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MPG mystery

Old 01-11-2018, 10:44 PM
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MPG mystery

Hey all,

Been a lurker for a while trying to look for information to solve my issue, which is this:

About a year ago, very suddenly, my '09 Fit's MPG dropped precipitously, like 20-30 percent. I went from averaging 29-32 MPG on a normal tank (live in L.A. and commute in moderate traffic, mostly on surface streets) to 22-25. So whereas I used to get 280-300 on a tank, now I'm lucky to clear 250.

Since then I have perused various gas mileage tips threads here but nothing has managed to get my MPG back up. Here is a summary of every to my mind relevant piece of info:

Car is an '09 with just over 70K. Issue started when odo was around 60K

I am an extremely MPG-conscious driver. I coast at every opportunity, accelerate slowly, try to navigate every traffic jam without touching my brakes, etc. If anything I've become even more so since the drop happened.

The issue seems to resolve itself almost completely when I stick to freeway driving. I recently averaged around 40 MPG on a round trip from LA to Bakersfield, up and over the Grapevine. This is purely a city problem. I feel like this may be a key insight.

Have not noticed anything performance-wise. No odd noises.

Car is well-maintained. All 4 tires are less that 2 years old, and pressure is checked often (I keep one of those little pumps in the car). Air filter and oil changed regularly. Just did the spark plugs. No issues with emissions. It's been almost 10 years and I've never even seen a warning light other than tire pressure.

I have tried several different gas and oil additives. None made a difference.

Hot and "cold" (it's L.A. after all) weather don't seem to move the (literal) needle much either way.

Have run several different brands of gasoline, regular to premium. Better gas does seem to improve things somewhat but usually I fill up at (ducks) Arco.

I have not done a valve adjustment. My mechanic told me it was unlikely I needed one, and I would be able to hear if the valves were out of spec. He took a spin in the car and said he didn't notice any noises or performance issues either. Also, I feel like if it were the values the drop would have been gradual, not sudden, right? It was literally like one tank I just noticed that when the gas gauge hit the half-full mark I'd only gone like 130 miles and normal I'd be at 180 or so. It's been like that ever since.

So that's it. Any and all suggestions are welcome, including "Give up and trade it in for the late '80s Ford pickup you've had your eye on!"

Thanks and cheers.
Old 01-11-2018, 11:25 PM
Join Date: Nov 2017
Location: MI
Posts: 67
Might be good to state if auto or m/t
Some owners have had troubles with the auto/trans. (not sure if that would affect mpg)
Old 01-11-2018, 11:54 PM
Join Date: Nov 2010
Location: New York
Posts: 1,133
Just a thought- I have a pump that has a pressure gauge built in. I used a tire pressure gauge and got a different reading.

Might you check the pressure with a different gauge?

Another thought- perhaps it's staying in a lower gear but on the highway it eventually upshifts to the right gear?
Old 01-12-2018, 12:31 AM
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Location: Cleveland, GA
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I don't have an exact answer, but have a couple of thoughts...

Both the O2 sensors and the ignition coils can degrade over time. Either of those could be weak, but not bad enough to throw any codes.

Other thought...just how mileage conscious do you drive it? Has it spent the 9 years being driven really gently? If so, the possibility of carbon deposits and fouling is pretty good. Have you tried giving it the "Italian tune-up?" If not, pour a bottle of Techron fuel system treatment in the tank and go drive it as hard as you can stand. Run it hard for a tank and see what happens.
Old 01-12-2018, 03:34 PM
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It's an auto trans. If it were staying in a lower gear I'd notice, performance-wise right? Like it would be obvious from the revs/tach. Don't think that's happening but worth checking. Thanks, Steve.

Hi Brain. Good thought but I have a gauge on the pump plus a separate dial gauge. The tires are 100% not low.

GAFIT, if it were the O2 sensor would that show up in smog though? Any way to test or would you recommend just replacing?

