Drop in MPG after spark plug and air filter replacement

[Hi Ho]

My 07 Mazda 3 2.0L was due for a spark plug replacement and I had some free time last week. While I was at it I replaced the air filter. I bought four Autolite XP Iridium plugs from a local auto parts store where I was assured they were properly gapped. I installed the plugs and the air filter and all seemed good.

Fast forward a week and 1.5 tanks of gas. My MPG has gone from 33.5 to 30.1 with the same commute to work that I have driven every week for nearly the past 5 years. I have never seen the MPG readout that low. I added some Techron fuel system cleaner with the last fillup just for kicks.

For the first few years, the car averaged about 36-38 MPG I haven't gotten those numbers for at least a year. I will manually calculate the MPG on my latest tank just to double check but historically the computer has been accurate to withing .2 MPG.

Could an incorrect spark plug gap cause this drop in mileage? Does the brand of plug make a huge difference? After some research I found that the stock plugs, even though they said "FoMoCo" on them, were made in Japan by NGK and are very high quality (and pricey at $12ea) iridium plugs. Did I just waste $30 on junk plugs? Visually, the Autolite plugs look nearly identical to the stock plugs.

The air filter I installed is a Napa Gold which, from my research is a good quality filter. I can't imagine that would be a problem?

[HAL9000]

Hey... at least it's just a drop in MPG, did you ever read what some crap plugs did to me last year?

GM Twin Cam (Quad 4) miss

[glc]

Put a set of OEM plugs back in. Today's engines can be VERY sensitive.

[auen1]

Here is my guess.
Three main things make an engine run. Air, fuel and fire.
I'm thinking that you are getting more air flow with a new air filter.
You are getting better combustion with new plugs.
Your engine has a computer that see's this and determines that you need more fuel.
You might not notice, but you may be getting better performance.

[rwest]

I was thinking it maybe computer related. Just didn't have the thoughtput to get it out. Maybe try disconecting the neg battery terminal for a couple a minutes and force the computer to do a reset. Or does that not work anymore?

[David M]

You probably want to check the gap yourself.

[birddog_61]

Anytime you do a air filter change clean your MAF sensor. It gets dirty and stops reading the correct amount of air. It tells the computer that it is dumping more air in than it really is and so the computer dumps more fuel to keep the mix right, which makes the car run very rich and you get worse millage. I make it a point to clean mine every 15,000 or so.
BTW dont use carb cleaner on it, they make a special spray for MAF sensors that you can get at any autoparts store.

[glc]

Don't try to regap iridium plugs - it's too easy to break the electrode.

Rockauto.com has the OEM NGK's for $9.77 each.

Did you use dielectric grease in the COP boots when you changed the plugs?

[Preston]

Originally Posted by rwest I was thinking it maybe computer related. Just didn't have the thoughtput to get it out. Maybe try disconecting the neg battery terminal for a couple a minutes and force the computer to do a reset. Or does that not work anymore?

I would start with this. Unhook it for 30 mins or so...

Then give it a couple of tank fulls for the computer to 'relearn' its idle settings and your driving style.

Also, I assume you are buying gas at the same place?

[bob]

My guess is a sparkplug wire may be damaged. One trick is to spray water on the plug wires when the engine is cold. If it misses it is the wires. At night you will see the sparks to find the exact wire.

[acr98disc]

Originally Posted by bob My guess is a sparkplug wire may be damaged. One trick is to spray water on the plug wires when the engine is cold. If it misses it is the wires. At night you will see the sparks to find the exact wire.

There are no wires on spark plugs on this engine. It is a coil - over plug system. Each plug has it's own coil. I have the same car.
Spark plugs are better be factory, NGK, or good quality plugs.
Autolite - is a complete cheap junk.


And yes, different brands of plug can change MPG drastically.

[glc]

Spark plug wires are now a thing of the past, bob.

[thefultonhow]

You want to replace the plugs with the appropriate NGK model. I have never heard of a modern Japanese car of any make running well on parts-store plugs, be they Autolite, Bosch, or something else. NGK run best on Nissans, they run best on Subarus, and I'd be very surprised if Mazdas, Toyotas, or Hondas were any different.

[bob]

Excuse me. I must have made a big mistake.
07 2007 Mazda 3 Spark Plug Wire Set - Engine Electrical - Denso - PartsGeek

[thefultonhow]

Originally Posted by bob Excuse me. I must have made a big mistake. 07 2007 Mazda 3 Spark Plug Wire Set - Engine Electrical - Denso - PartsGeek

Bob -- two items that show there are no wires:

Image is not vehicle specific

Notes: COP Boots -- Coil On Plug Boots;

It's just some pieces of rubber, no metal. Modern cars have eliminated all the extra tuneup parts possible. No wires, no distributor cap/rotor, a lot of times the PCV is a lifetime part, coils are a lifetime part.

