2000 740 Fuel Transfer

rsrmike · 08-05-2008, 06:24 AM

Hello, does anyone have information willing to share about the fuel tank in a 2000 BMW 740?

It's my understanding there is a fuel sending unit in each side of the tank but the fuel pump is on the right. Is this correct?

How does the fuel transfer from the left to the right side of the tank?

I have a car that quits running from fuel starvation when the tank gets down to one quarter. It doesn't seem to have used the whole tank so I believe it to be a problem with transfer rather than a gauge/sending unit problem??

The problem started 20 minutes after backing into a wall doing only minor body damage, by the way...

Thanks, Mike

3.2 CAB · 08-05-2008, 08:16 AM

The wall bump, may have knocked a pick-up tube loose, and now it is not able to draw fuel below where that pick-up tube was connected. Just a thought.

rsrmike · 08-05-2008, 12:14 PM

Thanks 3.2, We're on the same train of thought. I really don't know much about this car. It belongs to a P-car friend who was told by a prominent shop in town it was "unfixable".

I tend to disagree, which usually gets me in trouble so hear I probably go again...

I'm just trying to find info, resources, common problems, an explanation of the "venturi fuel transfer system?" etc. before I dive in to explore.

I'm guessing there are twin access holes to the sending units under the seat or in the trunk?

Jeron · Jeron's Garage

It probably has an internal suction tube that picks up from the bottom of the secondary (left/drivers side) lobe and runs to the base of the pump on the right side. The same system used in the 3 series.

Take a look at these to get some ideas
http://www.realoem.com/bmw/partgrp.do?model=GG83&mospid=47528&hg=16&fg=15
http://www.pumpkininc.com/content/doc/guide/ag-8.pdf

I would pull the pump and the mechanism will probably be apparent.

Jeron · Jeron's Garage

Its probably the black tube an the top of the last pic on this webpage.

http://www.bimmerboard.com/forums/posts/110831

A few more articles:
http://www.bimmerboard.com/forums/posts/110475
http://www.bimmerboard.com/forums/posts/73930

Where I found them:
http://www.e38.org/

rsrmike · 08-06-2008, 06:22 PM

Wow, thanks Jeron!! That's exactly the kind of help I was hoping for. Thanks much
Mike

Jeron · Jeron's Garage

Youre welcome. I hope it works out.

rsrmike · 08-19-2008, 07:34 PM

Okay, the saga continues...

The car quit today virtually within minutes of the fuel light coming on.

On Board Computer indicated approx 45 miles of fuel left in tank.

The first time it sputtered and I made a hard left across a four lane into a gas station saving momentum expecting to need it.

The car suddenly ran better, and idled up to an area adjacent to the pump.

I revved it a few times and it seemed ok??

I shut it off and removed both sending units ( back seat was out, covers off, pending starvation)

There was close to 2-3 gallons in the tank based on eyeball calculations...

I cranked the car with just the right sending unit lifted where I could watch the fuel flow.

I had good flow from the return line dumping back into the little fuel pickup sump through that little white valve on top.

I didn't see any fuel coming from the black tube.

The car seemed to behave at the moment so I put the covers back down and pulled back on the road.

Full throttle acceleration was fine through first and half of second gear before it started to misfire...

I killed the ignition to hopefully preserve the fuel state in the tank and coasted carefully without any G load to a safe spot to remove covers again.

This time the sump was nearly empty. I could see the sock for the pump sticking out of what little fuel was there.

I then pooured a couple of gallons of fuel in from the can in the trunk and buttoned up and came home.

Closer inspection under the car reveals something strange on the driver's side.

There are two nipples sticking out on the underside of the fuel tank facing the front of the car.

Someone has obviously removed whatever was there originally and placed a loop of fuel line in it's place connecting the nipples.

Looking through the left sending unit hole into the tank I can see metal tubing connected to these nipples on the inside. One tube goes to the bottom of the left sump, the other goes towards the passenger side and disappears- I can't trace it to the right side.

I'm assuming this is part of what scavenges the left sump to feed the right when the level drops to empty but I still don't know how it works.

The fuel level in the left sump was at least an inch above the pickup tube when it quit so something tells me the transfer is not working at the very end??

I need to know what is missing where some hack placed a piece of fuel hose, I'd also like to know what it does, and if that is possibly my problem. (I don't see how it's not) I can't find that part in any catalog. It doesn't come up on the BMW catalog that I've seen

The fuel pump is quiet as is the plastic valve above it and both sending units are new.

It just won't pickup the last few gallons of fuel like most modern cars will these days and needs to be fixed. If it were consistent it would be no big deal (just keep it at least 1/4 full, but the level that it quits at is unpredictable and could get worse.)

Please help!!! Thanks much