Fuel tank telling fibs

Hi Folks Quick question, a 996 Turbo has a 63-64 litre tank right? Last night i filled up with the needle just sitting off the bottom and the computer showing i have 47 miles to fill up. I go and fill up and the maximum i can put in is 43 litres. So either my car has massive reserve or the fuel sensor needs changing. I'm thinking the latter but just sanity checking with the rest of you as its getting slightly annoying now filling up every 4 hours (i don't drive that fast Cheers Neil

Could be a few things. Favourite is it just needs re-calibrating, second favourite is imploding fuel tank. Was your orange light on? Should have around 2 gallon left in the tank (just over 9 litres) when it comes on. My light comes on with 4 gallon left in the tank (just over 18 litres). I keep meaning to address mine but after 6 years I've sort of forgotten about it.

Its just had major service at East London OPC and before that i had some other work at Fearnsport, nothing flagged up. Red light was on and comes on with 75 miles left in the tank, i took it down to 47 miles then filled up to the top (always do) but it only took 43 litres not the 55-60 i thought it would which probably means its really between a quarter and halfway i would have thought.

A service will never pick it up. Probably just needs recalibrating by Porsche.

Thanks Alex, East London OPC round the corner so I'll probably take it in on Sat morning and get it sorted. Have a good weekend :thumb:

They'll not do it there and yhen. Its quite a process. They have to drain the tank then fill it ezactly halfway, then tell the ecu its half full.

Given this is a turbo, the bottom half of the fuel tank "saddles" over the drive shaft for the front wheels. Once the tank is just over half empty there is no actual measurement of fuel level. Av fuel consumption is calculated from your driving history up until the level goes below the measuring device. So it is a guess at best. The actual tank can "implode" but this rarer than just a pessimistic calculation of the bottom half of the tank's capacity. I would go on a long drive with 5 litres of fuel in a separate fuel carrier. Once the light/50 mile warning comes on, see how far you can drive before it really runs out.

I ran out of fuel in my turbo once, I put 5 litres in and drove 1/4 mile to petrol station and it took another 60 litres till the pump clicked off. So fuel tank is 65 litres. I usually only fill up when gauge gets to empty line and it normally takes 60 litres. The calculation for the bottom half of the tank works very well on my car regardless of how I drive it I've always ignored the fuel range reading, I almost never get more than 210 miles from full to gauge showing empty and computer always predicts 300 and something. I just think 50miles for each 1/4 tank!

The calculation is not done from averaging how you drove for the first half of the tank then applying it to the second half otherwise if you drove faster for second half you'd run out. It done by integrating the fuel injector opening time to work out total fuel used. And it's very accurate! People who've changed injectors or fuel pressure often have problems for this reason! Mac

Thanks for your correction Matt. I think I was more trying to explain that the bottom half of the tank is not measured, and the fuel range/low fuel light are "calculated" not measured, as opposed to focussing on the method of calculation. One other anomaly of this setup is that when the low fuel light comes on, with a 50 mile range, if you put 25 litres of petrol in to the car, the gauge and range do not update, so continue to show you as low fuel and 50/40/30 mile range, as you travel 120 miles! Paul