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Discussion Starter #1 (Edited)
I have a 40 mile one way drive to work and have a CVT. This morning I needed to gas up, about 30 miles into the drive. The temp was around 40F, damp roads but no spray to windshield, sides of car are wet. My MPG over the first 30 miles was around 77.7 MPG, so I was happy with that under the conditions. I filled the tank, logged the numbers, and left the gas station. A mile or so down the road I looked down at the FCD and it was sitting around 125 MPG. The road there is flat and level, and I was running my usual 40 MPH. Under the weather conditions I would have expected about 80 MPG. It ran around 125 MPG for about 4 miles, I then hit a hill, and afterwards was running about the normal MPG I would expect. At the end of the 4 miles the trip computer indicated over 95 MPG since leaving the gas station. The whole time it was running at 125 MPG I kept looking at the OBDII C&C gauge. Everything looked normal, except the injectors were running around 0.9x to 1.0x. The AFR also seemed normal, with the numbers flickering between 14.5 and 14.6. After the 4 miles or so the injectors went back to what I would expect, around 1.6x to 1.7x. I had a similar experiance about 3 months ago. I filled up at a different gas station, and in an uphill stretch that I would have expected around 80 MPG I was getting 100 MPG. The last 10 miles or so to work are the hardest on the MPG, and I got to work today with 76.1 MPG for the drive, 75.9 for the 10.4 miles from the gas station.

Anyone know what is up with this? Nothing else looked out of place on the OBDII C&C, and the car seemed to be running normal. On the 5 speed cars, while running in lean burn, I assume that the AFR goes up on the gauge, correct? I did not over fill the tank, let it shut off, pulled the handle until it clicked once again, then was done. I always shut off the car when filling up. MPG for that tank was 70.9 according to the trip computer, 68.5 according to my gas fillup (but the car is tilted at this station, my numbers are always slightly low when I fill up there).

What was up with this? Any ideas? I want to do this all the time!
 

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I had something similar happen about two years ago.

Scenario:

Warmer temps outside
No MIMA or FAS at the time
No OBDII C&C gage
Performing key-off FASing down a stretch of flat road
After one of the key-off's, the FCD reported 150 mpg, solid
About 1/4 mile down road, keyed-off again, and the glitch stopped

I have no explanation for what happened.

Jim.
 

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So you've got a few things going on here, first it is cold and damp so the air is very dense with high humidity. I have a theory that this dense damp air can actually increase your compression thus giving you better engine power, this accounts for the good MPG before you filled up. When I was in the Air Force some of our older tanker aircraft would inject water into the jet engine exhaust during takeoff to increase the thrust of the engine, kind of like a poor mans after burner.

Once you filled up then you got to experiance the other thing others here on the forum have seen, increased MPG after a full filling of the gas tank. The leading theory for this is that as you fill the gas tank you are pushing fuel saturated vapors into the evap canister. Once you get back on the road you are pulling and burning these enriched vapors from the canister for an extra MPG boost.
 

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Usually it's extra gas got into the charcoal vapor canister and is being burned in the engine outside the normal path... you get "free" gas vapors from the canister and the engine doesn't have to inject as much fuel.

Sam
 

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Yep, vapors from the charcoal canister. I've had cases where the FCD was pegged at 150 mpg for about 6 miles after a fill despite some generous throttle applications. Ive been trying to think of a way to exploit that little quirk and try to get it to happen on a more regular basis.
 

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Discussion Starter #8 (Edited)
Other than the 4 or so miles after leaving the station my mpg was not real good today (I did make it to work today at just over 75 mpg). The colder weather hurts the mpg, driving with the headlights on does not help (sucks down the battery), and then on top of that the damp humid air also hurts the mpg. I had a number of tanks this summer over 77 mpg, and with the cold, dark, damp weather of winter I have already dropped about 10 mpg since October. Then add on top of that the stupid car has a mind of its own lately. I left work and about a mile down the road the charge hit 72.0 (as expected) and would not charge any higher, 10 miles later it got to something in the 67% range and started to charge at 6 amps while I was going up a slight hill, dropping the fcd to 55 mpg instead of the usual 70mpg for that hill. During the summer I was able to go as low as 42% without it charging. I was lucky to get over 71 mpg on the way home tonight.
 

