Vacuum Guage and Digital Fuel Gauge - GasSavers.org

krousdb · 02-05-2006, 04:29 PM

In my intro, SVOBoy asked me to post more info on the vacuum gauge and digital fuel gauge install.

The 2 5/8" Autometer Vacuum gauge is installed in a pod on the A pillar.

I tapped into the vacuum line that runs between the throttle body and the valve on the fuel rail.

When fully warmed up, the car idles at about 650 rpm, 22-23 in/hg. When driving, I try not to drop below 10 in/hg during acceleration and I upshift at 15 in/hg. When going downhill with foot off of the accelerator, the gauge shows 25 in/hg. According to the shop manual, the ecu shuts off the fuel injectors when the throttle is fully closed and the engine rpm is above 870. I try to take advantage of that as much as possible.

Again, during accel I shoot for 10 and upshift at 15 in/hg. In Pittsburgh, it is quite hilly so I find myself approaching uphill climbs in 4th gear at 10 in/hg, then downshifting to third and holding 10 in/hg. Some hills require 5 in/hg to hold speed but I have never had to drop to 2nd gear.

On long downhill stretches I typically shut off the engine completely and coast for 1 to 2 miles. On my drive to work, I have three occasions when I can coast for about 2 miles so this contributes to the 50 MPG tanks. The trip back only has one 2 mile coast. I have spent lots of time optimizing my commute to maximize fuel efficiency. My route is slightly different for the del Sol than for the Prius because, well it would take lots of typing to explain. Suffice it to say that much of my fuel efficiency can be attributed to staying off of the interstate and using the service roads that run along side it. The speed limits on these side roads range from 25 to 45 mph. I do spend about 6 miles of the 25 mile commute on the interstate where the speed limit is 55. This alternate route takes an additional 5-10 each way, but the distance is actually shorter by a fraction of a mile.

Here is a picture of my digital fuel gauge. It is a DMM set to ohms which lives in the center console. In del Sols, you have to remove the rear storage compartments to get access to the fuel pump/sensor. In civics, just remove the back seat. There are two sets of wires, one to the fuel pump and one set to the fuel sensor. I just cut the fuel sensor wires and extended (by splicing more wire) them into the center console, basically adding 2 ft of length. In the middle of the wire extension I cut them and reconnected them using a connector. I also installed a matching connector on the DMM. So I can use the analog gauge in the dash by connecting the two connectors on the extended wires. Or I can use the digital gauge by connecting the DMM to the connector that goes to the fuel sensor.

As you can see from the picture, the resistance is at 2.7 ohms, which is nearly full. At 2.0 ohms, the sensor is maxxed out. I always fill to 2.5 ohms to get the most accurate tank MPG calculation. This is much more accurate than guessing or topping off.

Here is the fuel sensor map that I have developed by filling my tank in 1/4 gallon increments on three separate occasions. I take a reading before leaving work every morning to get an idea of how my tank mileage is progressing. My garage floor is perfectly level so I get a good reading every time. I also use the same pump to fill the tank and the ground is level there also.

So far I am able to use this method to determin e tank MPG and can get within 1 MPG. It ain't perfect, but it helps.

Does anyone have an OZTrip Computer? That would make the Vacuum Gauge and Digital Fuel Gauge unnecessary.

Dan

SVOboy · 02-05-2006, 04:51 PM

VERY COOL!

How cheaply do you think I can get a DMM that will serve this purpose? Also, how small do they come? I'm wondering now if someone more electronically inclined could turn that display into something a bit more self contained (could be mounted on the a pillar or something)...

Very very cool. The line you tapped for the vacuum gauge actually goes from the fuel rail to intake manifold though, if I'm looking correctly.

One of the things that can be done with chipping is to use your CEL light as a shift indicator so that instead of reading the gauge you will see the flash at like 2500 and know to shift.

MetroMPG · 02-05-2006, 05:35 PM

Quote:

Originally Posted by SVOboy

How cheaply do you think I can get a DMM that will serve this purpose?

if you watch for sales at your favourite parts store, you can get them cheap. i've bought a couple at $9 each over the past few years. the ones i got normally retail for $30-$40 (all figures $CDN @ .85 $US). door crasher type deals.

you could probably eBay yourself a good deal also.

ps - nice work, dan.

SVOboy · 02-05-2006, 05:41 PM

There are mini ones on ebay for 9 shipped,

, I'll get one soon once I finish closing down my ecu business.

