Fuel gauge issues

[Nipper4761]

Hi, my 2000 S40 2L petrol started playing up a couple of weeks ago. I try and drive it carefully to keep the mpg at a good level so I often look at the readout. It suddenly dropped from about 29 to 26.3 and stayed there for a few days without moving (the fuel gauge was reading correctly at this point). Then, the fuel gauge dropped to just above empty although I knew the tank was 3/4 full. I have seen posts relating to the fuel sender unit but given how this problem 'evolved', I wondered if it might be something else. Any ideas anyone?

[bimmer49]

Nipper, I've tweeted before that the fuel gauges (I have 2) are erratic in operation. I was out of gas once and the petrol guage was just under half. Went into a garage for gas...no gas, oh well Ive got plenty of petrol....I got about km and it spluttered to a stop. Still showing just under half. After all the electrical and fuel rail checks and finally removing the fuel tank sender....it was indeed out of petrol. 5 litres of fuel later and the sender back in.....oh look guage on the red line.
So I'm of the opinion that yes they do stick.
Dont get me on the LPG guage, that pleases itself, temp., altitude and atmospheric sensitive.

[Technician]

The way I see it you have got two strands to look at.

1). Sender unit/guages reading incorrect.
2). Actual consumption is higher

1). Fuel senders are know to fail, not every car guaranteed, but enough to be noted. My personal experience over 3 cars is one of them mis-behaved fpr 5 mins once. A failed guage would be a bit rarer than that, paticularly on the latter phase 2's. However with anything electronic solder joints can wear with age and changes in temp.

To maintain good operation you need to use the full range of movement of the float and the guage as they are electrical contacts.

Also where do you fill up? Not all petrol is the same, or even clean and with the change to the products contained within the fuel it may be that its upsetting your sender.

2). A drop in MPG when your doing esentially the same journey points to an issue somewhere. First thing to check is if the mechanical movement of the car is ok, basically check for no sticking brakes. Simple to drive for a bit and check the temp by putting your hand near the disc (don't touch). They should all be warm but not too hot. If one is hotter than the others that brake may be binding.

Next you need to think about your combustion. First thing to check is your air system. Is you MAF ok (give it a clean), is the air filter and box clean. Are there any obstructions or split pipes in the system. A good way to check is to spray some brake, carb or MAF cleaner very carefully (dont want to spray on exhaust an burn your face) adound the various air pipes. If the engine pics up.and runs better then air is leaking in and upsetting the fuel map in the brain.

Also don't forget to check your exhaust for leaks, they can affect MPG.

3). Remove the brand new heavy shoes you brought for driving.

Basically its probably your sender unit, but do check a few other things before you buy one.

[Donalall]

I'm experiencing the same problem. Fuel gauge is stuck on 3/4 full. I filled up to full but gauge stayed where it was. It should be about a 1/3 full but still reading 3/4.

I got a new fuel pump installed about a year ago. (was on hols so can't go back to garage). Is it an all in one unit or would the garage have used some of the old parts? Any ideas on problem/solution ?

[Technician]

Aside from the big plastic lock and sealing ring the pump and sender is one unit, so they should not have re-used any parts. If you lift your rear bench seat and pull the carpet back you should see the metal cover for the sender. You can undo this to look at the top of the unit and see if it looks new and check the wiring connection etc...

Its a fairly easy part to change by yourself and the only real akward bit is the giant plastic ring. Tapping it round gently with a blunt chissel will get it loose. Don't hit it so hard you will crack the plastic. Also battery off in case of spark, shich would be highly unlikely.

As an general rule any decent garage should show you the used and old parts they have removed...paticularly a obvious part like a fuel sender, oil filters you wouldn't generally need to see.

[Donalall]

@Technician - Thanks for the info. They showed me the old unit and I'm assuming they put in a new one. (just seems strange after a year the fuel indicator is stuck).

When you say check the electrical connections do you mean to check for corrision and that everything is connected? Anything else?