guno89

89 convertible 6 speed. This is a very low mile car 41,500.

Lately my gas mileage has been bad, I haven't been driving the car to much, so im not sure when this started. ( I hurt my leg, it's healed now so driving stick shift more than 10 minutes presented to be a challenge.)

Today the car said 40 miles to empty, I drove around maybe 12 miles street driving almost no lights it was down to 10 miles. I went to get gas I filled her up and she took 16 gallons. The car really has 4 gallons left and should of gone 80-100 miles. Weird thing is the ECM is about a year old and my old one was bad it was causing sever idle issues. the car runs like a champ except for the ECM putting out false MPG readings. I will track the old fashioned way this tank.

CMiller95

My 94 does the same thing. After you get to a low enough mileage left it displays reserve instead of miles. I've just assumed the "reserve" is defined by four Gallons being left. It's more a safety factor to tell you there is a lower mileage left. Once mine hits reserve it takes 16 gallons to fill up

Tom400CFI

The miles left to empty feature isn't very accurate on my '92 either. I never use it.

antfarmer2

ran mine out of gas once still said I had 48 miles

whalepirot

You may be seeing an accurate computation, based upon a moving average consumption, but the quantity is faulty. I cannot recall any time since I bought my car in '85 that the fuel gauge was real reliable.

What did the quantity show? I bet that's the root. If so, there's a thread here that has a fix.

guno89

I had 1 green bar!

Tom400CFI

One thing has always puzzled me; why they use the sending unit, in the factors to determine range. If you notice, if you can get your gauge to go UP or DOWN (by braking, accelerating, etc), your range changes correspondingly. This is a bummer, b/c the sending unit is not accurate (it's not linear) nor is it repeatable, IMO. I wonder why they dont just used "fuel used" calc's, subtracted from a "full tank", times ave mpg?

Our '05 Caddy functions the same as my '92 'Vette -the gauge is a factor (if not the main factor) in the range calculation. Also like the 'Vette, the Caddy's sender is totally unreliable in it's low range. You can park the car on "E", w/"LOW RANGE" being displayed on the DIC, then come out later, start the car, and the gauge now reads 1/4 tank, and you have 80 mile range! Stupid! What's worse is the Caddy actually shows you "Fuel Used" in the DIC options, and does NOT use the sending unit in this calc; only inj. pulse width data. So that calc is extremely accurate. SO, the car has a virtually worthless "range to empty" feature, but all the data to calc it very accurately. Since it won't, I have to; when low, I scroll to "Fuel Used", see how many gal's I've burned, I know it's a 17.5 gal tank, I know what I've got left, and I know my ave MPG...there is my REAL range to empty!

I believe the ('92 at least) has "fuel burned info in the CCM/BCM, since it calculates engine oil life. I believe that fuel burned is one of the factors in that calculation.

That was a rant w/no purpose.

Dakotaz

Agreed the gauge will go up and down on range and fuel in the tank when it gets low. Also I have noticed that the miles per gallon it shows on the gauge is low when I figure it with a calculator using gallons to fill and miles driven. Didn't buy it for the mileage but for the fun...

aklim

But if you pushed the car 48 miles, it might get you to the gas station.

aklim

IIRC, it is a separate computer that goes off RPM and injector pulse width with the size of injector programmed and fuel level gauge. Since the gauge isn't linear, I don't think it will work that well. RPM is probably accurate as is pulse width but size of injector didn't change when I went to 42pph so it isn't accurate.

aklim

How would you know how much fuel is left WITHOUT data from the tank sender?

FASTAZU

Many years ago I was bored and wanted to find out the quantity it took for each bar. The fuel bars are not liner the larger bars at the top represent about 3-4 gals. and the smaller at the bottom as low as 1 gal. Just fill up at a 1/4 tank and never worry about running out..

aklim

I have run it to reserve and some more to see how far it goes. Wife is of course anxious but such is life. I never fill up at a quarter unless I am in a strange area and don't know where gas it. OTOH, I have a GPS that can find gas so I know where to get it. Might not be 93 octane but unlike diesel, I never found a filling station without gas.

And no, I don't believe the old wives tales about how much cooling the pumps need from the fuel around it, etc. etc. I would say the fuel running thru the pipe cools it though. Besides, if there was something so hot in the tank that it needs fuel sloshing around to cool it, I'd have removed it and put an external pump because it would be constantly vaporizing the fuel. IF there is no fuel, can the pump burn? I doubt it since the engine would be off and there would be no power to the pump

Tom400CFI

Easy; Tank is 17.5 gallons. ECM and I know that 15 gallons has been shot through the injectors. 17.5-15 = 2.5 gallons left. Car averages 22 mpg, so my ranges is 55 miles.

This method works really, really well.

aDigitalPhantom

I don't know about your tank, but the one in may car is not 17.5 gallons. The lowest I let it get took 18.5-19 gallons to fill it back up. Also according to page 311 in the 93 owners manual it is a 20 gallon tank.

whalepirot

I use my motorcycle-taught technique, as it has only a 'reserve' light: push the trip odometer reset at each refill. this also gives me a rough idea of mileage.

The problem I see with computer driven range is the assumption that the tank is filled each time; not always true. The current methodology, used by all manufacturers, AFAIK, is the same. The crux is an accurate quantity sender. The one thing BMW provided, outside of a their sensor that determines when your wallet has repair money available, is a very accurate sending unit. The MTG is, therefore, very accurate and I've become comfy running the '89 vintage car down to reserve.

The Vette has me looking for premium below half a tank!

FASTAZU

The fuel tank capacity was changed sometime in the 90s by adding a longer filler neck, Don't remember why. But if I remember right it was by 2-3 gals.

aklim

And if partway through a tank you fill up?

aDigitalPhantom

This is about 2-1/2" tall from the top of the tank. I don't see how you could make it longer with out making the tank smaller. You would also need to make the filer neck a few feet longer, or about 1-2+ feet in diameter to hold 2-3 gallons.

PS
Also I understand the aftermarket tanks you can get from various places are 17ish gallons, but the factory tanks are not.

PSS
I had a 2008 Impala for a while and the range/mileage was normally wrong on that too. It was more accurate then my C4 when this worked, but still wrong.

QCVette

I always thought they were 20 gal. tanks.

The parts manuals that I have show all from '84 to '96 used part number 14048936 tanks.

I have owner's manuals from '87 and '93 to '96 that all say 20 gal.