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Old January 15th, 2015, 12:25 PM   #1
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Drastic MPG drop! 10mpg WTF?!?

Ok, before anyone starts with me for starting a new thread about such a common topic as MPG issues, I did my research and nothing was relevant to my issue so...

I bought my car in June of 2014 and have run only premium (93o) since I've had it. I do my calculations based on miles divided by gallons (always from empty to full) and then average those by adding all of them up and dividing by the number of fill ups getting an aggregate total of around 26mpg. I am now getting 16mpg. Sixf**kingteen miles per gallon! I do probably 35% highway driving and 65% city driving and there is no shortage of elevation changes. I drive fast but have not altered my driving habits to warrant such a drop. I use Mobil 1 5w20 @ 3k-5k miles and when I do I rotate my tires which are Goodyear Eagle GT 205 60 16's inflated to 32psi F and 30psi R (per Honda's recommendation). I have not made any modifications to the drivetrain or aerodynamics. I fill up at the same Sunoco station. I have no CEL. Air filter is new. ATF flushed and filled approx 2k miles ago. There are no issues with the car as far as driveability. The one thing I have done before I noticed the drop was changed the plugs. I did not use Denso or NGK but the Autolite laser iridium plugs (the ones recommended for this car). I can't imagine a 10mpg drop from a plug change. I've previously replaced calipers due to seizure or not backing off the rotor properly and even when the brakes would drag it only went to around 21mpg. Is there something I am missing that would cause this drop? I live in the north east and it has been below freezing for a few weeks but again, 10mpg? Any thoughts would be appreciated.

Edit:I totally spaced out and forgot a main question I had. Like I said above I live in the northeast and it is cold so I usually start my car before I leave to warm up and melt some of the ice on the windows. So the question is how much fuel is consumed while it sits idling? Right around 800rpm. And high averaging a total of 6-7 hrs of idle time per week what does that translate into gallons consumed for a properly tuned stock engine?

Last edited by StevilKnievel; January 15th, 2015 at 01:07 PM.. Reason: Derped
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Old January 15th, 2015, 01:16 PM   #2
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My car gets about 16-20mpg but I drive all city and hit v-tec when someone thinks they are getting in front of me Check your fuel trim with a scanner, maybe your a/f ratio is rich? Bad o2 sensor(s)?
Oh and fix your sig for gods sake, it's a j30a4, they retired j30a1 after 2002 model year.
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Old January 15th, 2015, 02:07 PM   #3
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Originally Posted by michaellowrider View Post
my car gets about 16-20mpg but i drive all city and hit v-tec when someone thinks they are getting in front of me check your fuel trim with a scanner, maybe your a/f ratio is rich? Bad o2 sensor(s)?
oh and fix your sig for gods sake, it's a j30a4, they retired j30a1 after 2002 model year.
lol.
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Old January 15th, 2015, 02:29 PM   #4
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I've done the generic obdII scan with no results. I think I've found someone with a snap-on that can get it done for me without trying to charge me a ****load of $ for a scan. I'm baffled by this. I will post my scan results when I get them. And LOL I must've fat fingered the number pad when I did my sig. Good looking out.
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Old January 15th, 2015, 02:31 PM   #5
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check your spark plugs, o2 would be my best guess. reset the ecu?
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Old January 15th, 2015, 02:44 PM   #6
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check your spark plugs, o2 would be my best guess. reset the ecu?
Plugs are brand new. O2 would throw a code even on the autozone blue light special scan tool. I fear it is something much more dubious than that.
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Old January 15th, 2015, 04:37 PM   #7
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O2 sensors get stuck lean and dump fuel on these cars so check that first.
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Old January 15th, 2015, 06:16 PM   #8
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Agreed , Primary O2 sensors are probably the culprit....
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Old January 15th, 2015, 06:56 PM   #9
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Looks like I have a starting point. Any guesses as to the idling fuel consumption?
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Old January 15th, 2015, 08:10 PM   #10
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1/10-4/10 of a liter per 10 minutes is a range I read somewhere, that's a range for all types of vehicles so in the middle of that range is my guess.
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Old January 15th, 2015, 09:39 PM   #11
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Quote:
Originally Posted by thisaznboi88 View Post
check your spark plugs, o2 would be my best guess. reset the ecu?
Quote:
Originally Posted by StevilKnievel View Post
Plugs are brand new. O2 would throw a code even on the autozone blue light special scan tool. I fear it is something much more dubious than that.
Quote:
Originally Posted by michaellowrider View Post
O2 sensors get stuck lean and dump fuel on these cars so check that first.
Quote:
Originally Posted by NVA-AV6 View Post
Agreed , Primary O2 sensors are probably the culprit....
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Originally Posted by michaellowrider View Post
1/10-4/10 of a liter per 10 minutes is a range I read somewhere, that's a range for all types of vehicles so in the middle of that range is my guess.
New different brand sparks will spark differently. Our ECU auto adapts trim so it may be confused right now, pull the ECU/FIC fuse under the hood and/or remove battery negative and hold the brakes.

