Why an Aveo automatic gets poor city MPG

[Thymeclock]

Our 2009 Aveo5 is only a few months old with less than 2000 miles on it. We like the car very much except that the city MPG has been disappointing. The city EPA estimate is 25. The best we have been able to get is 22 MPG. After installing a scan gauge and driving very conservatively I was determined to find out specifically why the fuel economy is so inadequate.

After considerable research I found the transmission is made by Aisin, a Japanese manufacturer with a good reputation for quality transmissions. It is an ordinary 4 speed, with no overdrive. The same transmission is found on all Aveos made from 2004 to 2010. I recently found the service manual for this transaxle, Aisin 81-40LE. It can be found on-line at: http://www.anagabriela.ro/.../2.%20AISI ... NSAXLE.pdf

The graph on page 5A2-12 shows the manufacturer’s specification for shift points. (The graph is stated in KMH, so I’ve converted the figures to MPH for this discussion.) The graph shows the shift points will vary somewhat according to the TPS (throttle position) reading. On my car a scan gauge registers the TPS reading at idle at about 16; very conservative (slow) acceleration would be about 25; brisk or above average acceleration would be about 35 TPS.

According to the graph, at 25 TPS the shift points should be at about 11, 20 and 27 MPH. At 35 TPS it should shift at about 12, 22, and 32 MPH. I compared what I am actually getting on my car, at about 35 TPS, which would be considered average or brisk acceleration. I found it actually shifts at about 15, 25, and 45 MPH. In other words, the upshift into the highest gear (fourth) is much too late. Most 4 speed transmissions typically shift at approximately 10, 20 and 30 MPH. The delayed shift into high gear at around 45 is a major reason for delivering poor city FE. I live in suburbia. On major roads, typically the speed limit is 40. Unfortunately, when there is traffic it is impossible to break 45 – thus the car is spending most of its time in first, second and third gears at best, and it virtually never goes into high gear, which would yield the best fuel efficiency.

The transmission shift points are entirely determined by an electronic module called the transmission control module (TCM) which was designed and pre-programmed by Chevrolet. The shift points for which it is programmed are widely different than the documented specifications of the manufacturer, Aisin. I visited a Chevrolet dealer and spoke with the service manager to see if the TCM could be reprogrammed. He told me that they could do nothing to change it, unless Chevrolet had issued a service bulletin update for the program (which is has not done in the past five years that Chevy has been using this transmission on the Aveo). In other words, unless and until Chevy engineering rewrites the programming of the shift points and offers it as an update for dealers to install, nothing can be done to rectify the existing discrepancy. I then visited my local transmission shop (AAMCO) and asked the same questions. They confirmed that there is no way to reprogram the TCM unless Chevrolet rewrites the programming for it.

From what I have read on the web this is a very common complaint for Aveos. Most owners are unaware that their cars are late in shifting into fourth gear and frustrated with their poor city mileage. Seeing that there is documentation supporting the fact that the TCM programming of Chevrolet is greatly divergent from the specifications of Aisin, the manufacturer of the transmission, I believe Aveo owners have a valid grievance against GM and should press to have the situation rectified.

Who can we complain to? How can we unite to complain effectively and have Chevrolet correct this situation?

[MetroMPG]

Unfortunately, when there is traffic it is impossible to break 45 – thus the car is spending most of its time in first, second and third gears at best, and it virtually never goes into high gear, which would yield the best fuel efficiency.

Presumably when you finish accelerating, to let's say 40 mph, you back off the throttle from 35 TPS to cruise at a constant speed, and your TPS value would drop.

Are you saying the transmission still won't upshift at ~40 even with the lower TPS value at cruise?

[Thymeclock]

Unfortunately, when there is traffic it is impossible to break 45 – thus the car is spending most of its time in first, second and third gears at best, and it virtually never goes into high gear, which would yield the best fuel efficiency.

Presumably when you finish accelerating, to let's say 40 mph, you back off the throttle from 35 TPS to cruise at a constant speed, and your TPS value would drop.

Of course, the TPS reading does drop as you press on the accelerator less.

Are you saying the transmission still won't upshift at ~40 even with the lower TPS value at cruise?

No, it won't - but it makes no difference. It won't go into 4th gear at all until 45 mph is reached, no matter what the acceleration rate (which is gauged by the TPS) is.

[MetroMPG]

That sucks - pardon my French.

On the off chance you don't get any satisfaction from GM, I wonder if you'd consider going so far as to look at spoofing the TCM. Modifyng the TPS and/or speedometer signal comes to mind as a way to force an earlier upshift.

