My 1996 w210 has a "float on an arm" type fuel sending unit instead of the "float in a tube" earlier style.

The "float in a tube" sending unit eliminated fuel gauge needle bounce by having very small holes in the tube so the fuel level in the tube could not change very fast.

The new, and I assume much cheaper to produce, "float on an arm" sending unit would cause the gauge needle to change as the fuel sloshed around from corners, hills, and bumps. To prevent this, when the engine is running, the gauge is only allowed to move a very small amount over time. I would guess the rate is between 10 and 20 times the fuel use rate of the car. So if your fuel use rate was 2 gallons/hour it would take the gauge 30 to 60 minutes to register a 20 gallon tank that was drained from full to empty in a few minutes while the engine is running. If the engine is not running, the gauge reads the actual fuel level almost instantly.

How did I find this out, and what's the problem ?

I converted my wife's new 1996 E300D to WVO. I use a MB tank and sending unit so instead of installing a separate fuel gauge for the WVO tank I just have the sending unit signal switched from the diesel tank sending unit to the WVO tank sending unit when the fuel source is switched. Static testing showed that when the fuel source was switched, the gauge quickly moved (about 2 seconds) to the fuel level of the fuel source tank.

First road test: I put 4 gallons of WVO in the tank. Primed the filter and lines. Drove until the coolant was up to temp. and switched to WVO. Nothing happened. That is a good thing. You just want to keep driving along as if nothing has changed. But I noticed that the fuel gauge reading did not change and the difference in fuel levels between the two tanks was at least a 1/4 tank. I switch back a forth a few times and then just continued with my test drive. I figured something was wrong with my wiring. About 10 miles down the road I noticed that the fuel reading had dropped to almost where the WVO tank level should be. In another 5 miles it was at the actual level of the WVO tank. So, 15 minutes for the gauge to move a 1/4 tank.

When I got home I purged and switched back to diesel. The gauge was still at the WVO tank level when I turned the car off. I turned the key back to the run position without starting the car. I switched back and forth between the tanks. The gauge would register the new tank level within 2 seconds. Start the car up and switch back and forth and the gauge would not move.

Knowing this, I can deal with it no problem, but... it annoys by wife.

Do you think there is a way to make the fuel gauge operate as if the engine is not running ?

__________________
Greg
2012 S350 BlueTEC 4Matic
2007 ML 320 CDI
2007 Leisure Travel Serenity
2006 Sprinter 432k
2005 E320 CDI
1998 SLK230 (teal)
1998 SLK230 (silver)
1996 E300D 99k, 30k on WVO
Previous:
1983 240D, on WVO
1982 300D, on WVO
1983 300CD, on WVO
1986 300SDL 237k, 25k on WVO (Deerslayer)
1991 350SDL 249k, 56k on WVO - Retired to a car spa in Phoenix
1983 380 SEC w/603 diesel, 8k on WVO
1996 E300D 351k, 177k on WVO