16 replies to this topic

[nvexplorer]

I've always wanted to install a flow meter somewhere on the fuel system on my 93 Loyale to find out exactly how much fuel I'm using at any given time. Anybody do anything similar and know a cheap and easy way to do it? I've thought of monitoring the tps or even the fuel pressure regulator, but I don't think that I'd be able to actually monitor how much fuel is going through the injector. I'd only get an idea of how much should be going through after some calculations. Just an idea I've had brewing for a while. Thanks.

[chef_tim]

GL10 trip comp gives alot of that type info. You could try and put one in your current car or, easier, get a GL10. Tim

[Phizinza]

I don't see this working very well as the fuel line is a circuit flow, so you'd have a lot of fuel just flowing past and would cause an inaccurate measurement. If you measured after the regulator that takes the 20 odd psi for the injector I would be worried about it messing with the pressure and maybe giving it a pulse or dips in pressure.
Still, I could be wrong.. I, myself, just do mileage counts after filling up. Can't get much more accurate then that.

[Loyale 2.7 Turbo]

...I, myself, just do mileage counts after filling up. Can't get much more accurate then that.

Well... Me Too!

I Just Overfill the Fuel tank to its Limit, put the Miles Counter on the Dash on "0" and Do a Trip, at its End, I Refill the Tank and count how many Gallons where Used, then Calculatin´ how Many Miles I´ve done whith ´em, and That´s my Miles per Gallon Measurement.

Not very Accurate, but Since City / Road \ Highway Mileage and Drivin´ conditions Vary; it Gives me a Good Average.

For Example, my EA82 Weberized White Wagon, does around 40 Miles per Gallon in Highways, but just Around 30 in Cities...

Maybe this Idea could Help you...

[joostvdw]

if you really want to use a flow meter, you can mount one in the supply line and one in the return line, then calculate the difference

you just have to use some electronics to do this calculating for you and display the results

flow meters are expensive though, just thought I might warn you.

[nvexplorer]

I was looking into the flow meters and saw that they were expensive, so thats why its just a thought... I always calculate my mpg based on amount put in vs. miles on the trip meter, but I think it'd be interesting to see what is happening while I'm driving, and possibly to tune my driving habits for maximum fuel efficiency. Just by changing how I drive I went from 25-26 mpg to 28. I was also trying to take it easy on my practically bald winter tires. Great for grip on dry non sandy roads, but I'd hate to hit a rock or tear a hole in one while taking off. No worries about them anymore though, they came off a couple weekends ago. Just thought I'd throw that idea out there to see who else might have had a similar idea or theory on something that might work.

[DaveT]

I don't think that I'd be able to actually monitor how much fuel is going through the injector. I'd only get an idea of how much should be going through after some calculations.

Since the pressure is regulated, it shouldn't be a variable.
Since the manifold vacuum isn't what draws the fuel in, it should make a big difference.

Theoretically, if you measuring the width of the pulse going to the injector should tell exactly how much fuel went in. You just need a computer - analog or digital - to convert to a usable reading.

Longer the pulse = more fuel.

Pretty simple analog circuit to average out the pulses for a flow rate meter.

For MPG meter, it would be better to go digital = computer.

[4x4_Welder]

SPFI uses a constant fuel pressure, but multiport uses a varying pressure to keep a constant over intake pressure.
If you want and accurate meter, use a vacuum gauge. Less vacuum means more air in the intake, more air needs more fuel-
Other than that, just keep your foot out of it.

[ausubaru92]

Seeing that air fuel ratios are always constant, if you read the voltage of the Air flow meter, that would give you an indication of how much air and thus fuel you are using.

[robm]

I have been thinking about this too. It would be nice to have a near-instant fuel consumption read-out, so we can settle once and for all the question as to whether it is cheaper to coast down hill in neutral, or not. (I suspect it depends how steep the hill is.) There are plenty of other "tactical" driving decisions which may or may not save fuel, but but it is really hard to tell when averaged over a whole tank of gas.

I have thought of using the signal to the injector, as DaveT suggests. A simple RC circuit should convert this to an analog signal proportional to flow. There is also an input to the ECU from the VSS (vehicle speed sensor I presume), but I have no idea what this signal looks like, or how it could be converted to a usable signal for either a digital or analog computer.

Anyone out there with an oscilloscope who wants to have a look for us?

A digital computer would be the best, and perhaps easiest means of calculating the MPG, but I wouldn't mind playing around with an analog one, if I had some idea what the VSS signal output looked like.

[GeneralDisorder]

Seeing that air fuel ratios are always constant, if you read the voltage of the Air flow meter, that would give you an indication of how much air and thus fuel you are using.

Actually they aren't.

GD

[ausubaru92]

Actually they aren't.

GD

How so?
I know that they vary a bit, but fuel input must be proportional to air input otherwise it would run lean or rich and not run very well at all.

If i am wrong, please explain how cos i love learning about these sorts a things.

[msteel]

According to the diagrams I have, the vehicle speed sensor is a reed switch in the speedometer. One end goes to ground and the other to the ECU. I presume that the ECU has a pull up inside, so you'd get a digital waveform. It seems logical to expect that the frequency is proportional to speed, but I've never tried to view it with an oscilloscope.

[robm]

The CPU allows for a richer mix at full power. And there is still some air getting through when coasting with a closed throttle, even though the fuel is shut off.

Better to measure the actual fuel flow. It is probably easier than measuring the air flow signal, and trying to calibrate that back to fuel. At least we know what the fuel injector will flow when open, I don't believe we know how much air comes through the system.

The VSS signal would be about what I would expect. There is a simple tach chip that would turn it into an analog signal, very cheap, and been around for years.

[psylosyfer]

Question for "Loyale 2.7 turbo"
You get 40mpg? Is Honduras mostly level ?
I have never seen that kind of mileage out of 7 subaru's I've owned, But I cannot go ANYWHERE that's not up/down hill.

[robm]

I can get 38 MPG with my Loyale 4WD. It is hard work, and slow, but it can be done. I can see a carefully-driven, well-maintained 2wd with Weber tuned to the nuts being able to beat that. I remember Loyale 2.7 has also recommended disconnecting the secondary throttle linkage, to get better fuel economy. With that kind of an attitude, almost anything is possible!

[nvexplorer]

You get 40mpg?

I get 40 mpg consistently between Carson City, Nv and Markleeville, Ca. Thats also the only drive that I do get that kind of mileage. About 80 miles round trip. I'm not sure what the elevation change is, but it does take me into the Sierra Nevadas.