O2 sensors will cause a hailstorm of problems before they will throw a code, especially in mpg concerns. The O2 tells the engine how well it's doing, if you impede that at all the engine will neither know what to do nor be able to tell that it's doing something wrong(until they get stuck or break).

NGK has the 2nd best sparks IMO, for performance, but best for reliable/cost/durability/performance.

Also verify that you got the correct heat level for your spark plugs, wrong heat level can drive an engine as crazy as O2 being distorted.

(Summary) Pull ecu fuse, check the rest of list items visually, plug in ecu fuse, try again.

One of those options, or your battery is about to die and your alt is on always on mode trying to constantly charge.
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Old January 15th, 2015, 10:40 PM   #12
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If you are making a lot of short trips with enough time in between for the engine to get cold again after the initial long warm up, that could explain it. Always running rich as the engine warms up.
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Old January 16th, 2015, 05:55 AM   #13
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Oraclem brings up a VERY IMPORTANT POINT, never, ever, use any spark plugs or O2 sensors from anyone other than NGK, NTK (O2 sensors), or Denso.
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Old January 16th, 2015, 10:36 AM   #14
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1/10-4/10 of a liter per 10 minutes is a range I read somewhere, that's a range for all types of vehicles so in the middle of that range is my guess.
So based on what you are saying I came to 8.4gal of fuel wasted while idling. Someone check my math here:

1 gal = 3.78L so the median 2/10L or 1/5L = .758L by dividing 3.78 by 5.

So, .758 every 10min multiplied by 6 to make an hour = 4.55L or 1.2gal per hour.

1.2gal x 7 = 8.4gal

Hmmmmm..... I think if the initial info provided is reliable I might have found the culprit. I never imagined idling would burn so much fuel.

I'm still going to take yall's advice though and swap the plugs out for some NGK's and clear the ECU. If I clear the ECU will that lose the data that a pro scan tool would compile? E.g. fuel trim etc...
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Old January 16th, 2015, 12:35 PM   #15
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Your math isn't adding up. I am approximating our cars burn about 1/5 liter every 10 min. So 1/5 liter x 6 equals 1.2 liters per hour of idling. That sounds more like a legitimate estimate, 1.2 gallons seems very excessive. Should be able to cruise @ 30 mph for about a hour on a gallon, hence the 30 mpg highway rating, heck even further at that speed.
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Old January 16th, 2015, 01:00 PM   #16
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Your math isn't adding up. I am approximating our cars burn about 1/5 liter every 10 min. So 1/5 liter x 6 equals 1.2 liters per hour of idling. That sounds more like a legitimate estimate, 1.2 gallons seems very excessive. Should be able to cruise @ 30 mph for about a hour on a gallon, hence the 30 mpg highway rating, heck even further at that speed.
I see where I went wrong now. It's 8.4L which is only 2.2gal. Thank you for double checking that it's been a long day lol. I'm hopefully getting it scanned tomorrow so I'll be armed with some more info. I will post the results when I have them. I priced out O2 sensors and I'm hoping it is something cheaper to replace
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Old January 16th, 2015, 07:41 PM   #17
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So based on what you are saying I came to 8.4gal of fuel wasted while idling. Someone check my math here:

1 gal = 3.78L so the median 2/10L or 1/5L = .758L by dividing 3.78 by 5.

So, .758 every 10min multiplied by 6 to make an hour = 4.55L or 1.2gal per hour.

1.2gal x 7 = 8.4gal

Hmmmmm..... I think if the initial info provided is reliable I might have found the culprit. I never imagined idling would burn so much fuel.

I'm still going to take yall's advice though and swap the plugs out for some NGK's and clear the ECU. If I clear the ECU will that lose the data that a pro scan tool would compile? E.g. fuel trim etc...
If you decide to do sparks first, either go with NGK iridium or NGK ix. Iridium might fit better for stock tuning.
Or just pull the ECU. If you have the past data already recorded, and or if it's irrelevant because the new data is scrambled from something, it won't matter if you erase it.
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Old January 17th, 2015, 07:15 AM   #18
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These guys are pointing you in the right direction.

To add some anecdotal evidence to the idle fuel consumption question; when I let my car warm up for 15 min before work every day I typically see about 50 miles less out of a tank.

