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As many of us do, I track my fuel mileage closely. I’ve noticed about a 5 mpg decrease in fuel economy during my commute lately. I’m struggling to get my 25 mile average into the 50’s, when mid-50’s was the norm over the same stretch of highway. First I attributed it to “winter fuel” and colder temps. But I noticed something today that makes me think there is an issue with my car.
Typically my Economy Trend page has all segments lit up with 50+ mpg trends. I have been seeing this sawtooth pattern over the past few weeks and today it hit me.
Seeing the repeated pattern every 3 miles made me think something in my computer is resetting on a regular interval. I started going over items that could cause this. The first thing I thought was fuel trims getting out of whack by an air leak and resetting. But I haven’t noticed any driveability issues or abnormal idle.
With the holidays, the car has been sitting for days at a time. I noticed that if it sat for two days it would crank real slow, so I have been keeping a battery tender on it to keep the battery up. I finally put two-and-two together and seen the battery voltage sticking at 15.0 volts during cruise, it is usually 12.2 volts. I believe the battery being weak is putting a higher workload on the alternator causing the fuel mileage to cycle the way it is.
Have any other Eco owners seen a similar trend?
 

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Discussion Starter · #2 ·
I just had the battery replaced with an AC Delco 94R AGM at my dealer. Not sure if this will fix the fuel mileage issues, but it was needed.
 

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My car had set for a week, the weather in central Indiana has been really cold, part of commute it was -8 for part of the trip, I would say my mileage is down at least 4 mpg. My car sits in a garage so this morning when I started it was 37 in the garage when it was like 4 outside.
 

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Discussion Starter · #4 ·
My car had set for a week, the weather in central Indiana has been really cold, part of commute it was -8 for part of the trip, I would say my mileage is down at least 4 mpg. My car sits in a garage so this morning when I started it was 37 in the garage when it was like 4 outside.
I definitely expect to see some decrease in mileage during the cold months. It just didn’t make sense that the 3-mile segments would be that consistent back-and-forth above 50 mpg. The mileage graph usually looks more like a sine wave, lately it looks more like a square wave.
 

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I can't even interpret the gauge so i don't even bother with it.

If your car can't sit for 2 days without a charge. It's time to check things out. As in battery going bad. And from your voltage readings. It sounds like your battery is no good.

It also effects your electrical circuits. The dealer just replaced mine with 2500 miles on the clock. My car sat for 13 months before being sold. Usually when it reads above 15 volts. It's because the charge isn't soaking from top to bottom. It's sitting around on top so your reading a surface charge. Mine went up to 15.4 before the battery got swapped. Now it's steady around 14.6

There are 3 tests to determine if a battery is bad. 1st test is a 50 amp fast charge for 3 minutes. Battery shouldn't exceed 15 volts. 2nd test is 30 minutes. Battery shouldn't exceed 15 volts. 3rd test is load test. 1/2 cold cranking amps for load. Battery shouldn't drop below 11 volts. (((Testing MAY be somewhat different for the glassmat types )))
 

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Discussion Starter · #6 ·
I can't even interpret the gauge so i don't even bother with it.

If your car can't sit for 2 days without a charge. It's time to check things out. As in battery going bad. And from your voltage readings. It sounds like your battery is no good.

It also effects your electrical circuits. The dealer just replaced mine with 2500 miles on the clock. My car sat for 13 months before being sold. Usually when it reads above 15 volts. It's because the charge isn't soaking from top to bottom. It's sitting around on top so your reading a surface charge. Mine went up to 15.4 before the battery got swapped. Now it's steady around 14.6

There are 3 tests to determine if a battery is bad. 1st test is a 50 amp fast charge for 3 minutes. Battery shouldn't exceed 15 volts. 2nd test is 30 minutes. Battery shouldn't exceed 15 volts. 3rd test is load test. 1/2 cold cranking amps for load. Battery shouldn't drop below 11 volts. (((Testing MAY be somewhat different for the glassmat types )))
To read the graph:

The vertical columns represent a 3-mile segment. The column on the left is 30 miles ago, the one to the right of that is 27 miles ago, and so-on down to the column on the far right which is the last 3 miles. The height of each column represents the average fuel economy for the respective 3-mile segment.
Here is what a typical graph looked like as a regeneration cycle would start. You can see that the previous 15-30 miles had been around 50+ mpg average. At about 12 miles before the picture was taken the regen started and the average fuel mileage started dropping off. This is what normal driving conditions look like on the graph. It is typically a gradual drop and rise. The picture in the OP with the alternating 50 and 40 mpg 3-mile averages are not likely caused by natural driving conditions. This is what triggered me to think something was wrong.
 

