I just purchased a 1989 740 GLE Estate. Fab car! There are a few minor ****les that I could use some advice on.
At the most, the fuel tank will only hold 7-8 litres (2-2.5 gallons). I can go about 55 miles, and I run out of petrol. I go the petrol station, and it will only take 7 litres to fill.
The chap I bought it from said it might be a problem draining from the auxillary tank. As far as I can tell, there is no auxillary tank. At first I thought it might be a blocked breather hose - pressure from the vapour in the tank building up as I fill it would prevent more petrol going in. It will take more fuel, but it takes ages to do so; literally drops at a time.
I pulled off the breather hose at the tank and blew into it - surprise, fuel came out of the tank! I removed the fuel filler cap and I could hear the air rushing through unrestricted when I blew into it again.
Which made me think 'hey! I fixed it!' I went to the garage... no deal, it still takes less than 10 litres.
Does anyone have any idea what it might be that's giving me this grief? A few other details which might be pertinent:
The car apparently sat for a few years untouched - could it be sludge in the tank? If it is, why does it run fine? Albeit for only 50 miles.
The manual says the tank holds 60 litres - could I have a full tank and the pick up is not working properly? What would make it stop working after 50 miles?
Any ideas would be gratefully received.

 

I'd be very careful...if gasoline was left in the tank it'll turn into a laquer within two years....any longer and it'll stiffen up. Not so good for your engine. Maybe investigate the fuel lines and see what's up with them?

 

Some 740 models do have an auxiliary tank holding 8 - 10 litres. It is mounted directly above the main tank and fills only when the main tank is full. The filler pipe actually branches so that as the fuel backs up the filler pipe it climbs into the aux tank. Only when it is full will the blowback system cut off the pump.

I agree with cbevis (nearly everyone does because he writes wise words) about the fuel turning to lacquer but I think you may also have a blockage in the fuel filler pipe past the junction of the main/aux pipes.

Can I assume that, like me, you are from England? If so a possibility is that some one fitted an anti syphon device at the time when petrol theft was rampant in the UK. They were basically a perforated metal disc which allowed petrol to enter the tank but stopped the little towrags getting a tube into the tank. A side effect was that they could easily block and stop the fuel getting in.

Just a thought which will not apply if you are not from the UK.

 

Thanks billofdurham, I am a fellow UK motorist!
I have removed the panel from the boot giving me access to the fuel tank. I can not see a seperate fuel tank, so I'm quite sure I haven't got one. The only reason it came up was the previous owner thought that's what it was - his logic being if there was only 8-10 litres being put in, it must be auxillary tank. He couldn't find one either.
I also agree with cbevis; I have seen what can happen to old petrol. But this wouldn't affect my range or my ability to fill the tank.
How far in are these anti-theft devices mounted? I cannot see anything fitting your description when I look down the filler neck. Are they mounted at the tank? Are they easy to remove?
Thanks for your help!

 

The anti syphon device was intended to be a push fit in the filler tube and had to be far enough down to stop thieves getting it out easily. It was never foolproof and could be removed if necessary. It just made it harder for the villains. If one has been fitted you may not be able to see it but you should be able to feel it if you push a length of rubber or poly tube into the filler.

However, your not finding an aux tank really scuppers my theory as there is no place for 8-10 litres to go in the filler pipe and, even if there were, the fuel would not be able to get to the pump and thence the engine. If an aux tank had been fitted 10 litres would fill it and there is another pipe for it to drain into the tank.

Sorry, but it looks like we are back to the drawing board.

 

Hi Bill, Thanks for that.
I've been thinking, too, that there probably isn't a blocked anti-syphon device, because if there was, I wouldn't be able to put any fuel in.
I dont' know if this has anything to do with it, but my fuel guage doesn't work. The mount holding the three leads off the tank I believe works as a ground - I've rewired a ground by drilling and sanding a new hole and new wires from the very corroded original ground wire. The guage still doesn't work; it changes from showing full to half a tank, and anything in between. Sometimes it shows empty.
Could this be an issue?
Cheers
Mike

 

This could be an issue but not the reason why you can't get fuel into the tank.

Have you tried gently inserting a softish probe down the filler neck? I am interested to find out if you can reach the bottom of the tank and see if there is fuel there.

I am also interested in the three wires as the fuel sender usually only has two. Could you make out the colours at all under the years of grime?

I do know that if you take the 'live' wire off the sender and touch it to earth (ground) the gauge should register full. That was a simple test to see if the gauge or the sender unit was faulty.

 

Alright, I will try that when I get home tonight. I will have a good look at the wires on the weekend and try your trick with the earthing.
I'm not back at the office till Monday, so wish me luck with this!

 

No chance this weekend, and we moved to Brighton. Made for a long journey, having to stop for petrol every 45 minutes or so. Luckily the sat-nav told me where the petrol stations are!
I had some trouble starting the car yesterday, I think the battery was a bit knackered - the glove box light was left on for a day or so. Seems to be alright now, but we'll see.
I do have a length of hose, so I will try the old hose-down-the-filler-neck-and-see-how-far-we-get trick this avro.

 

I have tried the hose-in-the-tank thing, and I can get about one metre of hose in before it hits the bottom of the tank. When I pulled it out, about one centimetre of the hose was wet - I had just topped it up with 8 litres of fuel.
Do the fuel tanks have baffles inside? Could a baffle be blocked?

 

That amount of fuel on the end of the hose would be about right for 8 litres bearing in mind the size of the tank.

I don't think that there are baffles in the tank and I would not have thought that one being blocked would create your problem.

I would suggest now that you get access to the top of the tank again and remove the inspection cover on the tank itself (4 nuts). This gives you access to the in-tank pump and the fuel gauge sender unit. Using a match for illumination would then end all your problems, but I would suggest shining a powerful torch inside, hopefully without having to remove the pump etc., and see if there is anything which could be causing the problem.