OBC MPG Calculations

[rogue38]

I have installed a 13 button OBC in my 90 325i. The OBC came out of an 325e, so I changed the coding plug to the proper 325i plug.

After a few tanks and a long drive to Georgia and back I have noticed the OBC MPG calculations are almost 3 mpg lower than the miles divided gallons method. The manual way seems to be pretty close to what they always have been.

Also the range to empty acts funny. With more than a 1/4 tank indicating the miles to go drops to 21m. It seem very inconsistent as the tank goes from full toward empty. Not just the usual variations with driving changes.

I know the calculation go through the SI board and there are some issue with the board. My Speedometer, Tach, Fuel and Water Gauges work fine, but the inspection/oil service and brake lining indicator are messed up.

Is this most likely and issue with the SI board or should I look elsewhere. My e30 is purely stock, no modifications.

[Gregs///M]

There will always be some variation between actual vs calculated MPG. 3 MPG difference is not to be concerned about.

[rogue38]

OK, thanks Greg. The engineer in me thinks that 10% is a bit excessive. The range though is quite strange though.

[AndrewBird]

Not sure on E30s, but E34s actually have an adjustment factor you can set. There is a secret menu you go into where you can look at and tweak all kinds of things.

[AndrewBird]

Just found this after a quick search:

"Inaccurate Fuel Range
The range function can be wildly optimistic at times, and needs to be recalibrated to give more realistic readings. To do this, brim the fuel tank and then remove the plastic grommet from the bottom left of the obc. Press the 1000 and 1 buttons at the same time to give a reading of how much fuel is in the tank and then turn the screw behind the grommet until it reads whatever capacity your tank is when full."

From here:

http://www.e30zone.net/e30zonewiki/index.php/On-board_Computer

[15Michaeljoseph]

^You'll have to convert from gallons to liters to do that correctly btw.


--Mike

[rogue38]

Thanks, I will check and adjust the the tank capacity setting.

[MoreMayhem]

I tried that procedure with my e30 and it didn't work. Maybe I have the wrong coding plug. Tank is the later type, 63 liters I believe. The adjustment on my obc wouldn't go above 56 liters.

[g.ruff561]

I know that this question has been sorted a number of times but while the topic is open about MPG. Could we touch on the subject of how much gas is left in the tank when the fuel light is illuminated. Gallons / liters........ Thanks...........G.

[rogue38]

Well I brimmed the tank and went to adjust the amount of fuel. I have the 62 liter tank but the OBC was set to 54L. So I adjusted the screw, but the most the OBC would go to is 55L.

[15Michaeljoseph]

I know that this question has been sorted a number of times but while the topic is open about MPG. Could we touch on the subject of how much gas is left in the tank when the fuel light is illuminated. Gallons / liters........ Thanks...........G.

not sure how many gallons/ liters but i believe it comes on when the 'range' function gets to be around 30 miles... iirc


--Mike

[Gregs///M]

Low fuel light comes on when less than 6 liters in tank as per ETM manual.

[rogue38]

Well after a road trip and burning 3/4 of a tank the OBC still said I had 43L left. I then went to fill up and before pumping I rechecked the OBC and it read 52L left. After pumping the tank to the brim the OBC showed 51L. There is definitely some input from the SI board and/or the Instrument cluster that is not right. Probably look at replacing or having them rebuilt.