Trouble filling tank on '91 740 sedan

[magoo]

Hi Everyone,

I have the above car & always have trouble filling the tank. I understand it has a 2 tank system, so you fill the upper tank & it then drains to the main tank. It seems like it'll take enough to fill the upper tank, but it mustn't drain into the main tank as fast as it fills because it'll rarely take more than about 25-30 litres at a time.

Is this normal for this car? Anyone else have this problem?

Thanks,
Shane

[jonesg]

I've never heard of the 2 tanks in a 740, mine only has one. (1990).

Did you happen to change the fuel pump lately?

My guess is the pickup is not in straight, you probably still have half a tank of gas when the guage reads E. The pickup isn't on the floor of the tank.
Its cocked sideways.

[billofdurham]

Some 740s were fitted with two tanks. The auxiliary tank actually sits on top of the main tank as shown in this diagram:

so you fill the upper tank & it then drains to the main tank

The main tank fills first and then the aux tank takes its fuel as an overflow. As the main tank empties fuel drops out of the aux to replenish it.

That breather tube on the top of the aux tank is the only one for both tanks and they sometimes do not breathe properly. Check the tube for restrictions. It is fairly easy to get at. I have also heard that the pump nozzle can restrict the breather whilst the tank is being filled if it is held so that it covers the end of the breather.

Bill.

[jonesg]

Is that for head pressure?
or to use a lower pressure pump?

[billofdurham]

As far as I know it is to give extra range as it increases the tank capacity by about 1/3rd. The pump is the same as for a car with only one tank.

My 740 had the aux tank which made for fewer visits to petrol stations but when I did go I needed a second mortgage..

Bill.

[magoo]

Thanks for the responses.

Bill - your drawing helps a lot. Our car has the main tank in front of the rear left hand wheel & the upper tank is in the boot directly behind the back seat. I had assumed the fuel would run through the upper tank into the lower. But with a setup like in your drawing I could see how the main tank would fill easily, but when you're filling the upper tank the fuel may not flow up into the bottom of the tank as fast as the fuel flowing into the filler neck, so it could be fiddly to get the last bit in. The top of the upper tank wouldn't be much below the level of the filler anyway.

The fuel gauge has been erratic for a while & the odometer isn't working (Bill gave some advice on that a month or so back, but I haven't had a few days where my wife doesn't need the car so I can fix it yet), so for all I know the main tank is full most of the time so I'm filling the upper tank a lot of the time.

I did try to gauge the level of fuel in both the tanks by tapping them, but I wasn't having much luck. Maybe if the tanks were steel it'd be easier, but the plastic tanks sounded the same all over to me.

[magoo]

I'll check those breathers too.