Also, if there were fouling would it have shown up on the old plugs? I replaced them at the suggestion of my mechanic but TBH the old ones looked perfectly serviceable when I took them out. No fouling. No pitting.

I'll try the Techron thing though. By drive hard what do you mean? Just, like, find a freeway where I can keep it above 75, say?

Thanks all.
Old 01-12-2018, 05:00 PM
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If you have an expressway around you, get the vehicle up to full operating temp and run it wide open throttle up the entrance ramp up to the speed limit and then maintain expressway speeds for 15+ minutes. I'd try to do that a few times over the course of a tank of fuel.
Old 01-15-2018, 10:18 AM
Join Date: May 2017
Location: Wilmington
Posts: 105
City and not highway points to Transmission, IMO.

Take a whiff of the fluid. Does it have a chemical odor, or a burnt odor? Is it clean or burnt/brown and used looking?

Put the car in Sports mode, then use the paddles to shift/check engagement while up/downshifting in each gear.

With windows down, listen for a moderately loud, high-pitched groan while under acceleration.

Might also have a look at trans temp.

Also, you say tires are under 2 yrs. Did the issue start WITH these tires? I had a set of Dunlops one time that were so grippy, they lowered my MPG by 12-15%

I notice my auto gets crap mileage compared to my manual, but I still see ~288 miles to a "tank." (Almost always when refilling, it is full after 9.4 gallons, so I'm right near 30.5mpg mixed. )
Old 01-15-2018, 04:05 PM
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Coilpack & valve adjustment may not 100% be the issue, but neither would be a bad idea on a car with your mileage.

(I'm at 85k and haven't done them, have noticed the car is not as perky as it once was.)
Old 01-16-2018, 06:05 AM
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I do not drive much so screw mileage. I have a couple of really nice on/off ramps which are newly paved. I cannot resist dropping my auto into third and accelerating on them. Fun.
Old 01-18-2018, 02:55 PM
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Update: I ran the transmission through its paces in Sport mode. I didn't notice any issues. Shifts all happen smoothly and there were no odd noises. I should point out that I own/have owned several Volvos, so I am eminently familiar with transmission issues, fluid smells, weird sounds, all that.

Tires issue is interesting. However the tires were swapped out in two stages. So I had two replaced and the two best old ones rotated to the front, then had those replaced recently.

Currently have the car gassed up with Chevron premium and a bottle of Techron fuel cleaner. Dragged it up and back down the CA 2 at 75-80 MPH last night. Pretty solid elevation change there so it was workin'. Will probably do it again while on this tank.

Also I'll consult the Haynes guide about the Coilpack and O2. I am determined to solve this.

One last thought: It could just be my imagination but one thing I MAYBE have noticed is a more pronounced deceleration while coasting. This would seem to point, again, to trans or tires, but could it just be that my wheel bearings need some attention? Like if one were damaged to whatever I assume I'd hear it, and this also doesn't jibe with the sudden as opposed to gradual reduction in MPG — but I ALSO assume that after 75K miles the travel in your bearings takes a hit.

Maybe I just need to bust out the grease?
Old 01-18-2018, 08:21 PM
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Dragging brakes? Coworker had this issue with his Corolla. Changed calipers (bought it loaded) and discs since they needed it anyways and he netted a few mpg.
Old 01-21-2018, 09:38 AM
Join Date: May 2017
Location: Wilmington
Posts: 105
What was your avg last tank? I managed 28mpg last week while driving around in the snow like a maniac, but it was all city.

If a bearing was creating enough friction to slow the car, you'd likely hear it grinding (especially if it was a sudden deterioration).

A stuck/sticky/dragging brake pad could possibly be at play, per 2rismo above...

Take her for a quick spin at highway speeds and then stop the car in a safe spot and feel each wheel. If one is way warmer, or even hot to the touch, it is likely getting more friction in the brakes than the other 3 wheels.

Again, though, you'd probably hear that at moderate speeds...

Good luck!

Last edited by TIGHT-FIT; 01-21-2018 at 09:43 AM.
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