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Old 05-06-2008, 05:26 PM   #1
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Fuel consumption at idle?

Queston for those with scan guages, how much fuel do you burn per hour at idle? I read that for the average 4 cylinder it was about .25 gal per hour.

In my VX, if I read the thread correctly, it messes with the lean burn when you shut the engine off, so I leave it running. I wonder what the consumption per hour, at idle, is in a VX?

One different technique I use in mine is to use the tranny to slow down because it kills all fuel delivery, as long as you keep the revs above 1000.

regards
gary
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Old 05-06-2008, 06:06 PM   #2
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.25 or .3 seems about right.
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Old 05-06-2008, 06:38 PM   #3
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Quote:
Originally Posted by R.I.D.E. View Post
Queston for those with scan guages, how much fuel do you burn per hour at idle? I read that for the average 4 cylinder it was about .25 gal per hour.

In my VX, if I read the thread correctly, it messes with the lean burn when you shut the engine off, so I leave it running. I wonder what the consumption per hour, at idle, is in a VX?

One different technique I use in mine is to use the tranny to slow down because it kills all fuel delivery, as long as you keep the revs above 1000.

regards
gary
I think the RPM at which the no fuel cut off point is varies gear to gear. I thought it was around 1500RPM? Is it really as low as 1000?
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Old 05-06-2008, 07:06 PM   #4
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So wait, are you saying the for every hour you let your car idle, a typical 4 cylinder that is, you use up about 1/4 to 1/3 gallon of gas? Hardly seems like much.
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Old 05-06-2008, 07:26 PM   #5
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If you watch the tach on mine as you decelerate(foot off gas) in any gear you can feel the fuel delivery begin with a surge about 1000.

I had to alter my driving technique to take advantage of the high gearing in the VX, which allows you to coast in gear much further than my 94 Del Sol.

The VX had lower idle speed than the Del Sol, so I guess with lean burn (assuming it works at idle) the idle consumption would be even lower. Could be less than .2 gph.

Basically it means under ideal conditions my P&G is about 1 to 3, using 5th gear (would be less pulse time with 4th) at 35-45 Mph. This means I am using about .15 gallon of fuel to keep the engine running, assuming 3/4 of the time I am gliding.

If I average 40 MPH and get 60 MPG, thats 2/3 gallon per hour. If I shut the engine off ideally that amount would drop to .667-.150=.520gph at 40, or 77 MPG.

I once did some serious engine off testing with my brothers 91 Civic and that mileage is about what I got, but that was on a deserted road with no traffic to worry about pulsing from about 28 to about 42 MPH. Averaging just over 1 mile every two minutes, it took 4:15 pulse in 18:30 total time. I used a DVOM to measure the fuel consumpition precisely, made a graph of the voltages per gallon used, and my calculation for idle consumption was .24.

In my situation, considering the VX's unique characteristics, I don't think it will help much to turn the motor off in the glide phase. My average speed in my normal daily driving is about 40 MPH and my P vs G ratio is much lower than 1 to 3.

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Old 05-06-2008, 07:28 PM   #6
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Frankenstripe, I generally use no more than 1 gallon an hour at speeds up to 65 if I use drafting.

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Old 05-06-2008, 07:41 PM   #7
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Gary, does your idle fluctuate at all while your moving? When I'm at a stop, my idle is between 500 and 600 (usually not as high as 600) When I'm in neutral coasting, my idle is up around 800, sometimes 900. Does your idle stay near 500 while you're coasting in neutral?

I thought I felt the gas at about 1300 in my VX when coasting in gear.
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Old 05-06-2008, 09:33 PM   #8
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Factory figures from FORD Australia state the locally produced Territory SUV , 6 cylinder 4.0 litre engine uses 1.5 to 2.0 litres per hour at idle with every accessory turned off.

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Old 05-06-2008, 09:55 PM   #9
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The VX has a small throttle body, roller rockers, a lightweight flywheel, no power steering, and idles on 12 valves. I think the fuel consumption would be quite low. I wouldn't shut down a VX let alone any other car knowing that to start them up requires a mixture of 10:1 or richer, and then a short amount of time that it runs rich with more timing to settle the idle.

Factory tachs move much too slow to be accurate for RPM changes. I know the HX DFCO is all the way down to 850 (grasshopper posted this in my build thread). The VX should be very similar if not identical.
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Old 05-07-2008, 06:21 AM   #10
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My d16Y7 idles at 0.25-0.3 gph when warmed up. A VX should be lower, I'd think.

I've found that even at highway speeds, EOC brings a gain over neutral-idle. I don't drive a VX, but I've confirmed this in mine, as well as in a TDI. 250 miles of highway, average 65mpg using neutral-idle to coast. Then, the last 50 miles I started using EOC for the glide, and the trip total came out just over 70 mpg - meaning 75+ mpg for that last leg. And a diesel idles with less fuel than a VX.
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