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Sudden Decreased Fuel Mileage!

I own a 2011 Nissan Versa Hatchback with just over 83k on the odometer. Apart from scheduled maintenance, regular oil changes, and tire changes/rotations I have not done anything to the vehicle. Living in Southern California a couple months back I started noticing a marked decrease in fuel mileage. Generally I can get +/-305 miles on a 10 gallon fill up if I stretch it to the empty line. Suddenly one tank only made it to 285 miles before hitting the empty line. Then it went down a slight bit more. Nothing else seemed to be going wrong with the vehicle. No Check Engine Light, or other Warning Lights for that matter, no loss in power (the car isn’t powerful any way), idle feels fine and so does the throttle response/engine revs when I drive.

I brought it in for a Diagnostics test to be done for $120. I’ve attached the link to their report so that you guys can check it out. Basically they want me to do their Fuel/Air Induction Service for $300, then replace the Spark Plugs, the Mass Airflow Sensor, and the Air filter. I’m going to have a second opinion made but I also wanted to bring this up to the group here. I’ve heard before that this Fuel/Air Induction Service can be a waste but I think in my case it potentially might not be a waste. If it gets suggested by another mechanic then I’d probably consider it.

All in all I was looking at $1200 out the door but that also included an oil change, headlight replacement, and replacement of an engine mount that had failed (I guess) causing excessive engine shake. If I just had those Fuel/Air services done and parts replaced I was probably looking at close to $1000 out the door.

This link is to their Diagnostics report for my vehicle. It’s somewhat in depth and hopefully to someone who is more fluent in mechanic language it can tell a lot more than it did to me. Thanks for any and all input. I am currently in the process of getting a second Diagnostics done from another shop this coming Saturday. When they have a report I will come back here and share.

When someone complains of a sudden drop in fuel mileage, the two things I check are the coolant temperature sensor for the computer, and the thermostat.

If the coolant temp sensor tells the computer that the engine doesn’t come up to full operating temperature, when it actually does, the engine will use more fuel.

If the thermostat is stuck open and doesn’t allow the engine to reach full operating temperature, the engine will use more fuel.


The repair price sounds reasonable. But seriously you need to keep track of your miles per gallon correctly. Also running your vehicle low on fuel is not the thing to do. Are you stopping when the pump clicks off or are you trying to put as much in as possible. Also not good.

A 5% increase in consumption? Which way is the wind blowing?

The first thing stated on the inspection report is that different fuels will result in different results. To spend $1000 to save a dollar on gasoline doesn’t make any sense.

Follow your maintenance schedule for oil changes, spark plugs and filters, the oil change sticker shows that the oil change was due @75,700 miles.

The Powertrain Control Module monitors coolant temperature, if it is out of specification there will be a fault code.

I don’t see a broken engine mount in the picture, only surface cracks in the rubber. Does it appear split to you?

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The linked report was instructive, You apparently moved from Southern Ca to WA.
The report also note the you were getting pure gas in CA and 10% ethanol in WA.
That alone could account for your drop in mileage.

It also note that you replaced your tires. If you replaced low rolling resistance tires wit regular tires that would reduce your mileage more.

The temp is cooler in Wa. that will make your fuel mileage a little less.

I don’t think there is anything wrong with your car. I do think you should learn how to figure your fuel mileage.


A faulty coolant temp sensor will not always turn on the Check Engine light.


Thanks for the link. I’ve done a fair bit of Google research on what can cause my problem but what I’ve learned is that there are a TON of potential causes. What I am looking for from the mechanic is for them to start with the basics and move out from there. Rather than immediately suggesting a $1000 service that MAY fix the issue. I am hoping to hear of a couple potential routes I can take for getting the MPG’s back to where they were.

What I intend to do is make of list of suggestions that I receive here on this forum, and combine it with the report from the first mechanic so that when I bring my car in to the next mechanic, I will have some leads for him to follow along with whatever they would normally check out for this kind of problem.

