Fuel Tank Drain Help

I loaded a fuel tank drain tune and unhooked my return lines to drain my tank. It seemed like the car ran the tank dry a bit early. I put 10 gallons of E85 in it and it is showing full. Do you think it is possible that it didn't pump the passenger side of the tank completely empty? I have the fore hat and I know I hooked up the venture on the hat.

If it only took 10 gallons it was only 2/3 drained as the tank holds a little over 15 gallons when it’s empty.

If you have an E85 test kit drain a little out of the tank and see what the alc % is. Sounds like it didn't drain all of the gas out, though...

kcc0521 said:

I loaded a fuel tank drain tune and unhooked my return lines to drain my tank. It seemed like the car ran the tank dry a bit early. I put 10 gallons of E85 in it and it is showing full. Do you think it is possible that it didn't pump the passenger side of the tank completely empty? I have the fore hat and I know I hooked up the venture on the hat.

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Wouldn't you just undo the feed line before the filter/check valve?

I am going to drain some out and test it. I was curious if anyone else had an issue draining their tank.

I drained my tank out just fine with my fore system unhooking the return line

Did you figure anything out here? I’m switching mine over today and wondering if I have the same issue. It seems like there may be still ~5 gal in the tank. I jumped the Fore controller to run the pump dry. The car won’t start, so no fuel at the pump. But my gauge is still reading 1/4 tank.

Update: I unhooked the battery and hooked it back up and now the gauge is showing empty. It still doesn’t seem like enough pump gas came out, but maybe...?

I never checked it. I just ran a couple of tanks of e85 through it.

rts9364 said:

Wouldn't you just undo the feed line before the filter/check valve?

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That would get even less fuel out of the tank in this car.

You’re going to have trouble getting all the fuel out of this tank with a drain like that.

The fuel you are not getting out is stuck on the passenger side of the tank.

The fuel tank is divided into 2 sides by the driveshaft. The driveshaft hump is on the bottom of the tank, so fuel cannot level out in the tank unless the fuel level is higher than the hump. Fuel is free to slosh around over the hump, and when you fill the tank it will flow over from the driver side to the passenger side once the driver side is full.

The fuel pump rests on the driver side saddle of the tank. Built inside the stock fuel pump, and also inside the Fore fuel pump, is a Venturi pump that slowly transfers/recovers fuel from the passenger side to the driver side. That is what that hose is, inside the tank, that you have to disconnect when removing the pump assembly and reconnect when putting it back in.

The Venturi recovery action is a function of the fuel pressure on the feed side of the plumbing. If you were to try and drain the tank by opening the feed line instead of the return, there would be no fuel pressure for the Venturi recovery system (called a jet pump). So you would get nothing from the passenger side of the tank.

By draining from the return line, you are allowing the Venturi to still function during your drain procedure. However, the 450lph fuel pump(s) are out-running your jet pump. So you are draining the driver side faster than the jet pump can recover fuel from the passenger side. When the driver side gets empty, the system loses pressure, and the Venturi action dies, leaving fuel on the passenger side of your tank.

What’s the fix? It’s dynamic and will depend on your ability to understand what is happening inside the tank when you cannot see it with your own eyes. One option, you could drive the car until the tank is already pretty low and then take a hard right turn to slosh the remaining fuel to the driver side. Or you could take 1 step back for 2 steps forward... add a few gallons back into the tank after it goes “dry” when pumping out. What you add back will go on the driver side and allow the system to build pressure and activate the Venturi action. But probably the easiest thing you can do, is to just drain more slowly. Much more slowly, by using a much smaller drain outlet. The longer it takes to drain, the more time you have given the jet pump the pressure it needs to create a Venturi action and recover all the passenger side fuel.

As you can probably tell, it’s not super easy, but it’s not super complicated either.

Also, for anyone curious... this system is not unique to the s550. This is standard procedure on nearly every RWD car who’s fuel tank is forward of the rear axle. Japanese cars use a return side Venturi jet pump, American cars use the pressure side.

That was a really good explanation.

This is an interesting topic that no one talks much about! I looked into this a bit yesterday as I was working on draining 93 out of my car. I came to many of the same conclusions as phunk explained (great post!).

One question I still have is how does the level sender system work from both sides of the tank? From the workshop manual, there is a second module with level sensor/float on the passenger side. How do the two floats work together (e.g., if one side is empty and one side is partially full)?

In addition to making a hard right turn, I presume jacking one side of the car up would accomplish the same thing.

I drained mine yesterday and may have inadvertently emptied most of the passenger side when I was getting the car in the air. I only unhooked the feed at the filter and ran the pump dry, but it took all 15 gallons of E85 that I dumped in (I only had 15; it likely would have taken a bit more). For me, unhooking the return line is much more difficult. I can unhook the feed before the filter right by the frame rail, but the only option for unhooking the return is at the hat (I won't explain the obvious extra steps that adds to the process). Maybe in the future I will plumb a connection into the return line under the car that I could undo easily. For now I will play around with the options I have available. As we all know with E85 in a place that gets cold and a car that doesn't get driven much, I'm going to become familiar with fuel tank draining methods.

I would appreciate knowing more about how the level gauge system works to better use the data that I can see while draining (i.e., the gauge).

I have never explored the electrical circuit of the twin floats in the S550. I do know that in Nissans (z-car, gt-r, etc) the 2 floats are in a series circuit. The sum of resistance between the 2 potentiometers is the final value used by the level gauge. Adjust my terminology as necessary, I am not an electrical engineer. One interesting thing is that fuel has more gauge value on the pump side than it does on the passive side. This is done with a different calibration on the potentiometer on either side (different ohm reading at bottom and top). Again, I am talking Nissan, but this very likely applies to other makes. I believe that the purpose of doing this in the sensors, rather than just gauge calibration, is so that the gauge is able to determine if fuel is only on the passive side. By allowing the passive side sensor to read much lower ohm, I believe this means the gauge can see a resistance sum only possible by having fuel on just the passive side. Once it sees this condition for enough time to know it’s not going to fix itself with a little slosh, and that the car is now inoperable... it will drop the needle to totally empty.

And the point of that is... if you are taking a long left sweeper with a low fuel amount in the tank, you run risk of sloshing your remaining fuel to the passive side of the tank and entirely stalling the car, leaving you on the road side despite having up to a couple gallons in the tank. They want the gauge to fall to zero in this scenario to give you an indication of why the car will not run.

Ford probably does this also, which is why your tank read empty after a power cycle. Whether it was the power cycle itself, or just the time elapsed in this condition, I don’t know.

I know I originally said that you could not keep the jet pump active if you drain on the feed side. That is only part true though. You can drain from the feed side, so long as your drain is slow/small enough to allow the feed side to remain pressurized while doing it. The more pressure, the stronger the venturi action.

All makes sense. So based on all of this I am certain I left some amount of gas in the pass. side of my tank, but based on the 15 gallons I was able to put in afterwards, it could not have been a lot. (I'm assuming I sloshed some over to the LH side as I was lifting the car onto blocks.)

I may look at some way to rig up a necked-down outlet for the unhooked feed line for the next time I drain fuel.

I don't want to make a big deal out of draining fuel, but it is great to fully understand what is going on. Thanks again.