TiCoyote

I picked up a 2021 A4 Premium Plus 45 about a month ago. I've been filling the gas tank when the fuel gage is at 1/2. The specs say that the car has a 15.3 gallon tank, but when the gage is at 1/2, it takes 9 gallons. My previous car was a VW, and the literature said that there was 1 gallon left in the tank when the gage was E. So I figure, when the car is at 1/2, it should only need 7 gallons. Do others have a similar experience?

Thank you.

uberwgn

It's not linear. I think many cars exhibit this. The last "1/2" of the tank disappears quickly.

TiCoyote

Yeah, that's kind of what I figured. Thanks!

Astra.xtreme

I've always wondered how cars measure the fuel level. If it's some sort of strain gauge or weight sensor of the tank, then yeah the accuracy is high when fully loaded and less accurate at light loads.

SMac770

The tank actually has two sensors, if quattro; one in the back left and one on the pump in the front right. They are just floats on potentiometers. The back left upper sensor is in play for about the top 1/3 of the fuel level. The front right lower sensor is in play for about the lower 3/4 of the fuel level.

For B8, they were connected to the J285 instrument cluster. For B9, they are connected to the J393 central convenience. You could track the resistance values across the two pins for that sensor on the fuel pump plug. The specs for that are in the workshop manual. Well, at least for B8. Not only did they remove that troubleshooting information from the B9 manual, you can't even replace the left rear sensor if it develops a problem; you have to replace the entire fuel tank now.

It also looks like they eliminated one of the wires from the G169 sensor 2 (back left) starting MY19. Yeah, they eliminated the redundancy check for the G169 starting MY19. How cheap of Audi. Not surprising at all. On the 6-pin plug, pin 2 is the common ground. Pin 3 and 5 are G169 sensor 2 and pins 4 and 6 are G sensor 1. The resistance from each pin to ground should add up to the same value across the two pins (opposite ends of the potentiometer). You'd do this from a low tank level every couple gallons as you fill and see if you have a non-linearity developing in one of the sensors, such as my left rear sensor has started to develop in the past year. Luckily, that just means my range remaining + distance traveled just dips around the 3/4 mark.

But my fuel level near the bottom is accurate to the gauge. 1/2 tank is always an 8 gal fill up, 1/4 tank is always a 12 gal fill up, 1/8 tank is always just under 14 gal. And red light will be just under 15 gal. This is for the 16 gal tank in my B8. One of the things that throws off the interpretation of the gauge in the upper half is that full is "over the top". And by range, Audi is always way over optimistic after a fill up.

So is your display (since B9.5 doesn't have a real fuel gauge) just off in calibration, or errant display due to bad programming in MIB3 or errant measurement due to bad design in general in B9, or do you have a problem with the sensor operation. No idea.

PghRich

Iitc, car companies received complaints that the fuel level dropping from F quickly (linearly and correctly) meant the car was getting poor gas mileage. So they calibrated the gauges to stay on F for a while so an owner could feel like the car could be driven a long way with minimum gas used - i.e. great mileage. Once it started falling from F, it's hard to track progress,