Definitely not the drop you are seeing but substantial none the less.
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Old January 19th, 2015, 08:26 AM   #19
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Ok, so I swapped out the Autolites for the NGK iridiums. So far I have noticed it is slightly smoother accelerating and seems to idle just a tiny little bit smoother. I will hopefully have the scan done tonight if this dude is done ****ing around. I will post my mpg results when this tank is dry. Seems like the second half of the tank is already lasting longer by the needle position and how many miles on the trip odo. I will keep you posted. And I really appreciate everybodys input here. This is the first forum I've ever been in and yall really help with the stuff the manuals don't tell you.
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Old January 20th, 2015, 02:17 AM   #20
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Most manuals were written before enough real world miles are put on a car to find the weird stuff.
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Old January 20th, 2015, 07:29 AM   #21
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Did a fill up today. Switched the plugs at half tank. Already up to 19mpg no scan yet -_- he said today or tomorrow.
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Old January 28th, 2015, 12:03 PM   #22
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:UPDATE: Scan showed no abnormalities. I could not get a print out of A/F trim so nothing to post there. From the mechanics interpretation he said they are within normal range. My last fill-up netted 20.5mpg. So that is definitely better than the 16mpg I was seeing. NGK & NTK from now on lol. On a side note, upon further researching I learned that when the defroster is on it turns the A/C on to provide dehumidified air to combat fogged windows. This may not be news to anybody else but I found that to be quite an interesting tidbit considering I'm running it literally non-stop when the car is running. I think with that, the plugs and the amount of idle run time that should put me back around the 23-24 range. I really appreciate everyone's input. Cheers!
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Old January 28th, 2015, 06:08 PM   #23
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would definitely still recommend an ECU reset with all the plug swapping you've been doing. The rear defroster should be an isolated circuit of only electrical use, however that doesn't help with the front. Best idea is to leave the rear engaged and engage the front defroster when the car warms up, then pulse as needed when driving.

At least I got the spark plug and something always on parts right. 2/3 isn't bad.
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Old January 29th, 2015, 10:54 AM   #24
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would definitely still recommend an ECU reset with all the plug swapping you've been doing. The rear defroster should be an isolated circuit of only electrical use, however that doesn't help with the front. Best idea is to leave the rear engaged and engage the front defroster when the car warms up, then pulse as needed when driving.

At least I got the spark plug and something always on parts right. 2/3 isn't bad.
I'm doing a 3x3FF and changing the filter on the transmission this weekend so I figured when I have to pull the battery, tray and airbox out that should reset the ecu. Right? I've read some pretty stupid sh*t on the forums regarding ecu resets. I've always been under the impression that by grounding out the positive (taken off the battery first, of course) cable will do a capacitive discharge to the whole system thus, "resetting" all electrical components. Of course, I am the type of person that likes to just taste for a hot wire rather than spend money on a voltage meter!
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Old January 30th, 2015, 07:45 AM   #25
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Here is your reset procedure.

1. Get in your car, it doesn't matter if you close the door or not.

2. Turn the key to the on (not start) position, the position just before the starter
turns over.

3. Press the gas pedal to the floor with the key in the "on" position.

4. Hold the pedal to the floor for five seconds, then turn the key back
to the "off" position (don't remove the key), then release the gas
pedal.

5. Wait 2 mins. for a full alignment.

6. Drive the car as you always do.

Depending on your driving style, this may or may not be a big factor. I do a lot of city driving so
I noticed a big change after erasing my history. It is not hard to do this, just follow the instructions -

Key on, pedal down, 5 sec, key off pedal up, wait 2
minutes.

Keep the key in for the entire thing.
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Old January 30th, 2015, 01:06 PM   #26
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Have you tried some methyl hydrate in the tank to get rid of condensation?

10mpg seems to me to be more than just ecu adjustments.

That being said I believe everyone is leading you in the right direction. But - 10mpg is a huge discrepancy. Maybe more than 1 thing is going on.
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Old January 30th, 2015, 03:35 PM   #27
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Here is your reset procedure.

1. Get in your car, it doesn't matter if you close the door or not.

2. Turn the key to the on (not start) position, the position just before the starter
turns over.

3. Press the gas pedal to the floor with the key in the "on" position.

4. Hold the pedal to the floor for five seconds, then turn the key back
to the "off" position (don't remove the key), then release the gas
pedal.

5. Wait 2 mins. for a full alignment.

6. Drive the car as you always do.

Depending on your driving style, this may or may not be a big factor. I do a lot of city driving so
I noticed a big change after erasing my history. It is not hard to do this, just follow the instructions -

Key on, pedal down, 5 sec, key off pedal up, wait 2
minutes.

Keep the key in for the entire thing.
Still not quite sure I believe this. Personally I just pull the fuse going to the ECU then put the key in the on position. Removed fuse means ECU has no power going in, car being on means power going out, viola reset ecu.
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Have you tried some methyl hydrate in the tank to get rid of condensation?

10mpg seems to me to be more than just ecu adjustments.

That being said I believe everyone is leading you in the right direction. But - 10mpg is a huge discrepancy. Maybe more than 1 thing is going on.
Check the posts bud, half of it was having the AC always on.
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