Fuel Tank Problem. Car only uses half of the tank!

[AMG]

Hi all.

Okay, this is going to be along winded write up, but i feel i have to explain everything, in order for everyone to understand.

I have a 98 740iL. Since i bought it, there was a small crack in the fuel tank on the PASSANGERS side tank (Its a RHD Car, in Australia). the crack was about 1cm in length, and was barley noticable. Though, when there was a full tank of fuel, it would constantly drip out. I am told, that it is a common problem with the E38's, as the tank contracts/ expends under the pressures, and thus is prone to cracking.

As we all know, the tanks in 740iL are 95L (or so), and it is essentially 1 big fuel tank, split over each side of the car.

The small crack in the tank, seemed to be getting worse & worse. I had tried everything to fix it. All the fuel tank repair kits, all the wives tales, etc, NOTHING WORKED. I tried everything.

Though having the fuel tank leak, i learned a thing or two. On the E38's, when you have a full tank of fuel, as you drive the car, and use the fuel in the tank/s, the PASSANGER side of the tank empties first, then the drivers side is used second.

Its weird why it does that, i dont know? But this can be proven also, (if you have the OBC unlocked), if you go to test #6, you can see as you drive along, that the LEFT hand side figure is the one that decreases first.

Also, if you have an empty tank, and you put like 40L of fuel in, its the drivers side tank the fills up first, then the passangers side 2nd.

So, before i got the crack in the tank fixed, i only ever filled up half tank, in order to help stop the dripping. Ofcourse when driving, fuel will go over to the passangers side, but it wasnt much of a problem.

Now here is the feaky bit. I had the crack in the tank welded, so it was fixed once and for all. They removed the tank for the car, flushed it out, etc, welded it up. NO MORE LEAKS! YAY!

Anyhow, in celebration (LOL), i decided to fill the tank up, thinking, wow, its going to be so good, not having to stop for fuel as often!

Anyhow, i filled up, and then i had to go on a big drive, which ordinarily, i could get to that place on 1 tank of fuel.

During my drive, i ran out of fuel! WTF? The fuel guage was reading just over 40L. I was thinking, what the hell is going on. Ofcourse, when on the freeway, and the car started spluttering etc, i didnt think i was out of fuel, as the guage was reading half tank. I thought it was something else. After racking my brain, i though, i will go into the OBC and go to test #6 to see exactly how much fuel was in each tank.

To my surprise, the DRIVERS side tank was empty, and the PASSANGERS side was full!

How did the car stop? I got a jerry can, and put 10L of fuel in the car, and it started working again!

So now, i am back to only being able to use half a tank of fuel at a time, but this time in reverse! LOL

So what do i do? how do i fix the problem. The car is not using any of the fuel on the passangers side for some reason now. I asked the guys at the shop, and they said that they plugged everything back in. I lifted the rear seat, and unlugged & plugged back in the fuel pump.

Any ideas anyone?

PS- under the rear seat, on the drivers side (Australia), thats the fuel pump, but on the passangers side, what is that? It doesnt have any hoses coming out of it, only 1 electrical connector, which is plugged in.

Thanks

[SupaFIN]

Any ideas anyone?

PS- under the rear seat, on the drivers side (Australia), thats the fuel pump, but on the passangers side, what is that? It doesnt have any hoses coming out of it, only 1 electrical connector, which is plugged in.

Thanks

It's that side tank sensor. One sensor with fuel pump and other side second sensor.
I got problems with fuel tank too. I have LHD car and sometimes i can fill tank only half. Only passenger side get full of fuel but it didn't get to drivers side. But now it has worked long time without problems, weird huh?

If someone could explain how these two sides of fuel tank are filled and balanced it would be great. Is there tubes inside that moves fuel from side to side or what?

[Route 66]

I always thought in theory that pressure would even the tanks, seek its own level.

[TxDarth]

It should unless there is an obstruction somewhere stopping the filling or draw down....

[Edwin NL]

+1

[AMG]

So no ideas anyone.

This is no frustrating! Im gonna have another look under the car, and see how it goes.

Will post up any developments. But if anyone comes up with any ideas, please let me know!

Cheers!

[imola740i]

never serviced bmw fuel pump or tank but merc has same looking tank and they are both fuel pump modules. both should be connected with hard plastic hose. if this is same with bmw your pass pump probably burned out due to lack of fuel.

[bimmerfixr]

The two sides of the saddle style tank are connected by an opening at the top center of the tank, over the tunnel hump. There is a suction jet attached to the bottom of the level sensor assembly without the fuel pump. As fuel is returned from the engine, it flows through the suction jet and fuel is transferred from that side to the pump side. The goal is to always have the majority of fuel available to the pump side. The suction jet itself cam become detached and stop working correctly, that one needs the tank to be replaced. It is also possible that someone has replaced the level sensor and not connected the fuel pipes correctly or an o-ring seal has become dislodged, I've seen both. Looking under the car won't help you diagnose this. You need to remove the rear seat cusion to gain access the access covers under the cushion. You can "watch" the fuel being transferred using the cluster test functions, do a search, I don't remember which test number it is, the display will show how much fuel, in liters, is in each side of the tank, check it with the key on , engine off and then start the car, you should be able to see the pump side level increase while the other side decreases. This test doesn't work well below a 1/4 tank and the car should be driven in a way that will splash fuel over the hump to the non pump side so you know that fuel is in that side to begin with. Maybe Q can chime in with some pictures and the cluster test steps.