JanGeo · 02-05-2006, 05:52 PM

Harbor Freight they go on sale once in a while and they also give them away for free on $40 orders sometimes. Usually you can get them for $2.99 - I have purchased a lot of them over the years for my scooter projects and once calibrated they hold up pretty well and do not drift much at all. Once I figure out the resistor network inside that is needed to make it function as a 20 volt meter I will be able to reduce the unit down to the display and battery size.

krousdb · 02-06-2006, 03:25 AM

Quote:

Originally Posted by SVOboy

VERY COOL!

How cheaply do you think I can get a DMM that will serve this purpose? Also, how small do they come? I'm wondering now if someone more electronically inclined could turn that display into something a bit more self contained (could be mounted on the a pillar or something)...

Very very cool. The line you tapped for the vacuum gauge actually goes from the fuel rail to intake manifold though, if I'm looking correctly.

One of the things that can be done with chipping is to use your CEL light as a shift indicator so that instead of reading the gauge you will see the flash at like 2500 and know to shift.

I have 2 more just like it I bought on ebay. I would give you one but it needs a mini 12V battery, which costs more than the DMM costed, about $5 at radio shack. You are probably better off just getting the one on ebay for $9 shipped. Just a few more notes about the digital fuel gauge, you can't use the 12V from the car to power the DMM, you get bad readings. Also, you get bad readings when the ignition or accessories are on. You need to turn the ignition off completely before a repeatable reading is obtainiable. I just take a look every morning before work, because the car is usually cooled to the same temp. As you know, temp also effects resistance readings.

krousdb · 02-07-2006, 06:20 AM

I just thought I would track this tank on this thread to give an idea what the data looks like and how the accuracy turns out. On Monday morning, 2/6 I was at 2.8 Ohms, 6 miles. This is slightly less than a full tank as i had to drive bach home from the gas station. This morning I was at 10.9 Ohms. Now the first gallon or two is not that linear so the data is probalby off a little. Here is the MPG calculation.

10.9 is 1/3 of the way between 10.5 (1 gallon used) and 11.7 (1.25 gallons used). So I calculate 1 + (.25/3)= 1.08 gallons used. The trip odometer shows 56 miles but I need to add 3.5% for the larger tires which I calibrated over a 50 mile stretch of I -79. So 1.08 gallons used for 56*1.035=58 miles equals 53.7 MPG.

I will be back tomorrow with another update.

krousdb · 02-08-2006, 05:46 AM

Quote:

Originally Posted by krousdb

I just thought I would track this tank on this thread to give an idea what the data looks like and how the accuracy turns out. On Monday morning, 2/6 I was at 2.8 Ohms, 6 miles. This is slightly less than a full tank as i had to drive bach home from the gas station. This morning I was at 10.9 Ohms. Now the first gallon or two is not that linear so the data is probalby off a little. Here is the MPG calculation.

10.9 is 1/3 of the way between 10.5 (1 gallon used) and 11.7 (1.25 gallons used). So I calculate 1 + (.25/3)= 1.08 gallons used. The trip odometer shows 56 miles but I need to add 3.5% for the larger tires which I calibrated over a 50 mile stretch of I -79. So 1.08 gallons used for 56*1.035=58 miles equals 53.7 MPG.

I will be back tomorrow with another update.

End of Day #1, 58 actual miles, 11.7 Ohms, 1.08 gallons, 53.7 MPG
End of Day #2, 110 actual miles, 15.9 Ohms, 2.125 gallons, 51.6 MPG

krousdb · 02-11-2006, 12:09 PM

End of Day #1, 58 actual miles, 11.7 Ohms, 1.08 gallons, 53.7 MPG
End of Day #2, 110 actual miles, 15.9 Ohms, 2.125 gallons, 51.6 MPG

Two days sick, no driving.

End of day #3, 163 actual miles, 20.6 Ohm, 3.125 gallons, 52.3 MPG

SVOboy · 02-11-2006, 12:17 PM

Looks like a good tank so far.

I just found a DMM in my basement, think it's my teachers but I wanna hook it up before I return it to make sure I can get the hook up working well for when I buy my own DMM (it's only 9 bucks after all).

Do you have any pictures of how it's wired into the fuel sender thing?

krousdb · 02-11-2006, 02:04 PM

Quote:

Originally Posted by SVOboy

Looks like a good tank so far.

I just found a DMM in my basement, think it's my teachers but I wanna hook it up before I return it to make sure I can get the hook up working well for when I buy my own DMM (it's only 9 bucks after all).

Do you have any pictures of how it's wired into the fuel sender thing?