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Yep, vapors from the charcoal canister. I've had cases where the FCD was pegged at 150 mpg for about 6 miles after a fill despite some generous throttle applications. Ive been trying to think of a way to exploit that little quirk and try to get it to happen on a more regular basis.
Do 'vapors from the charcoal canister' factor in to the FCD calculations of fuel economy?
 

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Ive been trying to think of a way to exploit that little quirk and try to get it to happen on a more regular basis.
Your still using fuel though, the computer is just not reading it.
 

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....Then add on top of that the stupid car has a mind of its own lately. I left work and about a mile down the road the charge hit 72.0 (as expected) and would not charge any higher, 10 miles later it got to something in the 67% range and started to charge at 6 amps while I was going up a slight hill, dropping the fcd to 55 mpg instead of the usual 70mpg for that hill. During the summer I was able to go as low as 42% without it charging. I was lucky to get over 71 mpg on the way home tonight.
Well, our "stupid" little car also get stupidly high gas mileage. :D

I did some coast down tests several years back and noticed that between colder air temperatures, and higher air density, that there is at least a 17% loss in efficiency in the winter, compared to summer.

Jim.
 

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On the condition where the FCD is working correctly and not stuck:
When the engne is burning the vapor from the canistor wouldn't the O2 sensors just correct the final mixture to the fuel/air ratio that the computer wants to see? i.e. reduce the injector pulse width slightly.

If so it would seem that the computer would then indicate a higher fuel mileage based on the reduced pulse width. But realistically how much fuel vapor mileage can the canister give you?

When you fill up the gas hose sucks the fuel vapors into the stations equipment as the tank fills up. So if you don't overfill the tank there shouldn't be an extra amount of vapors going into the canister. It seems the the vacuum might even tend to purge the cansiter somewhat.

If you overfill the tank can the canister have raw fuel in it? That might extend your indicated mileage for awhile. On my CRX the canister is at the front of the car and it would take a real force feeding to get raw fuel up there.
 

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I actually get worse mpg right after fill up. My engine usually runs rough and the car feels like it has less power. I always attributed it to the engine getting colder fuel than what was already in the line so the colder fuel doesn't vaporize as well right before combustion.
 

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Discussion Starter #16
I know that I did not overfill the tank. I have only noticed this twice, but I fill up at a number of different stations. Maybe the tank nozzle was not properly sucking back the vapors. The fcd was not pegged at 150, was running around 125 but moved up and down depending on acceleration.
 

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Yep, vapors from the charcoal canister. I've had cases where the FCD was pegged at 150 mpg for about 6 miles after a fill despite some generous throttle applications. Ive been trying to think of a way to exploit that little quirk and try to get it to happen on a more regular basis.
I think that this is a glitch in the ECM which pops up from time to time when you FAS the car. I've seen it myself. If you come to a stop and turn off the ignition for a minute, I think you will see your FCD return to a more "normal" display. I got this information from rightlancruiser, whom I think you know.
 

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I think that this is a glitch in the ECM which pops up from time to time when you FAS the car. I've seen it myself. If you come to a stop and turn off the ignition for a minute, I think you will see your FCD return to a more "normal" display. I got this information from rightlancruiser, whom I think you know.
Yes, that is a separate glitch though. What I mentioned above is only temporary (about 5-6 miles after a fillup), where the glitch you a speaking of is permanent until such time as the ignition is very briefly blipped off then back on.
 

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Yes, that is a separate glitch though. What I mentioned above is only temporary (about 5-6 miles after a fillup), where the glitch you a speaking of is permanent until such time as the ignition is very briefly blipped off then back on.
OK, I see the distinction you are making. I'll have to look for the full tank effect:)
 

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Discussion Starter #20 (Edited)
Stupid battery again. When I got to work today the battery was around 71.5%. Temp this morning was around 15f. For the last two months I have been watching the battery very carefully. With the cold temps and headlights it will force charge at 65%, so I do my best to keep it in the 69-72 range. It won't charge above 72.

So I leave work today and drive about 36 miles, hitting 72% at almost every stop. I stop T a cross road a few miles from home, again hitting 72%. Right after I take off I look at the obdii c&c gauge, now the battery is at 79.8%. As I watch it it is burning off an amp or more as I drive down the road. At least it seemed to be burning it down with the electric motor, great mpgs while it did that. Today might have been the warmest day in over two months, I never had to use the heater on the way home.

At about 76% it stopped burning it down, but still would not take any regen.

I know what will happen, I will get in the car the next time and it will probably indicate 65% or something like that and start a force charge again. Stupid battery.......
 
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