Fuel Consumption

[Gawernator]

Recently filled up my tank to full after a drill weekend, was at about 1/4 or less after only five days of driving. Normally I can make it 1.5-2 weeks on one tank of gas. What could cause increased consumption, I have been driving about the same? I have in the past few months installed new spark plugs, rotor, cap, vacuum lines, ignition wires, Bosch O2 sensor, DME temp sensor, etc.. Could poor quality gas affect the consumption? Mostly use ARCO these days, but I did go to a no name place recently, I think.

[ckelly78z]

You will need to actually calculate your mileage by doing some math before we can answer .

Drive it a few days and then fill up the tank again and keep the receipt, and write down the mileage on your trip meter (assuming you reset it upon the last fillup.

Take the mileage and divide by the number of gallons used to get your fuel mileage for example; 300 miles divided by 10 gallons of fuel equals 30 MPG.

[Gawernator]

I would normally do that, only issue is my odometer/trip meter is currently non-op until I can get the time to pull out the cluster and fix that, and while in there clean everything up and swap out a new volt/combo gauge, also need to replace the oil pressure sender still, I went to do it last week and ended up rounding off the nut with the tensioning strut because some PO replaced with SAE instead of metric and I didn't realize it, now I have a new bolt/washer/nut and just need a day off to work on the car. In the meantime, I was just wondering if there was maybe a major part I had overlooked that could contribute to high consumption. A contributing factor probably is that my fuel gauge isn't totally reliable, I am guessing it's the fuel sender. The gauge can jump up and down slowly or quickly but overall goes from high to low.. Then again, once I'm done cleaning all the grounds, especially behind the dash, that might do the trick. Because my temperature gauge also moves with electrical load

[Bukowski]

Originally Posted by Gawernator

The [fuel] gauge can jump up and down slowly or quickly but overall goes from high to low..

the gas level shifts with centripetal force and the fuel level reading will vary accordingly (on my '87 944 na). this is very clear if you take a long highway off/on ramp that circles around.

[flash968]

winter gas does not get as good mileage as summer gas.

colder temps will drop your tire pressure and increase rolling resistance

you use more electrical accessories, and these cost fuel

rolling resistance increases in wet winter weather.

lower air temps mean higher air density which means more aerodynamic drag

it is not at all uncommon for you to have a 10% drop in fuel economy in the winter

[Gawernator]

It's more of a "hover around 3/4" issue but I'll see what happens

[kdjones2000]

Originally Posted by Bukowski

the gas level shifts with centripetal force and the fuel level reading will vary accordingly (on my '87 944 na). this is very clear if you take a long highway off/on ramp that circles around.

If you want to see how much the fuel level actually changes as you go around corners, here is an example although this is racing on a track:



The curves are in order of items on the right from top to bottom, with Distance not shown.

[Califred944]

On my 944 87na the mpg dropped off to 20 and had long crank to start. Turned out to be the Fuel Pressure Regulator was bad.

Took a nut from a fuel rail, drilled and taped with 1/8 pipe threads and installed a fuel pressure gauge at the front of the fuel rail. 28psi when running but dropped to 0 when engine was turned off. Should stay at 14psi for 20 minutes. Replaced the FPR and all is fine. Mileage is back above 20 and starts right away.

[Gawernator]

I did fill up my tires today, only two were low though. It's more than 10%, but I'm logging my mileage on this tank and it's decent so far.

[Gawernator]

Originally Posted by Califred944

On my 944 87na the mpg dropped off to 20 and had long crank to start. Turned out to be the Fuel Pressure Regulator was bad.

Took a nut from a fuel rail, drilled and taped with 1/8 pipe threads and installed a fuel pressure gauge at the front of the fuel rail. 28psi when running but dropped to 0 when engine was turned off. Should stay at 14psi for 20 minutes. Replaced the FPR and all is fine. Mileage is back above 20 and starts right away.

So, the FPR CAN affect fuel mileage? The PO of my car mentioned "it might need new fuel pressure regulators" and that was about the only advice he had. Not sure why he used a plural either, thought there is only one. The car has been running really well after all the time and parts I've put into it, but maybe a new FPR could make a difference?

[flash968]

an FPR would only affect fuel consumption if it were stuck open. it's possible, but pretty rare. you should see it at the tailpipe if that were the case. with unleaded fuel, your tailpipe should not be sooty. if it is, you are running rich.

a vacuum leak could cause a loss in fuel economy. perhaps one of your new lines is not seated, or you forgot one.

definitely run a tank and track the miles, then do it again.

[Guest765]

OP - when you changed the o2 sensor did you buy the one with the full plug and wire or the one that had bare wire ends you had to splice? If you bought the splice one, did you use the metal clips provided in the kit to splice the wire ends or did you do it another way?

[Gawernator]

I bought the Bosch Universal and used the "posilock" wire connectors, black to black, white to white.