Good geezers,
I don't often ask but I need someone's brain to help me figure this out.
For the last couple of weeks I have been noticing that she's going onto reserve earlier than normal by mileage. Over the last few days it seems to have got worse.
This evening I pulled the tank off and emptied it right out. Exactly 10 litres left.
I filled up this morning so 23 litres full. She asked for reserve at 109 miles.
There is no crap in the fuel. The fuel filter above the tap is completely clean.
The tap obviously works and opens the reserve fuel inlet on demand.

I am absolutely stumped as to what's going on.
Have any of you guys ever had this happen?

If I'd lowered the front end something chronic or jacked the rear suspension I could perhaps understand it but nothing has changed that I can put my finger on.

Oh, the breather is clear. I checked that last night.

Ideas please......

H

 

For models with a fuel pump I'd start there. For gravity feed I would be suspicious of the tank vent.

 

Hi CB2, My suspicion also but having shoved wire up it and blown it through with an airline it is definitely working as it should without any blockage. I held the airline on the spigot under the tank for about 3 seconds until the tank actually started to change shape. The air coming out again seemed to flow freely. I think if I had a problem there it would manifest itself much earlier on a journey, especially if the tank's full no?
Perhaps I'm misunderstanding both fluid and aero dynamics.

I just put it all back together after checking the petcock internals. Absolutely no blockage in the tap and no problems with the diaphragms as far as I can make out.

I filtered the fuel when refilling the tank. The filter screen is completely unclogged and undamaged.
This is doing my head in.
The only thing I can think of now is that I'm using **** loads more fuel but that can't be right. I had 10 litres left at 109 miles. Enough for another 90-110 miles depending on wrist weight. That suggest fuel consumption is not out of the ordinary.

I'll post back tomorrow after the same journey completed and a refill first thing tomorrow morning.
Bloody weird.
Cheers CB2. Every little bit night help...

 

Even with a fuel pump I'd be suspicious of a vent problem. Especially since the tank pressurized as it did when the air hose was used. Um, unless the vent is a one-way item that blocks pressure from the inside and allows outside air in. Don't know about that, but guess it makes sense.

 

Fuel consumption must be normal, it's the point that you must switch to reserve that is not. At about 4L remaining the tall feed tube will be above the fuel level making you switch to reserve, the lower feed at tank bottom. Vacuum opening the feed is the same for both levels, normal/reserve so you should be able to eliminate that. Gotta say I'm at a loss too. Now that you have blown the vent out we will verify that. Keep us posted.

 

Henry,

If I'm understanding correctly, your fuel gauge is hitting the reserve mark, when in reality there is plenty of fuel in the tank?

If this is the case the problem may be electronic. The fuel gauge's circuitry has resistor's that prevent the gauge's needle from dancing around wildly with every single movement of the sender unit float.

Age takes its toll and suddenly you get erroneous readings.

My Mk1 has had this problem since I've owned it. From a full tank, my needle hits the reserve mark and when I gas up, it only takes three gallons to fill it.

My Mercury automobile had a similar problem. It would read full until I got down to a quarter tank and then it would read accurately. Ford/Mercury/Lincoln forum research pointed to a problem with the "anti-slosh module". It's a plug in module in back of the gauge cluster that controls the needle separately from the sending unit float to prevent the needle from dancing around. On the car the module runs close to the heater ducting and this is suspected to lead to failure. Went to a junk yard and pulled one from a similar car and then had to tear my dash apart to get to the back of the gauge cluster but it works fine now.

This is why I suspect the problem is with the gauge itself.

Also you might check your in-tank sender. Isn't it basically a submerged rheostat that's constantly exposed to the harsh chemistry that is modern petrol?

 

To me the gauge may as well not exist. This is about switching from normal to reserve too early, not what the gauge says. I only do that when you feel the engine start to run out of steam from fuel starvation. Switch over and 30 seconds later all is normal again.
When I get home tonight I'll know if any of last nights messing around did any good.

 

Well, no gold star by my name.

Good luck to ya'

it's a tech query, why's it in the Saloon

Mod's nappin'?