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Car Forum / Saab Cars / February 2005How does the CPU figure out how much gas?
Ok, this has probably been asked many times but does anyone have a good idea how the Saab computer figures out how much fuel is left and how much fuel gets added? Is their gas gage that good? > Ok, this has probably been asked many times but does anyone have a good idea > how the Saab computer figures out how much fuel is left and how much fuel > gets added? Is their gas gage that good? Well, the injection computer knows exactly how much gas it's injecting... They may do it that way, or they may just take an elapsed miles vs. percent of tank used calculation, similar to "download time remaining" calculations and dozens of other uses. I'd guess the latter. Dave Hinz >Ok, this has probably been asked many times but does anyone have a good idea >how the Saab computer figures out how much fuel is left and how much fuel >gets added? Is their gas gage that good? As Dave says, the ECU tells the SID how fast it's squirting fuel into the engine. It uses the same sender as the fuel gauge to find out how full the tank is. The MPG calculation only needs the ECU/speedo data and is quite accurate, but the DTE calculation needs the fuel sender info as well. Because of that, it isn't nearly so accurate - mine often reads more when I start up after parking on a hill than when I parked. Very handy as a rough guide though. Cheers, Colin. >It uses the same sender as the fuel gauge to find out how full the >tank is. The MPG calculation only needs the ECU/speedo data and is >quite accurate, but the DTE calculation needs the fuel sender info as >well. Because of that, it isn't nearly so accurate - mine often reads >more when I start up after parking on a hill than when I parked. Very >handy as a rough guide though. I'd noticed this effect - sometimes quite dramatically when the tank is close to empty, since in practice I use the DTE indication to see how low I really am once the 'low fuel' light comes on. I've also noticed the DTE increasing while on the move, particularly if you accelerate away from a stop and then cruise for a while. Martin > >It uses the same sender as the fuel gauge to find out how full the > >tank is. The MPG calculation only needs the ECU/speedo data and is [quoted text clipped - 8 lines] > noticed the DTE increasing while on the move, particularly if you > accelerate away from a stop and then cruise for a while. Indeed - any such system has to take into account current consumption as well as average. Therefore it can only ever be an estimate until the tank's nearly empty at which point you'll notice the low fuel indicator anyway. Graham For what it's worth, I check my gas mileage when I fill up, and the computer on the dashboard is always 8% optimistic compared with my measurements. So something is not calibrated right. Not that it really matters.  SignatureTom Reingold Noo Joizy > For what it's worth, I check my gas mileage when I fill up, and the > computer on the dashboard is always 8% optimistic compared with my > measurements. So something is not calibrated right. Not that it > really matters. Perhaps you are pessimistic Tom? ;-) Or perhaps those "joisey" pumps are calibrated on the optimistic side? That wouldn't surpise anyone, I don't think... -Fred W PS - good to see you posting. >>For what it's worth, I check my gas mileage when I fill up, and the >>computer on the dashboard is always 8% optimistic compared with my [quoted text clipped - 8 lines] > > PS - good to see you posting. I record and calculate every gas purchase, and each one is 8% worse than the car's efficiency, so no, I doubt every pump is ripping me off. SignatureTom Reingold Noo Joizy -- Tom Reingold Noo Joizy |
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