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    Thread: Why an Aveo automatic gets poor city MPG

    1. #21
      Administrator Daox's Avatar
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      Re: Why an Aveo automatic gets poor city MPG

      Alright, it looks like Darin got the attachments working again.

      Which map is your vehicle following?

      Here is the map for the economic mode.
      [attachment=1:hkhjhdg9]autoeco.JPG[/attachment:hkhjhdg9]


      And here is the map for the power mode.
      [attachment=0:hkhjhdg9]autopower.JPG[/attachment:hkhjhdg9]


      Attached Images Attached Images   

    2. #22
      Senior Member Thymeclock's Avatar
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      Re: Why an Aveo automatic gets poor city MPG

      Quote Originally Posted by Daox
      Alright, it looks like Darin got the attachments working again.

      Which map is your vehicle following?
      Yes, those are the correct ones. The link provided is working, too.

      I'm going by "economy" mode. The Aveo doesn't have any selector switch to control two different modes as some other Daewoo cars might.

      The map for "power" mode can't possibly apply here.

      As you can see from the map the TC lockup should occur at 45 MPH. But I'm not feeling any real drop in RPM's (nothing below 2000) that would indicate that it has entered fourth gear, until I hit 45. At 45 MPH, presumably at TC lockup, a noticeable drop in RPM occurs, and the MPG goes up.

    3. #23
      Administrator Daox's Avatar
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      Re: Why an Aveo automatic gets poor city MPG

      I know its not exactly what you are looking for, but have you tried using the hold control?

      The automatic features a “hold control” button. Selecting “hold control” turns the four-speed automatic into a three-speed manual, with the driver changing gears by moving through the gated shifter. The “hold control” mode also enables the driver to better control wheel spin at low speeds on slippery surfaces by manually selecting a higher gear.

    4. #24
      Senior Member Thymeclock's Avatar
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      Re: Why an Aveo automatic gets poor city MPG

      Quote Originally Posted by Daox
      I know its not exactly what you are looking for, but have you tried using the hold control?

      The automatic features a “hold control” button. Selecting “hold control” turns the four-speed automatic into a three-speed manual, with the driver changing gears by moving through the gated shifter. The “hold control” mode also enables the driver to better control wheel spin at low speeds on slippery surfaces by manually selecting a higher gear.
      Yes, and I think we had this same discussion on Ecomodder. If you think about it, hold control will do nothing useful for this problem. If you are in first gear selector position and apply hold, it will remain in first. Same thing applies for second position. But if you select drive position, and apply hold while in third or fourth gear, the hold function is not going to keep it in a higher gear: for the simple reason that if it really did 'hold' it in gear and it were not allowed to downshift, the engine would lug. Also the car can't start out from a standstill in any of the higher gears. (I tried it and applying hold seems to have no effect in "drive" position.)

      If it does what you say it probably does (meaning limit it to acting like a three speed manual), all that would do is reinforce my existing problem: remember, my whole complaint is that it is apparently not entering fourth gear substantially until the TC lockup presumably kicks in much later, at 45.

      I maintain that when it shifts you should see a substantial drop in RPM. I see the RPM drop for the first two up-shifts into second and third, but not the shift into fourth. Upon deceleration to below 40 MPH, whereupon the TC lockup comes off, it's either back in third gear or maybe a slushy fourth gear that I am unconvinced is working properly. The irregularity is noticeable in that the RPM builds to and stays at over 2000 RPM while gradually accelerating from 25 to 45 MPH. I should be seeing a pronounced drop in RPM somewhere in the 30's if or when it shifts into fourth. But I am seeing no real drop in RPM until 45 MPH.

    5. #25
      Senior Member ontarian_frog's Avatar
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      Re: Why an Aveo automatic gets poor city MPG

      Sounds like the transmission is operating just fine. The operation described sound perfectly normal. The TCM keeps the engine at somewhat a constant rpm before locking up. I do agree that lockup is occurring pretty late at 45mph. A lower lockup speed would be more desirable, such as 40mph (like more powerful GM transmissions).

      For one, I really doubt locking up the torque converter earlier would result in significant fuel economy gains. In the city, where the speed limit is closer to 35mph, the torque converter would still hardly ever be locked up. The power delivery of the car would be less predictable and "drivability" would suffer.

      Second, the engine's FE is probably best when the rpm is between a certain range of approximately 1000rpm. My guess is between 2500 and 3600 rpm (Peak torque is at 3600rpm 1st Gen Aveo). Then it would make sense to try to keep the engines rpms within that range by controlling gear change and the torque converter lockup to get the best FE.

      A good way to understand and get a grip on what I'm saying is to study CVTs (continuously variable transmissions). The reason why they are getting a lot more popular is because it's easier to control engine's rpms with them. I work with them everyday, not in automobiles unfortunately but in snowmobiles and quads. But the same theory applies.
      I leased Pontiac Wave from September 2006 to August 2011.

