather quickly.
 
  • Like
Reactions: a5m
Joined
Aug 20, 2003
Messages
1,641
Location
ottawa
I've poured an additive (usually Redline SI-1) into gasoline. I could clearly see that it wasn't quickly mixing. It requires some sort of agitation in order to properly mix. But if I shook the bottle I could see it evenly dispersed within seconds.

This is pretty common with miscible liquids, especially if they're of different weights. This demo is of potassium permanganate dropped into water, where it mostly sinks to the bottom. Even after 5 hours it's clearly mostly at the bottom.



However, in a moving vehicle, it's going to slosh the fuel around enough that it will eventually mix with an additive. However, ideally one would pour it in first and have the agitation from the fuel mix it more or less uniformly rather quickly.

With enough time , over night, osmosis will take care of any mixing.
Gotta love physics.
No absolute need to fill up after adding any additive.
 
Top