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Discussion Starter · #1 ·
I have a 2009 cobalt lt 2.2L, 75,000 miles. With in the past month ive noticed ive been going through 2 tanks of gas in a week. Ever since then i have been keeping a closer eye on my trip and seeing how many miles i am getting out of a tank. I am now getting almost 80 miles less out of a tank, when before i got a little of 300 miles on a tank, and now i am getting about 220-240.
Ive noticed i get anywhere from 1-13 mpg when i am leaving a stop light or the engine is under load, like going up a hill, etc... In hopes of fixing the issue i bought all new; iridium spark plugs, air filter, oil change, and some injector cleaner. none of these have fixed the issue. I dont have a CEL and only noticed this issue because i am burning way more gas then what i should, and my exhaust smells like its running rich. Today we cleaned the throttle body and got rid of the carbon build up, and that helped it a little bit but nothing anywhere close to what i should be getting. We then also took apart the Secondary Air Injection Check Valve and removed a decent amount of carbon build up from there, yet no changes.

We unplugged the air pump fuse and relay and took it for a ride, and noticed it was getting relativly the same mpg, except the car would not get 99mpg on the MPG INSTANT reading on the dash. and so we plugged the fuse and relay back in and got the same mpg.

Back to the mpg, ive been using the MPG Instant on the dash. before this issue occured anytime i let off the gas the MPG would read 99. Now when i let off the gas it will read anywhere from 75-95 mpg, and even lower at times. id say it does it about 50% of the time while im in the car driving.

Read some forums about fuel trim levels, so i went and got the readings. Fue trim long would be anywhere from +1% to +9.8%, and at time spiking up +35%. this is pretty close to the the fuel trim b1 and b2 read, execpt they where reading over +10% more frequently, going up to about +14%.
we then took reading on the o2 senors, and they where jumping around, but staying inbetween .05V-9.8V.

As of yesterday my exhaust weas smelling like rotten eggs, so we took the cats out and found that the first cat was plugged but it was plugged very bad. as for the second one it was completly fine. so currently running no cats, just resonator and muffler. Gonna sell the cats and get new headers and down pipe. After we took the cats out we did al;l the same test and monotring of the MPG INSTANT and its still the same. no difference.

So any help or points int he right direction as to what it could be would be greatly appritated. Thanks.
 

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Discussion Starter · #2 ·
I have a 2009 cobalt lt 2.2L, 75,000 miles. With in the past month ive noticed ive been going through 2 tanks of gas in a week. Ever since then i have been keeping a closer eye on my trip and seeing how many miles i am getting out of a tank. I am now getting almost 80 miles less out of a tank, when before i got a little of 300 miles on a tank, and now i am getting about 220-240.
Ive noticed i get anywhere from 1-13 mpg when i am leaving a stop light or the engine is under load, like going up a hill, etc... In hopes of fixing the issue i bought all new; iridium spark plugs, air filter, oil change, and some injector cleaner. none of these have fixed the issue. I dont have a CEL and only noticed this issue because i am burning way more gas then what i should, and my exhaust smells like its running rich. Today we cleaned the throttle body and got rid of the carbon build up, and that helped it a little bit but nothing anywhere close to what i should be getting. We then also took apart the Secondary Air Injection Check Valve and removed a decent amount of carbon build up from there, yet no changes.

We unplugged the air pump fuse and relay and took it for a ride, and noticed it was getting relativly the same mpg, except the car would not get 99mpg on the MPG INSTANT reading on the dash. and so we plugged the fuse and relay back in and got the same mpg.

Back to the mpg, ive been using the MPG Instant on the dash. before this issue occured anytime i let off the gas the MPG would read 99. Now when i let off the gas it will read anywhere from 75-95 mpg, and even lower at times. id say it does it about 50% of the time while im in the car driving.

Read some forums about fuel trim levels, so i went and got the readings. Fue trim long would be anywhere from +1% to +9.8%, and at time spiking up +35%. this is pretty close to the the short fuel trim b1 and b2 read, execpt they where reading over +10% more frequently, going up to about +14%.
we then took reading on the o2 senors, and they where jumping around, but staying inbetween .05V-9.8V.

As of yesterday my exhaust was smelling like rotten eggs, so we took the cats out and found that the first cat was plugged but it was not plugged very bad. as for the second one it was completly fine. so currently running no cats, just resonator and muffler. Gonna sell the cats and get new headers and down pipe. After we took the cats out we did all the same test and monotring of the MPG INSTANT and its still the same. no difference.

So any help or points int he right direction as to what it could be would be greatly appritated. Thanks.
 

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Is this a std our automatic transmission?
What state and brand of gasoline are you getting? How are you filling your fuel tank? Are you fueling until it clicks off the first time?

using a calculator, does your car calculate the same mileage you calculate by hand? I always log the mileage / odometer and use the trip meters and manually divide miles per gal and verify the car is calculating the same as I do…within a reasonable error.

i ask because if your transmission is an automatic, it could be it’s not going into lockup. That is, the torque converter is not locking up into direct drive. This could cause a 5%-15% reduction in fuel mileage, depending on how and where you drive. Also, different parts of the country change their fuel specifications in winter. This to can decrease fuel mileage.
Some of the dumber things receintly proposed by the idiots in charge is to increase the amount of alcohol allowed to be sold for street use. This too can cause a 5%-15% reduction in fuel mileage.
 

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Discussion Starter · #6 ·
It’s an automatic transmission, and I’m actually running 93 in it. I used to run 87 but since this issue, I’ve been running 93 and added fuel injector cleaner to it. Nothing has changed but I do get about 20 more miles out of a tank with 93 instead of 87. And I do calculate the miles by had to and it’s relatively the same. And that’s for the idea about the torque converter is there anyway to tell, if it isn’t locking?
 

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Best way is take it to a transmission shop and ask them. One way is watch for a 200-300 rpm increase under light load or a light tap on the brakes while in cruise control…if you have one. While in cruise, with the torque converter in lockup, a slight increase in throttle will cause the torque converter to come out of lockup and start to slip ( hydraulic couple) and should show a slight increase in RPM until the load is reduced and the torque converter goes back into lockup.

The other way is with a good diagnostic reader watching the transmission signals from the Transmission Control Unit.

what city is the car in?
 

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Discussion Starter · #8 ·
Best way is take it to a transmission shop and ask them. One way is watch for a 200-300 rpm increase under light load or a light tap on the brakes while in cruise control…if you have one. The other way is with a good diagnostic reader watching the transmission signals from the Transmission Control Unit.
Okay, well we where watching the o2 readings and we found out when pumping the brakes the o2 sensors will read a steady .002 volts. Then when they start moving the stick at higher voltages for about 2-3 seconds. They then take 2 seconds to go from 1.0volts down to .1 volts.
 

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Discussion Starter · #10 ·
I take it your watching the up stream O2 sensor change…..normal…..the rear sensor, or down stream sensor does not move as much. At 75k miles, I would not expect they are out of order.
They are probably about 6 months old, and I was watching both of them not just the single one, and what do you mean by out of order?
 

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If you watch both at the same time, you will see the upstream changing rather regularly while the rear or downstream changes much less frequently…….to a steady throttle / RPM setting as the ECU changes the fuel mixture.
 

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Discussion Starter · #13 ·
If you watch both at the same time, you will see the upstream changing rather regularly while the rear or downstream changes much less frequently…….to a steady throttle / RPM setting as the ECU changes the fuel mixture.
Okay, so they stay the same or pretty close to it when they are reading, does that indicate a faulty sensor?
 
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