Fuel gauge - strange readings, any ideas ??

Sonic ZS · 3rd January 2021, 16:08

I've never been entirely confident of the fuel gauge in my '04 diesel, and last evening confirmed something is definitely wrong.

The car was covered in ice when I finished work yesterday, so I left the engine running to assist warm up & demist a little whilst I got fuel (I know I shouldn't, but this was for experimental reasons...!!). The car was on level ground all the time and the gauge was showing about 1/8th of a tank, with the low fuel light having just tripped on - it wasn't illuminated when I parked up yesterday morning.

So with the engine running, I watched the gauge as I put about £30 in, approx 5 gallons or so. But no movement at all in the gauge.

Then drove about 25 miles (without turning the ignition off) and the needle gradually dropped from the 1/8th position towards the empty mark. Obviously I went up and down various hills, which may have caused this ?

Then on level ground, switched the engine off, restarted, and the gauge immediately shot up to just over half - pretty much the position I'd expect with £30 of fuel.

So it appears both the senders and gauge are working, but why is the system not recognising the increase in fuel, yet still seemed to drop a little whilst driving ?

Does the fuel system have an ECU or similar, to control the readout, which might have a glitch ?

I find it hard to believe the wiring could be at fault, as the gauge does appear to read correctly once the ignition feed is reset with the key.

Any help on how the system works, or what might be faulty, would be greatfully received

3rd January 2021, 16:08	#1
Sonic ZS Posted a thing or two 75 Conn CDT Tourer, 75 Conn SE V6, 75 Conn V6, 75 Conn CDTi Tourer, ZS 180 Join Date: Jan 2014 Location: Devon Posts: 1,986 Thanks: 1,287 Thanked 963 Times in 534 Posts	Fuel gauge - strange readings, any ideas ?? I've never been entirely confident of the fuel gauge in my '04 diesel, and last evening confirmed something is definitely wrong. The car was covered in ice when I finished work yesterday, so I left the engine running to assist warm up & demist a little whilst I got fuel (I know I shouldn't, but this was for experimental reasons...!!). The car was on level ground all the time and the gauge was showing about 1/8th of a tank, with the low fuel light having just tripped on - it wasn't illuminated when I parked up yesterday morning. So with the engine running, I watched the gauge as I put about £30 in, approx 5 gallons or so. But no movement at all in the gauge. Then drove about 25 miles (without turning the ignition off) and the needle gradually dropped from the 1/8th position towards the empty mark. Obviously I went up and down various hills, which may have caused this ? Then on level ground, switched the engine off, restarted, and the gauge immediately shot up to just over half - pretty much the position I'd expect with £30 of fuel. So it appears both the senders and gauge are working, but why is the system not recognising the increase in fuel, yet still seemed to drop a little whilst driving ? Does the fuel system have an ECU or similar, to control the readout, which might have a glitch ? I find it hard to believe the wiring could be at fault, as the gauge does appear to read correctly once the ignition feed is reset with the key. Any help on how the system works, or what might be faulty, would be greatfully received __________________