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Discussion Starter #1
Recently found this forum and I'm impressed, and I'm hoping you people would be able to help me with this problem.

Since I've had my Impala I have put regular gas in the tank and consistently gotten around 22 MPG in small town driving while moderately babying the pedal.

I recently (past two weeks) decided that using higher quality gas would be worth the extra ~$2 a fill up, and started putting Premium in the tank - which I believe the Impala SS is designed for anyway.

I also recently (two weeks ago or so) took the Impala to a local shop to get the oil changed and tires rotated.

After one or both or these (can't remember) I noticed that my average gas mileage was reading between 16 and 18 mpg, so I've somehow lost 3 mpg. I thought it might have just been the tank of gas I was on, but the mileage is still very low after refilling with another tank of Premium.

My car has 45,000 miles on it now, and I've heard that the spark plugs can go wrong around this time. However, I believe that one of these factors caused the very sudden large drop in MPG.

I'm thinking either the car isn't used to Premium (whatever that would entail) or, more likely, one of techs screwed up either my MAP/MAF sensors or a sensor that is dependent on my tires, resulting in a screwed air to fuel ratio that is killing my mileage.

Any ideas, and would work be covered by my 100,000 miles warranty?
 

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There's no need to use premium, you gain no benefit, only waste $$. So, I suggest going back then seeing what happens with your MPGs. Make sure to get as low as possible on gas, so you'll burn almost all regular. Then, if it doesn't change back look at the other things more closely.
 

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if i remember correctly, SS's are supposed to use Premium fuel. i know with my car, using premium increases the compression....which my engine wasnt built for. but for the vice versa side of things i dont know. using regular instead of premium decreases the recommended compression but i really dont know if its a bad thing.

i would say try to ride it out a while. i used premium a few times in my car months ago....almost 6 months ago...and my 'computer reader thingy', lol, showed a pretty good drop im MPG then it inched its way back up. but because of the higher compression of premium fuels, your mileage will drop regardless.
 

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The rule of thumb I've always gone by is to run the lowest octane fuel that won't ping. The reasoning being that higher octane fuels have a higher resistance to burning, which works out good for the higher compression engines, but may result in incomplete combustion in the normal or lower compression engines, leading to carbon buildup which may lead to artificially raising the cylinder compression over time, which may lead to the need to continue using the higher octane fuel. Not to mention the irregular edges of any carbon buildup leads to hot spots and resulting pre-detonation. Or, I could be using the wrong thumb :)

As far as the plugs - yeah, I was concerned that 100K miles seemed the stuff of fairy tales. However, I changed mine at around 85K - all looked good, gap had opened only 0.001 inches - so I guess the General knows what they're doing.

Can't see why the techs would be messing with the engine sensors on an oil change and tire rotation. Would suggest to have the MAF cleaned every 30K or so - whenever the air filter is changed works out well. Don't know that the tire pressure feedback is used in the fuel calculations - however, perhaps verify the feedback with a handheld gauge - mine tend to read 1-2 psi higher than the handheld that I trust.

Suggest to go back to the fuel originally used and see if the mileage comes back.
 

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It is possible that this is from the change in fuel. I would recommend contacting your local dealership for more specific information. I can also look into your warranty information with your VIN. Please feel free to contact me via private message as well. Thank you in advance.

Tricia, GM Customer Service.
 

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It is possible that this is from the change in fuel. I would recommend contacting your local dealership for more specific information. I can also look into your warranty information with your VIN. Please feel free to contact me via private message as well. Thank you in advance.

Tricia, GM Customer Service.
Is this what you copy and paste to *everyone* as a helpful response?
 

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^That's what GM makes her do. There was a whole thread about it in the Off Topic.
 

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the SS doesnt require PREMIUM, but does recommend 89 octane or above... most "regular unleaded" is 87 octance... often a stations "plus" fuel is 89.

While there are a number of things that could cause a drop in fuel mileage, it does sound like it was your fuel selection. Where did you fill up? I dont think you are using too high of an octane at all, in fact I think 91 runs better in the SS than 89. HOWEVER, if you got the super premium fuel (93) I have found that, becuase stations sell much less of it, that gas is sitting in their underground tanks a lot longer, which causes a lot more water to get into the gas. This could result in crappy fuel mileage and crappy engine performance.

Also, I know you mentioned that you had a tire rotation, but check the tire pressure... low pressure can hurt mileage.

best bet is to fill it back up with 89 or 91 from a reputable gas station and see if it helps.
 

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Discussion Starter #9 (Edited)
Good point on the 'sitting' premium fuel. I filled up at a Murphy gas station, and being a small town I bet that it doesn't get a lot of turn over for the premium fuel. I do not remember what the octane was specifically but I'd believe it was 91.

