Okay, so this post is about gas mileage, but not in the normal sense. I had been getting between 19-20mpg for the last 8 months or so, and now I am getting worse gas mileage than I ever have, around 16mpg. The last time my mileage dropped was when the coils were starting to go.

Now I've changed coils/plugs/wires about 6k miles ago, no longer have a cat, and everything else is up to date as far as maintenance goes, so what else could be causing this? I've been doing lots of reading and it seems like the cat going bad or coils failing are the two biggest concerns when looking at a drop in gas mileage, but being able to rule them out leaves me with no clue.

The car is basically stock, just with the coolant reroute, modified intake, and no cat.

Thanks for the help.

 

Also, I picked up a bottle of injector cleaner to run through with me next tank, figure it can't hurt ant helps to eliminate a fuel issue. Although I don't see how a fuel problem would cause WORSE gas mileage since pretty much everything would be restricting fuel flow, and there is zero difference in the way it drives. I was also going to go ahead and change my oil even though I still have 1k miles left just to check another thing off the list, but my drain plug is stuck. -_-

And to add to the service history, the coolant was completely drained and refilled about 4k miles ago, so some weird cooling issue shouldn't be the cause either.

 

Have you checked MAF sensor or air filter element? Are you getting a CEL?
Have you changed driving habits?

 

Actually meant to cover that, the MAF has been cleaned recently, and the air filter is still in good shape, I tend to get bored and rip everything down and clean it all pretty regularly. And when it started my driving habits were exactly the same, although I've been attempting to tone it down a bit and see if I can improve the mileage a bit. This spell of 16mpg has lasted about 4 tanks so far, through different weather and different gas stations so I assume I can rule all that stuff out too. Also, I always use 93 octane.

 

Oh, also just reset the MAF profiles and will see if that improves anything once it relearns. Maybe I got some bad gas or something and it made the maps go to hell?

 

How are you measuring MPG?
What was your MPG prior to "the last 8 months or so"?
What year is your RX-8 and how many miles?
Did your drive pattern change (more or less highway, construction zones, etc.)?
Did the ambient temperature change... Are you using your A/C more?

Think hard about some factor(s) extraneous to the car, that changed.

 

The mileage was around 17-18mpg before it went to the 19-20mpg range. It went up when I replaced coils/wires/plugs and removed the cat. It is a 2004mt with 76k miles. My driving patterns have not changed and although it it getting warmer, it has never done this when it has been warm in the past, I can't think of anything I or the world is doing differently.

I'm starting to suspect that it was a fuel trim issue, I did the reset this morning and the car seems to be driving a bit better. I'm not sure what would have caused it and I didn't think the fuel trims could change things enough to cause that big of a swing in mpg. I measure it by running it until the light comes on and topping off and comparing the miles I've driven to the gallons burned.

 

Alright, so now the oil change is done, fuel trims have been reset, and am currently running injection cleaner through.

Still no improvement, what else can I try?

 

First of all 16 mpg is not terrible. Many others in this forum including myself are getting about that much in mix driving (city & highway). From the info you have provided I think you get that mpg because of your modified intake (whatever this is) and running catless.

 

Well it's not the number that I'm worried about, it's the sudden drop that concerns me. Life has taught me that when something changes, there's normally a reason, especially when it comes to cars.

Now I would be inclined to think the same if those modifications had not occurred long before this drop in mileage happened.

 

Is there a possibility you initially miscalculated your mpg and the one you have now is correct? If your car performs the same, running strong, not starting issues, etc. I do not see a reason for being that worried.

 

No, I've calculated it the same way since I got the car and it has been consistent(or at least had good reasons for being inconsistent) until the last 3 or 4 tanks.

 

Well I will throw this idea, although if this was the case you should have already noticed it. Have you checked your fuel pump? Some members here have reported problems with the fuel pump (leaking). You should be able though to smell gas. I assume though that your car already has a strong smell as you run catless.

 

Hmmm, how exactly would I go about checking that? My car always smells like oil/gas/rotary for sure. But now that I'm thinking about it, it might actually have gotten a bit worse in the last couple weeks. Would it be leaking onto the subframe or something?

 

You could try reading the DIY how to remove the fuel pump...

 

I was planning on it if it needs to be pulled, I was just wondering if there was an easy way to see if it's leaking or not, those are pretty obvious but since it's a somewhat common problem it seems I figured there would be an easy, it's broken or not, test to determine if it's worth pulling.

 

https://www.rx8club.com/series-i-major-horsepower-upgrades-93/fuel-pump-solution-all-failure-modes-176491/

This might be a good start

 

How did I not find that thread? My google-fu skills must be lacking today.

 

If only people would read the stickies

 

I was using the site:rx8club.com method, but I must not have seen that thread. Not to mention that it's in the Major Horsepower Upgrades section, not exactly where I would look first for this kind of problem.

 

It is always easier to use google and put rx8 at the end of your sentence or word (e.g. fuel pump problem rx8)

 

Welp, unless I'm missing something, it doesn't look to be leaking anywhere.

 

So, just to make things concise and easy to read...

Different Gas: Tried.
Coils: Replaced recently.
Wires: Replaced recently.
Plugs: Replaced recently, also visually inspected last week.
Fuel Trims: Reset.
Air Filter: Checked.
Throttle body/intake: Checked and cleaned.
Cat: N/A.
Fuel Pump: Inspected and not leaking.
Tire Pressure: Checked.
Vacuum lines: Inspected and couldn't find anything out of whack.
Injector Cleaner: Currently running with this tank.

One other thing I read it might be is a failing battery causing excessive draw on the alternator. Any good way to test that theory without buying a new battery?

 

Okay, so it looks like it could either be the battery, thermostat, or upstream O2 sensor... any symptoms that would help me decide which to tackle first?