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Awful fuel consumption - up and down

About 4k miles ago I replaced the previous spark plugs in my 2007 mazda 3 2.3l hatch with bosch double iridium plugs. Since that was when I bought the car I had no benchmark for how much mpg I was getting and ended up getting about 200 miles per 12.5gal. I just assumed something was wrong but didnt have the time for it until about 2 days ago. I started getting 260 miles out of that same tank for some reason. A day later I found my coil packs backing out and pushed them back in and torqued them again. Since then ive been getting the same, awful mileage again. Any Ideas? There are no check engine lights or obvious signs.

Is 12.5 gallons your tank capacity or the amount of fuel it is taking to refill it after 200 or 250 miles. I am just curious because the car always taking 12.5 gallons of gas would be strange.

12.5 is the tank capacity, based on what ive observed its about 17mpg on the low end and 22mpg when it was doing well for a couple days. Sorry for not clarifying

Why are you mentioning tank capacity in a question of fuel mileage? You measure fuel mileage by filling the car, driving until you need gas and divide the miles driven by the amount of gas used to re-fill it.

Thats what I’ve been doing, the 17mpg is now and before it did well, and the 22mpg was during. The 200 and 250 estimates were what I would get for a given tank.

I suggest you start doing more precise measurements of your fuel economy. The fluctuations you’re perceiving might not actually be happening.

Every time you buy fuel, write down the amount of gallons you used and the amount of miles you traveled since the last time the tank was full. Record the full number on the pump and receipt, to the third decimal place. It will be more precise than the mileage, which will only be to the first decimal place, but use the most precise numbers you have from each measurement, and then round your answer off to the nearest whole number.

Until you’ve actually tracked your fuel economy, and confirmed there is a real issue, I can’t see dedicating the time and energy trying to figure out what might be wrong.

Are these plugs the OEM ones? Some cars seem to have trouble with “fancier” plugs, so I recommend sticking with the OEM ones.

Have you verified that your thermostat isn’t sticking? Are you keeping your tires at the right pressure?

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The rule with Japanese cars is to use only NGK or Denso plugs, of the original type.

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I agree. I wouldn’t use any other plug but those two on any Asian vehicle. GM is now using NGK in some of their vehicles.

My 05 4runner came from the factory with Denso on one bank and NGK on the other bank. I replaced all 6 plugs with NGK.

I’ll go look into the thermostat, I’m still trying to look into supporting components as well

Ok, I’ll try out some NGK plugs. I figured that might be the case.

I fill my Corolla’s tank when the gauge shows between 1/4 and E. It takes right around 10 gal. each time. Last night was 9.6, which is a fluke in my experience. Tank capacity is 13.2. If the OP is consistent I see no reason he wouldn’t add the same amount each time. BTW, his Mazda has a tank capacity of 14.5 gal. so 12.5 is what he’s actually putting in the car.

I just went over the gap specs on the plugs installed and the recommended ones. 0.06 to 0.08 is the recommended and currently the bosch ones have a 0.04 gap and said “do not gap” is it safe to try gapping these or should I replace them with NGK 0.052 gapped plugs? Would the 0.02 difference cause the awful fuel economy?

You have to be real careful gaping Iridium plugs. You could easily damage the tip.

I looked at the specs for your cars plugs. Were the original plugs Iridium? Doesn’t seem like they were. The only reason I’d use Iridium Plugs are. Gap size will change depending what type of plug is used.

#1 That’s what manufacturer suggests.
#2 The plugs are a pain to change. This Mazda is NOT. My wife’s Lexus and my Highlander are.

Is the fact that the gap is out of spec by .02" more of a problem or would the type also make a big difference?

The most common cause of increased fuel consumption is a faulty coolant thermostat that causes the engine to run too cool. Suggest to check that first. For a Mazda I’d prefer to see NGK or Denso plugs installed rather than Bosch. Unless Mazda requires multiple electrode plugs, suggest to go back to the normal one electrode version.

You might discover a clue by searching on the term Bosch multiple electrode spark plug w/the forum search feature, icon above right.