I ended up switching back to 87 octane and my gas mileage has gone back up to 21-22 MPG. If nothing else the next tank I may try 89 and see what that does.

Thank you all for the helpful feedback!

As far as the tire pressure, I checked that when I noticed it was getting low MPG and saw it was 34-36 between all of the tires, which is in the range that I've had the tires before.
 

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^Nope.....that's unusual, you got something wrong or drive with a lead foot.
 

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i have 18.5 mph driving 93. im going to try unleaded next time i fill up, but i thought it was bad for your engine to fill up with unleaded; thats where the ticking comes up....
 

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BlackNeSS-Monster
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i have 18.5 mph driving 93. im going to try unleaded next time i fill up, but i thought it was bad for your engine to fill up with unleaded; thats where the ticking comes up....

I'm not sure what you mean here?

"UNLEADED"....isn't just the 87 octane.

The 89/91 octane (Super)....and 92/93 octane (Premium)...or whatever....are also all UNLEADED.

Here in the states...."Leaded" gasoline...was pretty much discontinued in the 1970's (for usage on regular...street driven auto's)....so it's all been "UNLEADED"...since then (other than bio-fuels or blended fuels....which are also probably unleaded too..but mixed with things like ethanol/methanol....or some other formula and/or additives blended in as well)

I'm not any kind of a fuel/gas expert....by no means....but that's what I'm familiar with.
 

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Divine Autosound
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I'm not sure what you mean here?

"UNLEADED"....isn't just the 87 octane.

The 89/91 octane (Super)....and 92/93 octane (Premium)...or whatever....are also all UNLEADED.

Here in the states...."Leaded" gasoline...was pretty much discontinued in the 1970's (for usage on regular...street driven auto's)....so it's all been "UNLEADED"...since then (other than bio-fuels or blended fuels....which are also probably unleaded too..but mixed with things like ethanol/methanol....or some other formula and/or additives blended in as well)

I'm not any kind of a fuel/gas expert....by no means....but that's what I'm familiar with.
You didn't leave much to add. Kinda scary to think that someone put leaded fuel in a modern car....it isn't meant to work that way.
 

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Bigdawgz, Old thread is old; and he means regular unleaded. With regular unleaded fuel (87 octane), people report ticking with the LS4 engine. I thought it was pretty obvious to draw that conclusion, even if his wording is a bit limited.
 

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Pedal to the Metal
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I got an 08 Impala SS and just found out the car has to use 91 or higher, or you will here a knocking with 87... Soo i started to run premium and i feel a slight different. Feels like the V8 is should be.
 

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Impalaforums.com
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And for future reference this is what the manual says:

Gasoline Octane
For all vehicles except those with the 5.3L
V8 engine (VIN Code C), use regular unleaded
gasoline with a posted octane rating of 87 or
higher. If the octane rating is less than 87, you
may notice an audible knocking noise when
you drive, commonly referred to as spark knock. If
this occurs, use a gasoline rated at 87 octane
or higher as soon as possible. If you are using
gasoline rated at 87 octane or higher and you hear
heavy knocking, your engine needs service.
If your vehicle has the 5.3L V8 engine (VIN Code
C), use premium unleaded gasoline with a
posted octane rating of 91 or higher.
You may
also use regular unleaded gasoline rated at
87 octane or higher, but your vehicle’s acceleration
may be slightly reduced, and you may notice a
slight audible knocking noise, commonly referred
to as spark knock. If the octane is less than
87, you may notice a heavy knocking noise when
you drive. If this occurs, use a gasoline rated
at 87 octane or higher as soon as possible.
Otherwise, you might damage your engine. If
you are using gasoline rated at 87 octane or higher
and you hear heavy knocking, your engine
needs service.

And here is a link to the entire manual (this is for a 2007)
http://www.extendedgmwarranty.com/owners-manual/chevrolet/2007-Chevrolet-Impala.pdf
 

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Pedal to the Metal
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Soo us SS drivers should use 91 or higher. I noticed different's. when i first bought the car i use 87, but after i found out ive been running 93 and i feel a bit of a different's.
 

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You can use 87 in the SS, the engine's computer will just reduce timing and such (reducing power). In Utah our regular is 85 octane due to the altitude so I use premium which is 91. Unlike most places in the US it is $.20 a gallon more compared to $.30-35. That is around $3 a tank which shouldn't break anyone's bank.

We just drove it to Vegas and back. We averaged 24.5 to St George (southern Utah), then 22.5 from St George to Vegas and back to St George (40 mph headwinds going to Vegas, city traffic in Vegas) and then 27 mpg from St George to home (some tailwind). All this was cruising at 80 mph, matter of fact I averaged 80.5 mph on the last leg. So am I worried about using premium? Not at all. I wouldn't have been able to use 85 since Vegas is low altitude anyway (yes they sell 85 in St George).
 
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