    6. #26
      Administrator MetroMPG's Avatar
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      Re: Why an Aveo automatic gets poor city MPG

      Quote Originally Posted by ontarian_frog
      Second, the engine's FE is probably best when the rpm is between a certain range of approximately 1000rpm. My guess is between 2500 and 3600 rpm (Peak torque is at 3600rpm 1st Gen Aveo). Then it would make sense to try to keep the engines rpms within that range by controlling gear change and the torque converter lockup to get the best FE.
      I've done RPM vs fuel consumption tests in numerous cars, and have found that without exception in every vehicle I've driven (from a ~50 hp, 3-cylinder Geo Metro up to a 500 hp Corvette), maximum cruising fuel economy happens in top gear at the lowest possible engine RPM.

      It's a bit of a myth that best RPM for cruising fuel economy is close to peak torque RPM. I think the confusion may come from people's partial knowledge of brake specific fuel consumption (BSFC) (lowest does happen near the torque peak) ... but which isn't same thing as best fuel economy.

    7. #27
      Senior Member Thymeclock's Avatar
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      Re: Why an Aveo automatic gets poor city MPG

      I've done RPM vs fuel consumption tests in numerous cars, and have found that without exception in every vehicle I've driven (from a ~50 hp, 3-cylinder Geo Metro up to a 500 hp Corvette), maximum cruising fuel economy happens in top gear at the lowest possible engine RPM.

      It's a bit of a myth that best RPM for cruising fuel economy is close to peak torque RPM. I think the confusion may come from people's partial knowledge of brake specific fuel consumption (BSFC) (lowest does happen near the torque peak) ... but which isn't same thing as best fuel economy.
      I couldn't agree more; every car I've owned over the past 35 years bears that out.

      Here's a current example. I also have an '89 mitsu Galant - nothing fancy, just a very plain, ordinary, automatic 4-speed with a Jap transaxle. It typically up-shifts at about 10, 20 and 30 MPH (approximately) under moderate acceleration. It goes into fourth gear by 35 MPH at the latest. It's a 2 liter engine, a much bigger car, and it weighs at least as much as the Aveo. My FE in all city driving, even in winter is a consistent, dependable 20 MPG and has been for 20 years ever since I bought it new. But I can feel and hear each shift and also see it shift by watching the tach (although it's not harsh shifting by any means). And it was the same with my '89 Celica which would go into fifth gear by 40 MPH at the latest.

      On the Aveo, the best we have been able to manage is 22 MPG - and it seems that no matter how conservatively we drive it, it makes no difference. In fact, the sooner we get it into the higher gears, the better the MPG.

      The Aveo is like driving a pail of slush - not because it lacks power, but because it doesn't up-shift appropriately. From what I see of it, Chevy engineering tried to compromise and rig the TCM for power, more than for economy, by having it rev high before TC lockup - but it achieved neither power nor economy. Maybe they thought it would be an acceptable shift strategy for someone who drives long highway distances in a sparsely populated state with little or no traffic. But I live in suburban metropolis. The way this tranny is programmed it will never lug the engine, but it sure does consume more fuel than need be at speeds of under 45 MPH.- and there is no way I can change it.

    8. #28
      Administrator MetroMPG's Avatar
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      Re: Why an Aveo automatic gets poor city MPG

      Thyme: have you gone ahead and made contact with GM yet?

    9. #29
      Senior Member Thymeclock's Avatar
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      Re: Why an Aveo automatic gets poor city MPG

      Quote Originally Posted by MetroMPG
      Thyme: have you gone ahead and made contact with GM yet?
      Funny you should ask. I decided it would be best to take it first to my local, friendly, GM dealer if for no other reason than as a formality, to give them a chance to diagnose it. I would expect that if I hadn't, my writing to those higher up in the chain of command would be seen as premature, and automatically kick my complaint back down to a lower level. General Motors has as much top brass at their highest levels than the U.S. Army does. (Maybe that's the "General" part of General Motors?) Accordingly, dealing with a leviathan, top-down, big business structure is very similar to, and can be just as frustrating as dealing with government bureaucracy.

      They had the car for most of a day, and they called me back and told me there was a "memory code" set for the accelerator pedal itself, which apparently is an hybrid mechanical + electronic assembly. They want to replace it, so we're waiting for the part to arrive. I have no objection, but I hope they don't turn it into a FUBAR. (My Scantool showed no active codes set, however.)

      I spoke with the repair tech and asked him about his road test of the car and inquired about what he found with the shift points. He said everything seemed normal for a three speed transmission. I told him it's a four speed with TC lockup. He disagreed. I smiled diplomatically and left it at that. No point in arguing; but such blatant ignorance destroyed any confidence I might have had in them. Seeing that they lack essential knowledge of what the transmission design is, I get the impression that their replacing the accelerator assembly might only amount to 'grasping at straws' to show they are making some attempt at remediation. Read the code, replace the part, CYA, business as usual. I'm not surprised.

      Stay tuned...

    10. #30
      Administrator MetroMPG's Avatar
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      Re: Why an Aveo automatic gets poor city MPG

      They thought it was a 3-speed? That's disappointing.

      I think the last time GM had a 3-speed automatic was in the Metro days.



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