mondeo 130 55plate  just under 200000miles on clock.

I blanked the egr and removed the cat about 12month 15000miles back had no problems only improvements.

Now I can put in say £30 of diesel from empty and the fuel gauge will rise to the correct amount the on board computer reading will state 300miles worth of fuel (est guess)

so all is well after fuelling up,  I can drive the car for about 5 miles and literally watch the onboard computer reading drop from say 295miles left of fuel to 155miles then within in a minute even if stationary with the engine running it can sometimes drop to 10miles of fuel left, the fuel guage drops at the same time, low fuel warning light illuminates yet the comp shows I am achieving 47.3mpg.  Now I can park with the car facing downhill (has to be quite a steep decline) and the readings will go back to 300miles of fuel left and the guage will rise to the correct place. This doesn't happen all the time sometimes it stays saying nearly empty and I haven't dared to keep running it to see if I run out I chuck more fuel in. No leaks I have checked.

I am thinking fuel pump dodgy alas the car runs perfect never misses a beat.  I have replaced all suspension items wishbones shocks springs links etc with top quality parts had the car Hunter aligned so fuel economy should be at it's peak for the mileage of the car. I do run the car on Shell opitmax and chuck in some redex about every 6 months.

Help please advice welcome, I am experienced in fords and this problem has me stumped.  Thank you.

Tank sender unit faulty I suspect

11 hours ago, furry vandal said:

I am thinking fuel pump dodgy alas the car runs perfect never misses a beat.

It isn't the fuel pump. It is either the sender, or the wiring to it.

You are getting wrong readings for the fuel level. Sometimes the sender float leaks, sometimes the 'wiper' goes open circuit, sometimes it s the wiring that goes open circuit, or gets a short to ground. The fuel pump doesn't do that. Fuel consumption (as oppose to 'range') doesn't use the fuel tank level, so is unaffected but range is fuel remaining divided by fuel consumption, so is affected.

Your symptoms sound like there is an open circuit on a part of the sender resistive track, which means that the level reading goes 'bad' (zero) over a certain range of tank levels.

Do you want to confirm the fault with a DVM or do you just want to change the sender?
 

Thank you for the advice I did mean sender unit when I wrote pump I had being working on a petrol car all that day with fuel problems and my head was still in petrol mode.

I think the way forward is as BOF suggests dvm and see what happens. The open circuit theory sounds like the spot on answer, Thank you again.

I will be having a look tomorrow and will report back on my findings.

Be aware of two things; there is a lot of filtering in the fuel gauge so don't expect (anything like) instant response there. Secondly it may be just a portion of the track (sounds like) that is worn out so you may get entirely different results with different fuel levels.