Retrofitting 16.6 Gallon tank (late model) in early model chassis

[element6]

I've got a TRE340 to pop in there. My 14.5 gallon tank is apparently rusting like crazy and clogging my fuel system so I need to change it out. The main feed hose and return hose are also the OEM ones and kind of scare me.

No point in getting it cleaned and sealed and then repaint the outside of it when a new 16.6 tank costs $250/shipped.

So what differences am I dealing with here? Obviously I have another hole for a fuel pump/gauge sender/return line.. Can I block it off or is it necessary to move fuel between the two sides of the larger tank?

Anything else I should know when swapping between a 14.5 and 16.6 tank?

TIA

[jlevie]

You could block off the second fuel sensor port, but then you won't get accurate fuel level indications except when the tank is full or empty. A big difference between the tanks is that the early model has a crossover tube to equalize fuel between the side of the tank. The late model tank has an internal siphon operated by returning fuel for that purpose. So the amount of fuel in each side of the tank can be different. The best solution is to install the second level sensor, modify the sensor wiring for the second sensor, and change the fuel gauge in the cluster to a late model gauge. Otherwise, swapping in the later tank is pretty straightforward.

[element6]

So in order to get an accurate fuel level reading, I need two fuel level sender units?

Or do I just need one fuel level sender on the drivers side of the tank?

If I do need two, is there a way to wire it up using my existing wiring harness?

Is it still correct to use the same type of in tank fuel pump assembly as I already have? The one I am currently using has a larger single feed line and a smaller single return line.

[element6]

ahh ok.. two fuel level senders..

The stand alone: 16111176708 and the one that fits into the area where the fuel pump is mounted: 16141152266.

As for the wiring though, I'm still not sure how I would wire them up.. can I wire them in parallel or will that create issues?

[jlevie]

You will have to modify the existing level sensor wiring. The sensors are wired in series. Take a look at the ETM (http://wedophones.com/BMWManualsLead.htm) for how it should be. You can use the transfer pump housing from the old tank if you replace the pump with a high pressure pump. You'll have to plug the return line fitting on that housing as fuel return must go to the fitting on side of the late model tank.

[element6]

Thank you Jim! My saving grace. Hopefully I'll get the new tank in there next weekend.

Just an FYI for anyone else referencing this thread.. If you decide to use the original transfer pump housing like I am, you will need to chop about 1/3" off of the bottom of the metal feed line and redo the shaping on the lip of the line. This applies to the TRE340 and probably any of the 255lph walbros too. If you don't do this, you will not be able to get the pump all the way down into the tank and screw the assembly in at the top.

On mine, the plastic lip of the pumps feed line hits the metal feed line of the assembly before I can get it all the way down in the tank.

Don't take too much off of the feed line or you will be "empty" before you are actually empty. Just make it so that when you have the pump attached, the bottom of the strainer is level with the bottom of the return line.

[bmwstephen]

so if im reading this correctly. the late model fuel tanks use 2 sensors?

[Jean]

yes

[element6]

Yeah, number 9 in the pic.

http://realoem.com/bmw/showparts.do?model=1113&mospid=47309&btnr=16_0039&hg=16&fg=05

I grabbed the second sensor from a junkyard today for $10.

[bmwstephen]

is it possible that an early sensor can fit in a late model tank? Not that I'm trying to get one in my late model but I suspect the PO changed the fuel sensor at one point in my car but installed the incorrect version since I'm getting reading errors.

[jlevie]

The left side sensor is completely different from the right side sensor and is only used on late model tanks. The right side sensor is the same for early and late tanks.

[bmwstephen]

The left side sensor is completely different from the right side sensor and is only used on late model tanks. The right side sensor is the same for early and late tanks.

the right side being the drivers side? because that one appears to have been tampered with upon inspection

[element6]

no, left side would be the drivers side. Usually when referencing left and right with parts placement on a vehicle you would position your vantage point at the rear of the car looking towards the front.

The drivers side of the late model cars have a secondary level sender unit because the fuel level in the tank is not equalized the same as in an early model e30s tank. The fuel level sender unit on the passengers side is an interchangeable part on all e30s I have seen thus far.

[bmwstephen]

no, left side would be the drivers side. Usually when referencing left and right with parts placement on a vehicle you would position your vantage point at the rear of the car looking towards the front.

The drivers side of the late model cars have a secondary level sender unit because the fuel level in the tank is not equalized the same as in an early model e30s tank. The fuel level sender unit on the passengers side is an interchangeable part on all e30s I have seen thus far.

ahh i see. is there any way to test out the sensors? I am still having trouble visualizing the 2 tier system with the late model tanks (siphoning, dual sensors, etc) utilize

The problem I face right now is that when my gas neeedle is on the red mark, I know the tank still has 6 gallons left (because filling up at the gas station stops at 10 gallons) despite the indicator suggesting I have a lot less. However, it has no trouble recognizing when the tank is full. I suspect one of the sensors are faulty but am confused as to which one it owuld be.

[element6]

I will get back to this later.. but the first thing off the top of my head that springs into mind is that when one of those level sensors goes bad it is usually because they have dirt stuck somewhere inside them that is preventing the moving level sensor from going up and down its measurement guide on the inside of the track.

So for example, pull one of the level sensors out and let it drain. When it's drained, hold it vertically and flip it upside down and then rightside up. If you can feel or hear the level sensor getting stuck at certain positions on its track on the inside or if it has a hard time moving up and down the track it is on, you know that sensor is bad and needs to be replaced.

There are of course other things that can go wrong with the sensor to make it non-functional but I have come across that situation more often when I see a broken level sensor.

If the little level sensor inside the assembly can't float to the same level that the gas in the tank is at because it is getting stuck at a certain position; it obviously can't report the correct fuel level.

Maybe Jim knows of other things that go wrong with these. Maybe there are other complications of a late model tank that could cause the issue you have described. I just picked the first explanation that came to my mind.