[glc]

Those are just replacement boots and springs for the coils. A whole set of new coils is around $300 (OEM) unless you buy Chinese knockoffs.

[thefultonhow]

Originally Posted by glc Those are just replacement boots and springs for the coils. A whole set of new coils is around $300 (OEM) unless you buy Chinese knockoffs.

And theoretically they should not need to be replaced unless there is an actual failure -- and then you replace just the one that failed.

[MaxRat]

thats wrong... coils do wear with time and their inductive charge wears down...

two things probably happens...

the O2 sensor auto adjust the fuel maps and make memory of it to match the air coming into the engine... beings you changed the filter the map is now slightly out of wack...running a tad richer than it should...

Also those plugs actually take a little more voltage to make the spark jump the gap and place an additional load on the coil for the extra voltage... on worn coils this can lead to excess fuel consumption... beings the plugs need more voltage to make the spark jump the gap they also run hotter at the tip and can cause spark knock...

first thing I would try depending on you... either disconnect the battery for 15 minutes to clear the ECM memory or try swapping in the factory plugs...

Copper core plugs run the coolest and are the easiest on the Ignition system beings they need very little voltage to make a spark jump... the only downside is increased wear...

[thefultonhow]

Originally Posted by MaxRat thats wrong... coils do wear with time and their inductive charge wears down...

Well, at least on my Subaru, they are not official tuneup parts, and there are people with close to 200k mi on the original coils. Maybe they do wear down, but apparently not enough to recommend replacing them on a schedule like caps, rotors, and wires.

[glc]

Considering you need a $6000 stress tester to test a COP properly, it's a judgment call on whether to replace them or not - unless you actually throw a misfire code.

[thefultonhow]

Yep, misfire codes area surefire way to tell that either the coil or the injector in a cylinder needs to be replaced.

[SARGE]

Originally Posted by Hi Ho My 07 Mazda 3 2.0L was due for a spark plug replacement and I had some free time last week. While I was at it I replaced the air filter. I bought four Autolite XP Iridium plugs from a local auto parts store where I was assured they were properly gapped. I installed the plugs and the air filter and all seemed good.

My approach as a troubleshooter for all my life, is to start at the beginning. It worked fine until I did what?? Never trust an assurance from a parts place. What, they were gapped by some guy in China? I know 'cause every engine I had rebuilt by anyone I always had to torgue the bolts and thank God I did. Start at the beginning...it was running good and now it's not - what took place? Therein is your villain and don't let it get complicated. Remember KISS. These guys are over-analyzing.

[SARGE]

Back off guys, you trying to send this feller into your twilight zone of what ifs and all your wizard fixes. Start at the beginning.

[Hi Ho]

Thanks for all of your advice. I talked to my mechanic today and she said the same thing. Install OEM plugs. I guess I'll just take it as a $30 lesson learned.

The engine runs great. There are no knocks, rough idle, or decreased performance. It's just the gas mileage. I will feel better with the correct plugs in there anyway.

[SARGE]

Originally Posted by Hi Ho Thanks for all of your advice. I talked to my mechanic today and she said the same thing. Install OEM plugs. I guess I'll just take it as a $30 lesson learned. The engine runs great. There are no knocks, rough idle, or decreased performance. It's just the gas mileage. I will feel better with the correct plugs in there anyway.

As I wrote, start at the beginning. KISS

[glc]

Originally Posted by SARGE Back off guys, you trying to send this feller into your twilight zone of what ifs and all your wizard fixes. Start at the beginning.

Look at my first response...........

[thefultonhow]

Originally Posted by glc Look at my first response...........

And mine. "You want to replace the plugs with the appropriate NGK model."

[rwest]

Then disconect the battery and let the computer relearn. With all the older sensors and original parts adding new plugs (beit OEM or what have you) the air fuel mixture still wont be right untill the computer recalculates it.

We never shoulda' let 'em have a computer run the engine....give me a carb and distributor, and I'll make 'er run right.

[glc]

Don't disconnect the battery if you are due for an emissions inspection - it takes a while for all the sensors to achieve readiness and there will be a P1000 code stored for a while.

[rwest]

Didn't think of that. Ohio's not an inspection state. yay!