I had the same problem about a month ago with my 2000 540. After filling up, the gauge read about 1/4 tank. By the time I got to work, about 45 minutes later, the gauge read almost full. When I got home the gauge was reading full and I put the car on the bmw DIS computer and performed a short test on all the modules. I had no failures so I went a step deeper and tested the instrument cluster and the sending unit. Everything checked good and I've never had the problem since. A co-worker with a 98' 530 had the same problem 3 months ago and it hasn't come back since. Has the gauge gone back to full yet?
Keep an eye on your fuel usage vs. miles traveled or you may run out of gas before the gauge shows you're that low. The fuel level sensors (there are two, on in each side of the tank) get gummed up from the gasoline. Chevron Techron claims it will clean fuel level sensors and I've started using it every once in a while after having replaced 7 fuel level sensors in my 2000 M5 I bought new in 1999. A couple of dealers told me it's a common problem. Luckily, the sensors are not hard to replace (they're accessed from under the rear seat).
7?! bloody hell. I have had instances of E39 fuel sensors sticking. Might be exacerbated by ethanol fuels. +1 on the Chevron fuel, or any of the other Top Tier fuels.
Paul, All 7 of the replacement fuel level sensors were covered by warranty (the original factory and then an extended warranty) so I let the dealers change them. A couple of times they were both changed, the rest of the time only one was changed. If it happens again I'll probably just pull the sensors out and clean off the wire windings the arm of the sensor rubs on to set the resistance the computer reads to send the signal. In the past when I've pulled codes it would sometimes indicate a fuel level sensor problem but usually not becasue the sensor is tecnically operating, it's just not sending the right signal beacuse of the contamination on the wire windings. Since the two sensors operate independently in the two sides of the tank (the computer adds the two readings together to send the signal to the gas guage) sometimes an "implausable reading" error code will be set if the levels in the two sides of the tank are significantly different. There's only one fuel supply pump, in the right side of the tank with a scavenger-type pump in the left side that moves fuel to the right side where the supply pump can get it. The fuel level in both sides of the tank should generally be almost equal. FWIW, I have always used Top Tier gasolines, Chevron (which contains Techron) when I can get it. I think what casuses my problem is that the car can sometimes sit for weeks between drives. Lately I've been adding Techron additive to the tank every couple of months. Hopefully this will solve my problem as Chevron has advertised that Techron will clean fuel level sensors. Time will tell. At one time, and it might still be the case, the fuel system additive sold by BMW was Techron.
