DIY Fuel rate meter/injector duty cycle meter/GPH - Fuelly Forums

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Old 06-01-2008, 07:17 AM   #1
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Lightbulb DIY Fuel rate meter/injector duty cycle meter/GPH

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:
http://www.gassavers.org/showthread.php?t=5233
http://www.gassavers.org/showthread.php?t=4250
http://opengauge.org/diympggauge

If you want to combine fuel rate with vehicle speed to get MPG, see these threads:
http://www.gassavers.org/showthread....32#post6473232
http://www.gassavers.org/showthread.php?t=4250
They talk about using laptop computers, and the second one also includes some circuit diagrams.

Extremely cool project with really great information about OBD protocols, connectors, FE calculations, and how to put a MPG gauge in for cheap:
http://www.circuitcellar.com/avr2004/first.html

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...
http://www.gassavers.org/showthread.php?t=4258

If this is pasted to another forum or otherwise sent elsewhere, here's the link to the original (and please leave the link in):
http://www.gassavers.org/showthread.php?t=7714

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.
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Old 06-01-2008, 07:31 AM   #2
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I've already beat my previous best MPG in both of my main vehicles due to this meter. Things I've managed to learn about my vehicles with it:

2002 GMC 5.3l:

- DFCO (Deceleration Fuel Cut Off) is only useful if you can engine-brake for 30 seconds or more. It takes 6 to 10 seconds to get into DFCO but does hold it down to ~1100 rpm.

- Due to that DFCO issue, using N is far better than coasting in D at any time. Not only does coasting in D slow the vehicle down, but even with my foot off the gas the computer must inject more fuel at higher RPM to keep the air/fuel ratio correct.

- While stopped (at traffic lights and such), idling in N uses a very tiny bit more fuel than idling in D, but is worthwhile.


2008 Volkswagen 2.5l I5:

- DFCO is very aggressive whenever I'm above 40mph and will continue down to 1000rpm. If I'm going below 35mph when I take my foot off the gas, it will almost surely NOT go into DFCO.

- Like the GMC, when not in DFCO and foot off the gas, it still has to use more gas than if it's in neutral, to keep the air/fuel ratio correct...and of course still slows the car more.

- WOT (Wide Open Throttle) actually uses less fuel than 90% of the gas pedal travel while still making more power. It is actually more efficient with my right foot all the way to the floor than if I back off a little. This is useful knowledge for someone who uses higher gears and wide throttle openings to reduce pumping losses. (Note: The car is Drive By Wire and the gas pedal isn't physically connected to the throttle, but the data is as observed.)

- When I combined this meter with Ross Tech Vag-Com OBDII data collection, I found that the engine does not enter open loop under any condition, even at WOT. It only uses open loop for about 30 seconds after a cold start, and when in DFCO.
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Old 06-03-2008, 04:03 PM   #3
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N Coast

Quote:
Originally Posted by theholycow View Post
I've already beat my previous best MPG in both of my main vehicles due to this meter. Things I've managed to learn about my vehicles with it:

2002 GMC 5.3l:

- DFCO (Deceleration Fuel Cut Off) is only useful if you can engine-brake for 30 seconds or more. It takes 6 to 10 seconds to get into DFCO but does hold it down to ~1100 rpm.

- Due to that DFCO issue, using N is far better than coasting in D at any time. Not only does coasting in D slow the vehicle down, but even with my foot off the gas the computer must inject more fuel at higher RPM to keep the air/fuel ratio correct.

- Idling in N uses a very tiny bit more fuel than idling in D, but is worthwhile.
I too became a big fan of N coasting vs. D coasting with my 2004 5.3L Yukon XL when experimenting down a 2-mile hill. The onboard computer showed 99 mpg in neutral vs. 55 mpg in drive. I'm sure the actual mpg is significantly better than 99 as coasting in N as much as possible improved my city driving by 2 mpg, a huge percentage for these vehicles.

(Caveat: My front diff blew out recently and I just don't know if there's any way it might be related to this technique. I never noticed any obviously stressful situations when re-engaging drive and the tranny seems fine.)

I took your comment about idling consumption to be a rolling idle comment. At a stop, have you found idling in N uses more or less than idling in D?
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Old 06-03-2008, 05:25 PM   #4
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As I mentioned in the other thread, it's impossible for your diff to have gotten messed up by neutral coasting with the possible exception of if you were in 4wd when doing it.

The comment about idling was while stopped. I will edit it, as I really should have said that. While rolling, there's nothing "tiny" about the difference between N and D.
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Old 06-05-2008, 05:47 PM   #5
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As requested in another thread, photos. Click on the photo to see it at PicasaWeb where you can go to view it in a larger size.

A fuel injector wire in the VW:


The meter hanging on my dash, left-center in this photo:


In my truck (look at the red wire):


The red wire where it meets the injector:


Close-up of the injector wire:


Where the wire goes in to my dash. It comes out at the steering wheel, though I could also have it come out at the center console.
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Old 06-06-2008, 05:44 PM   #6
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Oops, I didn't notice this until just now. Cow, you did a really nice job of putting a lot of good info in one place. Thanks for doing that, and thanks for mentioning me! I was just sharing bits and pieces I've learned in various forums like this one.

You're coming up with lots of interesting observations, and I look forward to hearing more about what you're discovering.
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Old 06-11-2008, 01:43 PM   #7
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Great idea. Any thoughts on using this set-up with a throttle body injector?
('92 Tracker) I haven't investigated the system thoroghly yet. I have the same meter. Just thought I'd ask.
Thanks
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Old 06-11-2008, 05:24 PM   #8
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Either monroe74 or fumesucker could probably answer that question better than I can. It ought to work with TBI fine, I think. Since you already have the meter, all you have to invest is a few minutes of your time...I'd love to hear if it works for you.
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Old 06-11-2008, 05:46 PM   #9
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Thanks for the compliment, but I don't think I know any better than you. I figure it's worth a try.
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Old 06-12-2008, 08:48 AM   #10
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I assume I need to try to find a signal for an individual cylinder, just as you did for the injector? It has a Camshaft Position Sensor which pulses and breaks for each cylinder. I guess I'll just need to play with it.
Thanks for the help.
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