Fuel reserve light / gauge

[70sThrowback]

Mine appears to read correctly but I still don't trust it. I think its the scale, from full to 3/4 covers a large area on the gauge. when it gets to 1/4 its so close to empty I start to get range anxiety. When I fill up though, the amount of litres I put in is 3/4 the tank volume. So its good, why is 1/4 so close to empty!! Anyway, point is, I am like Ian and check the mileage as although it apparently works I can't trust it.

[911hillclimber]

I have a Skoda Citigo. The gauge is marked in a linear manner, but the shape of the tank is far from a simple form, and this is why when you get to 3/4 empty the rate the indication drops is very quick compared to the full to 3/4 empty.
The 911 tank is quite a regular shape, in comparison so maybe the effect is not so pronounced.
om my simple 73T once the big red light comes on during gentle cornering, time to fill up and that is about 3/4 of a tank too.

I think fuel gauges are indicators, not measuring devices that are accurate.

[Lightweight_911]

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I'm with Ian - I never trust/rely on the fuel gauge in any car.

I usually know what level of fuel consumption I can expect given the driving conditions so always zero the trip meter when I add petrol & have a good idea of when I need to get more fuel - & if I'm away on a trip I always carry a spare container of 5 litres of petrol ...


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[hot66]

my issue is more that it was working fine for the past 23 years and now it has changed... as always with these things, I want to work out why its changed and correct it

[Lightweight_911]

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Keith - have you taken the sender unit out of the tank recently ?

The central metal rod that the float slides up & down on can get corroded - especially at the top where the float often spends less of its time.


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[hot66]

checked sender and all is good there ... its deffo an electric or gauge issue . Sender float is still in the correct position which is why the reserve light comes on at the correct fuel level .

[911hillclimber]

Run a new wire from the sender to the gauge outside the car, simple to do and no messing about. If that works, thread the wire from the tank to the gauge, a short run on a 911.
If that fails, the sender is good then, the gauge is next.

Good luck, I hate electrics too.

[KS]

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Keith - have you taken the sender unit out of the tank recently ?

The central metal rod that the float slides up & down on can get corroded - especially at the top where the float often spends less of its time.


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Hi Andy - yes, I took it out a while back and cleaned all the contacts and central wire, etc, but to no avail. I'm guessing the gauge, so will try some 'direct wiring' to see what happens. I've only run out once and that was on the way back from Wiscombe about a mile from home! Fortunately had my spare 'race fuel' in a can...

[hot66]

So … if I remove the green wire from the gauge and then reconnect that wire the gauge reeds correctly… but if I give the gauge a knock it the needle jumps up a bit and reeds incorrectly

[Ian Comerford]

It’s so random, the performance. Mine is currently reading accurately, or so, but hasn’t for much of the current tankful. Are we assuming the gauge is at fault James? I’m considering messaging a gauge repairer/refurbisher with the details to get their thoughts.

[hot66]

it all points to being a dodgy gauge

[Ian Comerford]

it all points to being a dodgy gauge

It might appear to but....I've just called a company (https://classicsparesengineering.co.uk) to discuss sending it on for testing etc. Very helpful and seemingly knowledgeable guy said the gauges are generally very reliable and they either work or don't, there is no in between. They have had a couple of 911 gauges to repair where an internal wire has broken on one of two coils, but in that case the gauges couldn't work. He said the fault is one of three things; the sender, the wiring or a bad earth. I'm happy with my sender so I think it worth trying running an independent wire and also cleaning up the relevant earth. Thoughts?

[Nine One One]

You can but try? Bad or worn earth points or the earth wire always seem to be the problem, also if you have a multimeter check continuity between the various wires and the gauge, to see if you can isolate the problem quicker that way?

[911hillclimber]

James, I suggested the new wire posts ago..
It was the only way I could sort my oil pressure gauge. In my case the sender and gauge were good (as it turned out), the original wire not so, even intermittent, a fresh wire run alongside the harness fixed the lot once it was proved to be the loom.

This is a simple experiment to try, just need enough wire.
Remove the gauge, wire fresh to the sender and run a fresh wire to the earth (battery). Nice if you can put an in-line fuse from Halfords in the sender/gauge wire.