I carefully added fuel a gallon at a time to determine when the left side is full.
It was between 9.5 and 10.0 gallons when the left tank finally filled and started flowing over to the right side.
The gauge measured about 8.5gal on the left side but I wasn't on level ground.
In any event it's about 9/25 gallons when the siphon pump kicks in right about 1/3 of a tank. My needle is closer to 7/16
After driving 20 minutes:
Back to separate left & right tanks and car is operating solely from the reserve tank once the left side is less than about 32L.
so, the summary of all the research, and hands-on testing:
- The 'reserve' tank is almost exactly 5.0L
- The computer doesn't include most of the reserve tank in it's calculations
- The low-fuel light comes on at 8.0 or 10.0L calculated
- You should have 3-4L of gas remaining once the DTE is ZERO and the combined gauges read 0.0L remaining
- It takes about 32L of gas on the left to reach the level of the plateau between the tanks sides
- Once less than about 33-34L total fuel or about 1/3 of a tank, the E53 will only 'see' 5L of gas
- Learn what your car right sensor reads at flooding over level on the right tank
- Do this by taking note of the right side test six value when the left side is just below 32L or so and driving on level ground at a steady speed; it will become very clear where the value stabilizes; mine is 1.4L
- The right side value in 'test six' should be exactly the same from 32L on the left side down to 0L on the left side
- Soft failure of the main pump, or any other problem like a leak will show up as the right tank value dropping as the fuel level drops on the left side
- If you see the value get below normal (in my case 0.9L down from 1.4L normal: example 245009), find the problem before you get stranded
- If the right side value drops to zero, get fuel immediately, you have less than a gallon of gas remaining and will starve the engine of gas within 15 to 20 miles
- By FAR the most common problem will be soft-failing main fuel pump
- It's designed to last 4000 to 6000 hours; multiply your average mph over a tank of gas by 4000 or 6000 hours to get expected miles of life
- 4000 if you have pure gas, 6000 if you use crappy gas (reformulated with ethanol)
- If your avg. mph multiplied by 5000 is close to your current odometer reading it's time to change the pump
- If the right side value drops once the left side value gets below about 32L, there is a problem, resolve it before you have a hard failure or the soft-failure gets weak enough that you will infamously run out of gas with 1/4 of a tank
- As the soft failure gets worse, you will run out of gas with an increasing range on the distance to empty (DTE)
- Example: at first you may have 10 miles DTE and run out of gas, but it will eventually be 80 or 100 miles.
- This variability is because the weight of the fuel on the left side helps boost a weak pump and depending HOW weak the pump it needs a variable amount of fuel on the left side to help
- Lastly, the ONLY way to test the system is to drive down to zero DTE; it sholud be done periodically maybe annually or twice a year.