No pics. I would have to take all my trip pieces out to get to it. But you don't need pics. Especially if you don't want to switch between analog and digital guage all of the time. Just locate the fuel pump and sending unit. Access should be from the inside of the car directly on top of your fuel tank. If you can't tell which is which, the fuel pump is the one that buzzes for two seconds when you turn your ignition on. You want the other one which is the sending unit. The sending unit also has smaller gauge wires than the fuel pump.

Once you find the two wires going into the sensor cut both wires, leaving yourself some room to solder them back together if you want to restore the analog gauge. Next, connect the DMM leads to the cut wires that go to the sending unit and set to 200ohm setting. You should be at 2 - 5ohm for a full tank and around 130ohms for an empty tank, according to the 92-95 civic shop manual. I have never had it higher than 110 ohms however.

You should do your own calibration since your fuel tank may be a different size or shape. I started with an empty tank, dead on the "E". I added gas in 1/4 gallon increments, taking a reading afer a few seconds of letting the reading stabilize. When you get close to 10 ohms, it only takes a small squirt to move it an ohm or two. Continue small squirts until a squirt fails to change the reading, this is what I call the full point, because I can duplicate that exact level on each fill, ensuring accurate tank MPG calcs.

Now all of this is based on the 92-95 civic. Refer to your CRX shop manual as there may be differences in locations and sensor range.

And one more thing. Readings between DMM's will vary. If your teacher's is high quality and the ebay is low quality (most likely) then you may see a few ohms difference. The ebay one will be fine, just be sure to calibrate with the ebay DMM.

SVOboy · 02-11-2006, 02:23 PM

Thanks, I'm pretty sure I know what it is and since I've got no interior getting to it isn't a problem.

Matt Timion · 02-11-2006, 07:10 PM

Just as an alternative, a digital panel meter might be better than hooking up a DMM to your car. A digital panel meter could easily be made to fit and be more permanent.

And they're rather cheap too.

http://search-desc.ebay.com/Digital-panel-meter_W0QQbsZSearchQQcatrefZC6QQcoactionZcompareQQ coentrypageZsearchQQcopagenumZ1QQfposZ84104QQfromZ R10QQfsooZ2QQfsopZ2QQfstypeZ1QQftrtZ1QQftrvZ1QQfts Z2QQmppfqyZQ28n600Q2cQ20an600Q2cQ20z600Q2cQ20az600 Q2cQ20Q22hondaQ20600Q22Q2cQ20Q22nQ20600Q22Q2cQ20Q2 2zQ20600Q22Q29Q20Q2dcbrQ20Q2dbluetoothQ20Q2dmotoro laQ20Q2dsonyQ20Q2dnorcoldQ20Q2dQQsadisZ200QQsargnZ Q2d1QQsaslcZ2QQsatitleZQ22DigitalQ20panelQ20meterQ 22QQsbrftogZ1QQsofocusZunknown

http://www.action-electronics.com/panelmeters.htm#Digital

I would do this in a heartbeat if someone could convert the readout to be a percentage of the tank left instead of the voltage.

krousdb · 02-11-2006, 07:27 PM

Quote:

Originally Posted by Matt Timion

Just as an alternative, a digital panel meter might be better than hooking up a DMM to your car. A digital panel meter could easily be made to fit and be more permanent.

And they're rather cheap too.

http://search-desc.ebay.com/Digital-panel-meter_W0QQbsZSearchQQcatrefZC6QQcoactionZcompareQQ coentrypageZsearchQQcopagenumZ1QQfposZ84104QQfromZ R10QQfsooZ2QQfsopZ2QQfstypeZ1QQftrtZ1QQftrvZ1QQfts Z2QQmppfqyZQ28n600Q2cQ20an600Q2cQ20z600Q2cQ20az600 Q2cQ20Q22hondaQ20600Q22Q2cQ20Q22nQ20600Q22Q2cQ20Q2 2zQ20600Q22Q29Q20Q2dcbrQ20Q2dbluetoothQ20Q2dmotoro laQ20Q2dsonyQ20Q2dnorcoldQ20Q2dQQsadisZ200QQsargnZ Q2d1QQsaslcZ2QQsatitleZQ22DigitalQ20panelQ20meterQ 22QQsbrftogZ1QQsofocusZunknown

http://www.action-electronics.com/panelmeters.htm#Digital

I would do this in a heartbeat if someone could convert the readout to be a percentage of the tank left instead of the voltage.

Problem is that the sensor output is not linear. Yiu would need to set up a lookup table. If that could be done, sign me up for one.

