This technique was presented to me
by user monroe74 on the realtime monitoring tools for pre-obd2 cars
thread. Thank you, monroe74! See also fumesucker's work measuring the same thing with his laptop computer sound card
, which allows him to graph and log the data.
More with the sound card, using programs to calculate MPG:
If you want to combine fuel rate with vehicle speed to get MPG, see these threads:
They talk about using laptop computers, and the second one also includes some circuit diagrams.
cool project with really great information about OBD protocols, connectors, FE calculations, and how to put a MPG gauge in for cheap:
Another idea is to use an OEM MPG display or DIC (Driver Information Center) made for your model or a similar model, or one that might be adaptable to dissimilar models...
If this is pasted to another forum or otherwise sent elsewhere, here's the link to the original (and please leave the link in):
I've mentioned this in many threads, but I think it deserves its own thread. The result is a meter/gauge that shows you in realtime just how much fuel is being used. It's different from an instant MPG gauge, which shows you fuel rate compared to distance. This is a better way to directly and immediately compare one strategy to another in some circumstances -- for example, determining when you're in DFCO (Deceleration Fuel Cut Off) or if more fuel is used at a given speed in one gear than another. It also gives you fuel usage information when the vehicle isn't moving, such as at idle, which you won't get from an instant MPG reading.
1. Get a duty cycle meter, or a digital dwell meter. I have not seen a cheap/easy/appropriate duty cycle meter, but Harbor Freight
sells a digital multimeter (DMM) with Dwell function for $33
(plus $8 shipping if you can't get to one of their stores). That one has a very large display and is built well, though if you leave it under your windshield on a hot sunny day the label will peel.
You need a digital meter because you will need to easily see very small, precise numbers as well as larger numbers. It's tough to see such different scales on an analog display.
2. Tap one of your fuel injector wires and run the wire to wherever you'll want the meter (I ran mine near the center console). It doesn't matter which injector, but you must use the injector's positive wire. If you tap it with something like a 3M Scotch-Lok QuickSplice, be careful not to damage the wire. To avoid that, you can just disconnect the injector's electrical connector (either from the injector itself or the ECU, whichever is easier to reach), jam the stripped end of your wire into the female end of that connector, and re reconnect.
3. Connect the injector extension wire to the positive lead of the DMM. Connect the DMM's negative lead to ground (I used one of my car's power outlets).
4. Set the DMM on Dwell for 4 cylinder engines, regardless of how many cylinders you have. We're not actually measuring dwell (which is adjustment of old-fashioned points and condensor ignition systems); we're using it as a duty cycle meter. The 4 cylinder provides a 0 to 90 scale, which is almost exactly the 0 to 100% scale required.
5. Start the engine and look at the meter. You are looking at fuel injector duty cycle information. It is off by a relative 10% -- so if it reads 1.0, the actual measurement is 1.1; if it reads 20, the actual measurement is 22; if it reads 50, the actual measurement is 55. This is the percentage of time your injector is injecting; the balance of 100% is the time that it's closed.
If you want to translate that into an absolute fuel rate measurement, as in Gallons Per Hour, you've got some math to do. You need to find your fuel rail pressure and your injector size/flow rate, I think. I'm not entirely sure how to do this, as it hasn't mattered to me. I think it might be possible to get a decently accurate estimate by knowing the idle fuel usage, which you may be able to find online, and figuring that compared to your reading at idle.
For reference, warm idle on my 2002 GMC 5.3l V8 reads 1.0 to 1.5, and on my 2008 VW 2.5l I5 it reads something like 1.3 to 1.8. If you're idling and reading .1 or 10, you may have done something wrong.