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Acute drop in Mileage, what could be the problem?

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Old 01-09-2018, 05:21 PM   #1
spinalator
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Default Acute drop in Mileage, what could be the problem?

My 07 Automatic Vert with 95K miles recently saw a drop in MPG. She was averaging about 16.5 city and now she is running about 13.5 city. Nothing was done to her recently in terms of mods. The vehicle hasn't had a tune up or any major service ever. I was thinking of doing the 100k service including plugs, wires and change transmission fluid and filter. I did clean and oil my K&N air filter but that didn't help. Any suggestions?
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Old 01-09-2018, 05:31 PM   #2
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Plugs should help if they havent been changed in a long time, or the gas your getting could be shitty.
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Old 01-09-2018, 05:37 PM   #3
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Plugs, wires, O2 sensors would be amongst the first things I look at replacing with that type of mileage.
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Old 01-09-2018, 05:48 PM   #4
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The car has long tube headers. The front O2 sensor was replaced years ago
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Old 01-09-2018, 05:51 PM   #5
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Quote:
Originally Posted by spinalator View Post
The car has long tube headers. The front O2 sensor was replaced years ago
They can foul like plugs...do you run an fuel additives, octane boosters, etc.?

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Old 01-09-2018, 06:00 PM   #6
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I do not. I only use 93 Octane gas. Either, Chevron, Mobil or Sunoco

Last edited by spinalator; 01-09-2018 at 06:02 PM.
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Old 01-09-2018, 06:15 PM   #7
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How old are the plugs?
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Old 01-09-2018, 07:03 PM   #8
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Plugs are original. Everything is original. Car has never been serviced. It has lots of mods but never serviced.... LOL And she is my daily driver. 10 years old and 93000 miles. Not bad...
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Old 01-09-2018, 07:08 PM   #9
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You're definitely due for plugs/wires....esepecially if you've seen a drop in MPG like that in recent months.

A while back my buddy had his daily (GM work truck) with 120K miles tuned up...essentially plugs, wires, cap, rotor, O2 sensors, timing...picked up 10 MPG.
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Old 01-09-2018, 07:58 PM   #10
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I would do plugs, wires, clean the throttle body and MAF and see how it does. If that doesn't help I would do O2 sensors next.
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Old 01-09-2018, 08:07 PM   #11
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I appreciate the feedback forum members. I have the plugs and wires ordered. I will do thse first and clean the MAF although I have an oil catch can and the throttle body doesn't get dirty ever. I recently checked it when I had to clean my K&N air filter. If that doesn't help, I'll do the O2 sensors next... I'll let you know about the anticipated increase in MPG
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Old 01-09-2018, 08:45 PM   #12
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Quote:
Originally Posted by spinalator View Post
I appreciate the feedback forum members. I have the plugs and wires ordered. I will do thse first and clean the MAF although I have an oil catch can and the throttle body doesn't get dirty ever. I recently checked it when I had to clean my K&N air filter. If that doesn't help, I'll do the O2 sensors next... I'll let you know about the anticipated increase in MPG
I would bet that your using the DIC mileage that's not right EVER, in good tune even with mods my LS2 gets 26 highway and 19 around town IF I keep my foot out of it, you didn't mention the air filter? is this the first car you've ever owned?
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Old 01-09-2018, 09:05 PM   #13
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No sir. This is my 5th Vette and I've been driving nothing but Vettes since 1993. This is the longest that I've owned any one and it has the highest mileage. I do use the DIC and it used to be 16.5 City recently. Now it's 13.2. It was higher when I first purchased it and it did drop when I did mods but I don't have a heavy foot and I only commute from home to work 4 miles each way
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Old 01-10-2018, 02:06 PM   #14
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My first gas would be winter fuel formulations.

16 to 13 in town IMO is not so drastic and could be a heavier throttle foot.

I would start by inspecting my sparkplugs to look at how they are burning.

Also plug is a OBD reader and look for some codes.

Mis fire, struck injector, vacuum leak, fuel gauge issue (know issue), and of course a fuel leak...those are the easy off the top of my head DIY comments.

Good hunting.
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Old 01-10-2018, 07:00 PM   #15
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Hello,

Change plugs/wires, clean MAF sensor and throttle body!

Douglas in Green Bay
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Old 01-10-2018, 08:26 PM   #16
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The wife's Caddy had a vapor canister purge solenoid fail a couple weeks ago and her fuel mileage dropped about 1/3.

I tried to look that up for a Vette, but found nothing.

Check for codes like Kenny94945 said.

BTW, at 151K miles, I still have the OEM wires and get 38-40 instant mileage in 6th at 50 mph. It's been that way since new.
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Old 01-11-2018, 08:34 AM   #17
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With 95K, it about due for new plugs, wires, air filter, and even cleaning the MAF.

Also pull codes to check to see if is anything else going on as well.

Lastly, go to the gas pump and compare the amount of ethanol in it, to the gas you where running before.

Here in Denver, they run a higher percentage of ethanol for the winter gas blends (10%) than the summer blends (about 3%) due to temp inversion smog problems, and you will see if every winter with lower gas mileage when you are running fuels with the higher ethanol amounts.

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Old 01-11-2018, 04:37 PM   #18
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I ordered the plugs and wires. I will change them when I get them. The gas hasn't changed. I always use the same 3 stations. I live in FL so we don't have winters. My mileage was always consistent. I noticed a drop in MPG when I did a wide body conversion and went from 285/19's to 345/20's but only about .5 MPG. I changed the fuel pump about a year ago and that didn't effect mileage. Just past 2 months, I noticed the mileage drop from 16.5 city down to 13.2 city. If new plugs and wires don't help, I'll get a diagnostic done next.
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Old 01-12-2018, 09:12 PM   #19
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winter blend gasoline is my best guess ... check with your local gas station to see what's going on. You might need to check if the EPA requirements have changed recently.
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Old 01-13-2018, 12:24 AM   #20
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Quote:
Originally Posted by spinalator View Post
I ordered the plugs and wires. I will change them when I get them. The gas hasn't changed. I always use the same 3 stations. I live in FL so we don't have winters. My mileage was always consistent. I noticed a drop in MPG when I did a wide body conversion and went from 285/19's to 345/20's but only about .5 MPG. I changed the fuel pump about a year ago and that didn't effect mileage. Just past 2 months, I noticed the mileage drop from 16.5 city down to 13.2 city. If new plugs and wires don't help, I'll get a diagnostic done next.

Did you clean the Maf, and did you change the air filter? Next, depending on the fuel that you are running, my need to get some techron in the fuel to clean the injectors.

Also, do a data log on the motor running, and check the O2 sensors for one or both that is going lazy. The sensor should be bouncing up an down with a wide spreed, and if one of the sensors is holding steady instead, it has gone lazy and will need to be replaced.


Last edited by Dano523; 01-13-2018 at 12:28 AM.
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