(Side note: some Honda V6 owners - Ridgeline & Odyssey - report being able to trick the TCM into locking the torque converter earlier (a few MPH) than normal by briefly shifting into neutral, then back to drive.)

But let's not get ahead of ourselves. You've obviously done your homework and identified a significant gap between the programming and the hardware that the manufacturer should be made aware of.

[Thymeclock]

That sucks - pardon my French.

No kidding! (I could think of more vulgar ways of phrasing it, but I won't elaborate...)

My next step will be to contact and complain to GM - not just their customer service, but also the honchos in charge of engineering. I'm currently collecting corporate names and contacts. If ever there was a business model with too many top-heavy official positions, GM is it.

On the off chance you don't get any satisfaction from GM, I wonder if you'd consider going so far as to look at spoofing the TCM. Modifyng the TPS and/or speedometer signal comes to mind as a way to force an earlier upshift.

I doubt that fudging the TPS would work. Don't forget the shift point is just marginally dependent upon that factor. I don't see how you could change the speedometer signal without screwing up other parameters.

What I thought might work (but it's really a long shot) is that maybe there might be a different (non-Aveo) TCM made for another model car that could be transplanted, i.e. from a different make that has the same transaxle. The 81-40LE was also used on the Daewoo Kalos & Lacetti, possibly the Ford Fiesta <?>, the Suzuki Forenza ,<?>, and a slightly different but essentially the same Aisin transaxle is used on the Toyota Yaris and Echo.

But let's not get ahead of ourselves. You've obviously done your homework and identified a significant gap between the programming and the hardware that the manufacturer should be made aware of.

I presume you mean Chevy. Aisin made the tranny but they're just the supplier to GM-Daewoo. They have no control over the erroneous crap programming of the TCM running it, which is Chevy's involvement.

[NorthernAveo]

Ok, let's calm down here. The pistons rings are not even set yet and true fuel economy won't be seen till at least 10,000miles which is when the engine is broken in.

[Thymeclock]

Ok, let's calm down here. The pistons rings are not even set yet and true fuel economy won't be seen till at least 10,000miles which is when the engine is broken in.

I am calm, but I don't find that credible. Do you think the EPA runs all vehicles tested for 10,000 miles before establishing a fuel estimate?

Besides, piston rings have nothing to do with the shift points of an automatic transmission.

[Unidentifiable]

Can you show me any resource that says 10,000 miles is a valid break in point for a motor vehicle engine?

I don't believe in all the time I've been working on cars, I've ever heard that number. I've heard closer to 5,000 and I think that's about as high as I'd really be willing to believe.

[petrified.rabbit]

i thought with the newer materials and supposed "dyno" testing most engines were considered broken in leaving the factory? i am not saying this for the aveo, but the term "broken in" means different things though. its not like there is a break in layer on the rings, or that if you tore down the engine at 10001 miles it would not be as close to factory specs as one would think. I have seen cars with 30-40k with still signs of crosshatching on the cylinder walls..

If you go to a nissan or toyota dealer with 200k miles they like to tell you the car is barely broken in (until you go to trade it in). But if the engine is built wrong it wont last nearly 10000 miles either.

If we are talking "seasoned" i know a lot of people tell you to drive easier and not at a consistent high rpm for the first 500 miles of ownership to seat the rings. And also change your oil at the 500 miles mark ( i have always done the oil change). I don't know that i believe this with factory built car either though, i "break them in" the way i am going to drive it the rest of its life.

[NorthernAveo]

Can you show me any resource that says 10,000 miles is a valid break in point for a motor vehicle engine?

I don't believe in all the time I've been working on cars, I've ever heard that number. I've heard closer to 5,000 and I think that's about as high as I'd really be willing to believe.

I use 10,000 as a nice round number to make sure it is completely broken in. Different people have different driving styles and thus, break in will be different. I have tested many engine and compression is never complete at 5000-6000miles. At 10,000, everythign is usually broken in.

This engine is not dyno broken in at all. Any stating that thinks their mass produced 4 banger engine in korea is given the same treatment as a viper engine needs to get their info straight? They go on the dyno for one pass on the assembly line as they leave jsut as a quick check.

Even at 5000miles you stated, you are calculating the economy at 2000miles which means, the engine is not broken in yet and anyone stating that an engine doesn't need to be broken today needs to learn a lot more about engines.

People need to relax and stop getting all worried that their car isn't getting the exact fuel economy off of the dealer lot. Let the engine break in and the compression build and you will see it will climb over the next months.