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Discussion Starter · #7 ·
If your car can't sit for 2 days without a charge. It's time to check things out. As in battery going bad. And from your voltage readings. It sounds like your battery is no good.
I agree that the battery was going bad. I was having to keep a battery tender on it if it was going to sit for a few days. But it wasn’t until today that I realized the alternator was working harder to compensate for a bad battery and leading to a reduction in fuel mileage. I haven’t been able to drive my commute with the new battery to confirm the battery resolved the fuel consumption issue.
 

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I agree that the battery was going bad. I was having to keep a battery tender on it if it was going to sit for a few days. But it wasn’t until today that I realized the alternator was working harder to compensate for a bad battery and leading to a reduction in fuel mileage. I haven’t been able to drive my commute with the new battery to confirm the battery resolved the fuel consumption issue.
It's not your alternator working harder. It's the battery not taking the charge from top to bottom. It's all accumulating at the top.

The alternator climbs as the battery charges up. Say if it was dead, your alternator would probably read 13.0 and work it's way up as the battery charged up. Rough example. But if the battery can't absorb the charge completely. It's all accumulating at the top causing the alternator to read high because the battery isn't accepting what the alternator is pushing out.
 

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I had to go to 2 dealers before i got a new battery. First one told me nothing was wrong and to not leave my obd2 plugged in while parked. Everything worked better for a month before the heater started losing temperature again. Now i got a new battery and the car acts like a whole new 3 month old car.
 

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I had to go to 2 dealers before i got a new battery. First one told me nothing was wrong and to not leave my obd2 plugged in while parked. Everything worked better for a month before the heater started losing temperature again. Now i got a new battery and the car acts like a whole new 3 month old car.
I noticed the same thing as soon as the battery got replaced it drove like it was new. Im a little confused why the cars drive-ability suffers so greatly with a under rated battery. There should be a recall on the crappy OE battery for sure.
 

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Winter diesel, folks.
 

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My guess would be the battery. The Cruze has a variable output alternator. To get variable output you have to vary the amount of input power. That first graph is really strange and is a sure symptom of something else going on. With the voltage constantly reading 15 or higher the alternator is working a lot harder than it should be.

Please let us know if the graph smoothed out after you replaced the battery.
 

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the cruzes have programming built in that will change the voltage of the alternator output to better fuel mileage. im not sure of the specifics, my only reason for commenting is that your voltage variation might be normal operation, meaning theres a separate issue at hand.
 

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Discussion Starter · #15 ·
My guess would be the battery. The Cruze has a variable output alternator. To get variable output you have to vary the amount of input power. That first graph is really strange and is a sure symptom of something else going on. With the voltage constantly reading 15 or higher the alternator is working a lot harder than it should be.

Please let us know if the graph smoothed out after you replaced the battery.
The slow cranking issue was resolved with the battery replacement. The fuel mileage graph also seems to look better. The only picture I have of it includes standstill traffic at the 27-30 mile segment. It also has a 50 mpg spike at the 12-15 mile segment due to coasting off the mountain. Overall it looks better. It has been abnormally cold in Kentucky(6th longest time spent below 32° in recorded history), so my mileage is down and I don’t have many data points to compare to.

I will update when the temperature returns to normal.
 

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Help Required,

Tell me one thing. I have 2013 model Toyota corolla and my car is not giving me a good mileage on petrol. Why..... How can I increase my car fuel average?
 

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Well, it is winter, so that's definitely a majority of the issue here. Denser cold air, winter diesel blend, and increased rolling resistance are definitely all working against you.

I feel like ours doesn't do quite as well on the freeway as it used to, but it's also not new anymore, either. Then again, it'll surprise me out of nowhere, so I think it's just that the car sees almost entirely city that throws my perception off.
 
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