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2011 LT 4cyl 2.4L I posted earlier about this and since then have tried some different things. Currently I’m at 14.9 mpg city and when I hit the highway it’ll go up to 15.1. I’m just burning through gas and I work up the road; it’s ridiculous. Air filter is new, spark plugs changed. MAP sensor changed, professional fuel injector cleaning service performed. No change ... actually it dropped after the service. What else could this possibly be ?!? No codes. Runs a little rough when idle; doesn’t purr like when I first got it. ANY suggestions I’d appreciate it!
 

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2016 Malibu 1LT 1.5T
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Any fuel smells around the car? Any hard starts when the engine is warm?
 
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No hard starts at all. No fuel smells. If I sit and idle for a while I’ll get out of the car and there is the smell like when your car is overheating (but it isn’t).
 

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2016 Malibu 1LT 1.5T
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I was thinking about a leaking injector considering you said it seemed like mpg dropped further after the injector service. Cleaning maybe made it worse. But not enough symptoms for armchair diag on that. Others will be along with more guesses... good luck.
 
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#1 O2 sensor, the one in the exhaust manifold. That is what monitors fuel mixtures, it may be on the edge of failure or getting weak but not bad enough to set a code.
When I changed the O2 sensors in my Tahoe I got back 2 mpg highway on it, no codes, scanned fine and it ran fine on the old one.
2 mpg is a big deal on something that only gets 18 mpg highway normally.
Use only a AC sensor also, aftermarket ones known not to be reliable.
 
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#1 O2 sensor, the one in the exhaust manifold. That is what monitors fuel mixtures, it may be on the edge of failure or getting weak but not bad enough to set a code.
When I changed the O2 sensors in my Tahoe I got back 2 mpg highway on it, no codes, scanned fine and it ran fine on the old one.
2 mpg is a big deal on something that only gets 18 mpg highway normally.
Use only a AC sensor also, aftermarket ones known not to be reliable.
that’s exactly what I was thinking was the problem abd s as knits pukled the trigger on the part but so many people (including mechanics) were sayin no it would definitely throw a code. Like there was no way it was possible. Thank you for confirming my thoughts . That’s definitely gonna be my next step
 

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that’s exactly what I was thinking was the problem abd s as knits pukled the trigger on the part but so many people (including mechanics) were sayin no it would definitely throw a code. Like there was no way it was possible. Thank you for confirming my thoughts . That’s definitely gonna be my next step
#1 O2 sensor, the one in the exhaust manifold. That is what monitors fuel mixtures, it may be on the edge of failure or getting weak but not bad enough to set a code.
When I changed the O2 sensors in my Tahoe I got back 2 mpg highway on it, no codes, scanned fine and it ran fine on the old one.
2 mpg is a big deal on something that only gets 18 mpg highway normally.
Use only a AC sensor also, aftermarket ones known not to be reliable.
****UPDATE changed out my 02 upstream sensor and when I pulled it OH MY GOSH. Completely warped !(attached photo) I don’t even know how the damn thing was running still. Went on a road trip the next morning and went from 15mpg to 22mpg 🙌🏼 The only problem is that the check engine light came on after a day then turned off after a couple hours then back on agsin. Luckily there was an O’reillys near by. Used their scanner and it said that it was running lean ??? Over the next few days it would come on and turn off but no other problems and maintained mpg. It has been off for a couple days now. Could it possibly be the downstream now?
anyways wanted to update and reiterate how every mechanic told me it couldn’t be the O2 sensor but I went with my gut and thank god for forums like this where people encouraged me to check it out !
 

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Downstream is only there to verify catalytic convertor is working, no other purpose or effect on engine running.
Did you use a AC sensor up front ? Hopefully yes as other brands are not dependable and erratic.
 
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