Matt Timion · 02-11-2006, 07:35 PM

Quote:

Originally Posted by krousdb

Problem is that the sensor output is not linear. Yiu would need to set up a lookup table. If that could be done, sign me up for one.

Ah, I was under the impression that the output was linear. If it was then the conversion equation would be REALLY easy to do. Assuming 2.6Volts was "full" and 0 volts is "empty, the percentage would just be "(x/2.6)*100," or just "38.46x," where "x" is the voltage.

If it's not linear, however, I have no idea. Perhaps some of the electronics guys on this site want to make a LITTLE extra money and whip something up for us? Huh? Anyone?

krousdb · 02-12-2006, 06:03 AM

Quote:

Originally Posted by Matt Timion

Quote:

Originally Posted by krousdb

Problem is that the sensor output is not linear. Yiu would need to set up a lookup table. If that could be done, sign me up for one.

Ah, I was under the impression that the output was linear. If it was then the conversion equation would be REALLY easy to do. Assuming 2.6Volts was "full" and 0 volts is "empty, the percentage would just be "(x/2.6)*100," or just "38.46x," where "x" is the voltage.

If it's not linear, however, I have no idea. Perhaps some of the electronics guys on this site want to make a LITTLE extra money and whip something up for us? Huh? Anyone?

Take a look at the chart that was included in the first post. The sensor is quite linear from the full to half tank mark at about 4.8 ohms/gallon. But after that you can see a more of curve. You probably notice this on your analog gauge also.

Lets just keep our fingers crossed that the SuperMID will work with non-toyotas. If so, in addition to showing tank, trip and lifetime MPG, it shows fuel used in the lower right hand corner, and is extremely accurate, as is the ODO after calibration. It takes awhile to get used to the metric display. But just remember to multiply km/L by 2.35 to get MPG. So 50 mpg would be 21 - 22 km/L. The fairly commom 11.9 gal fuel tank is 45L. The 15.9 tank is 60L. Didn't you ever wonder why the odd tank size, well it's not so odd if you convert to liters.

JanGeo · 02-12-2006, 06:24 AM

The meter should have a calibration but it may be automatic - however all you need to do is go through a pot to adjust the voltage range so full tank reads 100 on the meter. Hell calibrate it to gallons!!

krousdb · 02-12-2006, 06:31 AM

End of Day #1, 58 actual miles, 11.7 Ohms, 1.08 gallons, 53.7 MPG
End of Day #2, 110 actual miles, 15.9 Ohms, 2.125 gallons, 51.6 MPG
End of day #3, 163 actual miles, 20.6 Ohm, 3.125 gallons, 52.3 MPG
End of day #4, 182 actual miles, 23.2 Ohm, 3.675 gallons, 49.4 MPG

OUCH! Running errands sucks. Lots of short trips, but only one cold start. So calculating backwards, my MPG for day 4 was around 38. I hope the WAI didn't cause any of that.

krousdb · 02-13-2006, 05:03 PM

End of Day #1, 58 actual miles, 11.7 Ohms, 1.08 gallons, 53.7 MPG
End of Day #2, 110 actual miles, 15.9 Ohms, 2.125 gallons, 51.6 MPG
End of day #3, 163 actual miles, 20.6 Ohms, 3.125 gallons, 52.3 MPG
End of day #4, 182 actual miles, 23.2 Ohms, 3.675 gallons, 49.4 MPG
End of day #5, 235 actual miles, 27.6 Ohms, 4.563 gallons, 51.6 MPG

Whew! Back over 50. Day 4 reading could have been a fluke.

krousdb · 02-14-2006, 05:49 PM

End of Day #1, 58 actual miles, 11.7 Ohms, 1.08 gallons, 53.7 MPG
End of Day #2, 110 actual miles, 15.9 Ohms, 2.125 gallons, 51.6 MPG
End of day #3, 163 actual miles, 20.6 Ohms, 3.125 gallons, 52.3 MPG
End of day #4, 182 actual miles, 23.2 Ohms, 3.675 gallons, 49.4 MPG
End of day #5, 235 actual miles, 27.6 Ohms, 4.563 gallons, 51.6 MPG
End of day #6, 286 actual miles, 33.8 Ohms, 5.583 gallons, 51.2 MPG
End of day #7, 339 actual miles, 39.1 Ohms, 6.313 gallons, 53.7 MPG

Nice and warm today, 44F in the AM and 55F in the PM

MetroMPG · 02-16-2006, 12:34 PM

glide question, krousdb...

i'm sure you've calc'd this already, but i'm curious to know: how much of your normal daily commute happens with the engine shut off?



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