Fuel tank capacity and gauge

[phatman5000]

Hello chaps! I have a quick question about your experiences of fuel gauges. My '89 350i 3.5 V8 appears to drop from Full to Damn Near Empty within about 22L of unleaded. My gauge is wooly (bounces around) and probably unreliable. I've never run out of petrol when pushing my luck to about 30L.

I've read that the capacity of the tank is 63L which a) sounds a lot, and b) sounds waaaay more than my gauge is indicating.

My question is, do you know the capacity of the tank, am I safe to keep driving without having to fill up every 150 miles, and has anyone improved their fuel gauge?

Thank you!

[gruffalo]

I sold my Wedge 22 year ago but I do remember the fuel gauge was hugely in accurate.

I normally filled up when it showed a quarter of a tank but I let it run out once and it must of had well over half a tank left when I eventually spluttered to a stop and retrieved the can from the boot.

I can't remember exactly what I put in it at the next petrol station but I think it was around 60 litres.

[adam quantrill]

I think it varies.... with my 350i the max record I ever got in on a fill (when it conked out pulling up at the pump!) was 61l, and that was brimming it to the filler cap. Also I used to get just over 220 miles max range.

With the 400SX I think I have got 54l in. Maybe 55-56.

I would recommend sticking a can in the boot (but NOT plastic, must be metal) and trying your luck. Also make sure it has a filler spout (I use a bit of old radiator hose). When filling from empty, only expect 4 miles from the gallon can before it cuts out, because it might slosh around a bit.

Also all of the wedges I have had, have pretty good gauges, you can tell down to the last gallon how much is in there.

Edited by adam quantrill on Thursday 18th March 12:12

[KKson]

Having recently recommissioned my 450 SEAC, including removal of the fuel tanks, when I came to refit and refill I put 4 gallons in before the gauge even flickered. In total I got around 56 litres. I always fill up when it shows quarter of a tank, but then only get around 30 litres max in, so the gauge is definitely highly inaccurate.

[KelWedge]

My gauge just moves from full to empty all the time, never has been any good. I just fill the car up, reset the trip after 150 miles, time to think about a refil.

This works until you fill up at a garage where the car is not parkred level and you fill up one tank only as it does not cross fill to the second tank ! Then you run out after about 80 miles

[colin mee]

I have 3 tanks on mine .dont know why

[Skyedriver]

On average, what is the range on a 350i?
what is the theoretical tank capacity.
I ask as someone with a guage that actually works but only from 1/4 tank to empty....

[Englishman]

I went though this recently with my '87 SEAC.

Capacity of my car is 47 litres (twin tanks), measured by draining completely and refilling a litre at a time. Ignore what the handbook says!

To calibrate correctly and to stop the waving needle when the fuel is sloshing about, I fitted a Gauge Wizard, https://spiyda.com/gauge-electronics/the-gauge-wiz... Now have a fuel gauge I can trust. Also added a low fuel warning light driven from the Gauge Wizard.

[KKson]

Thanks David. I need to get one of these. I have squeezed 56 litres into my SEAC. Went for a drive for my Covid jab 1 today. Gauge read just over 1/3 ,but tried to brim it and only got 23 litres in it. My gauge is seriously inaccurate. When it says zero, there's at least 20 litres left.....

[BlueWedgy]

I had been looking at some of these types of things as i thought my gauge may be suspect as well, but I have not let it get too low, and don't drive excessive miles from home.
It is just calibrating the gauge to the sender, there are articles that just use a couple of cheap pots to achieve this as well.

These are a couple of the projects that I found a while back, that I thought may do it:

https://www.aliexpress.com/item/1005001863941642.h...

https://www.furneauxriddall.com/acatalog/Fuel-Wate...

Although a little cheaper, it is always good to know one that has been tried tested and proven, good that it has anti-slosh as well nice one Englishman.

[ElvisWedgeman]

You can calibrate the gauge to the sender by using an inline resistor. I did just that on a previous wedge on the temperature gauge. It’s very simple to do and gives good accurate results.I see no reason why it wouldn’t work for the fuel gauge.

[rev-erend]

Talking of gauges..

Somehow despite my best efforts when refurbishing my fuel tank I managed to loose the 'bolt' that secures
the sender wire.

Does anyone have any idea what size or thread it may be?



Finally managed o answer my own question.

For anyone looking for this in future, it's a UNC 8-32.

I had to buy a bolt of every similar size

Edited by rev-erend on Monday 19th April 08:29

[Englishman]

ElvisWedgeman said:

You can calibrate the gauge to the sender by using an inline resistor. I did just that on a previous wedge on the temperature gauge. It’s very simple to do and gives good accurate results.I see no reason why it wouldn’t work for the fuel gauge.

I started with the resistor route to try to improve the accuracy, as the gauge in my car never registered full. This did work to some extent, but the gauge still moved down far faster below 1/2 full than above. The needle still danced around too of course when cornering, braking, or accelerating.

Next stage was to adjust the tank sender arm so the gauge showed just below zero with an empty tank. I also measured the resistance of the sender at minimum (3 ohms) and maximum (190 ohms) which just about corresponded with the values needed by the gauge to indicate empty and full. This was an improvement, but the gauge still went down rapidly under 1/2 tank. The reason is obvious if you look at the tanks (and why the fuel gauge is different to the temperature gauge), they are not a regular shape, so a linear sender will not reflect the volume of fuel in the tank accurately.

Hence why I eventually went with a device that can handle a non-linear sender input and translate that to a linear output for the gauge. Having fitted and adjusted it though, the best thing about it is stopping that manic fuel needle when cornering, braking, or accelerating!

[adam quantrill]

I think it's a part of the car's character that you get a fuel level boost when cornering left. ;^)

Personally I use that to tell when I'm down to the last gallon, if it stops wavering then I know I need to get to the petrol station pronto!

[phatman5000]

Thank you so much for all your advice gents.

I'm going to give the Wizard a try, and if it works I'll report back exactly what I did. If it doesn't work, I'll be stuck on the side of the M4 and probably won't mention this again...

[ElvisWedgeman]

phatman5000 said:

Thank you so much for all your advice gents.

I'm going to give the Wizard a try, and if it works I'll report back exactly what I did. If it doesn't work, I'll be stuck on the side of the M4 and probably won't mention this again...

Ok. Just one suggestion but it’s a bit of a long shot. The classic mini fuel sender is very similar, you may be able to modify it using the parts off yours to combine the two into a working sender with the moving parts renewed. Just a suggestion but no guarantee it’ll work.