The decrease in fuel mileage started about 2 months before I moved to Washington so while I will keep that in mind for future reference I don’t believe gas or weather were the initial causes. Like I’d said before I could easily average 300 miles on 10 gallons of fuel so 30 mpg. It dropped to 285, then 275. Now on our last refuel I think the odo was around 220 miles. This is over the course of 5 fill ups. Yes I know you aren’t supposed to run the tank that low but for the sake of research I wanted to know what I was totaling out of a tank and how much that was dropping each time. I also don’t over fill the tank when it clicks.

On a 10 gallon fill up I was averaging 300 miles so 30 mpg right?
Then it dipped to 285 so 28.5 mpg right?
Then it went lower to around 275 so 27.5 mpg.
On it’s latest refill I noted a total distance driven was 220 so 22 mpg right?

That’s an 8 mpg difference between when it was running normal and now. Am I figuring that right or is there something wrong?

When a person owns a vehicle long enough, they can pretty much sense when the fuel mileage drops off.

Every person who’s come to my shop for a drop in fuel mileage, never kept records of fuel mileage.

They just knew the mileage dropped off.


You need to do the actual math to calculate your fuel economy.

You should do the routine maintenance listed in your owner’s manual, but the “fuel/ air induction service” is bogus. They are NOT advocating the replacement of the MAF sensor, just cleaning it. Cleaning the MAF sensor is unlikely to help.

The motor mount does not appear to have failed, but that could just be the angle of the picture. Are you noticing any unusual shaking?

That’s quite the slick report they gave you. That slickness added to them hawking the BG induction treatment raises a bit of a red flag for me.

In regards to the motor mount I haven’t noticed anything in particular that would worry me. I can feel a difference when I’m pressing down the brake pedal in how smooth my car idles when in gear VS in neutral or park. That always annoys me if I’m stopped for a long period of time so I may put it in neutral or park depending. But it’s not violently shaking back and forth when I’m stopped or when I drive that I’m aware of. All they report is that the engine torque mount is “cranking” which I have no idea what that would look like so IDK.

Check Mechanics Files for another mechanic. This guy is taking your money without proof of a fix.

U can fill up and than refill at 1/2 tank and use that fuel amount to check ur mileage. U don’t have to drain the tank to get mileage numbers. 10gal at 300 miles is same avg as 5 gal at 150 miles.


I’m not sure if all cars are like this but from the Full line to the Half line on my fuel gauge I can make it about 220 miles easily. By that math I should be making 440 miles a tank. But what happens is once I go past that Half line it drops much faster than it does when going from Full to Half.

When I first bought the car I thought I was gonna be making 450 miles on a full tank but learned quickly that my fuel gauge was tricking me. My old car did this, and my motorcycles have all done this as well. It’s just easier for me to use a full tank to determine my mpg.

That means nothing, do it the right way or just don’t do it.

What means nothing?

The way to check your gas mileage is to divide the number of miles driven into the amount of gas used to fill the tank when you do so. Do this for at least two tankfuls and post the results.

225 miles driven, took 8.4 gallons to fill = 225/8.4 = 26.785 mpg

  • 243 miles driven, took 9.1 gallons to fill = 243/9.1 = 26.7 mpg

    avg of 26.785 mpg and 26.7 mpg = 26.7425 mpg

Once you have the results, you then need to compare it to your car’s EPA rated mileage.
Understand that a change in driving environment can have a serious impact on gas mileage. I visit my son every year in southern California, and the driving environment there is terrible on gas mileage. It’s way, way too populated, and everyplace outside of the San Fernando Valley requires driving squirrelly roads up & over the mountains in heavy traffic. Very heavy traffic. Very very heavy traffic. :relaxed:

Your air filter is filthy. Other than that and getting all the other maintenance up to date, I’d be reluctant to suggest anything until you get a good mileage figure.

The engine mount shows surface cracks in the rubber, but this is normal. Unless when you shake the engine by hand it thunks or there are actual tears in the rubber, I don’t think you need to replace it. Try shaking the engine vigorously by hand and if the mount has failed you’ll know.

One of the things I’ve found at some dealerships these past years is that they recommend many things that aren’t really necessary. They seem to recommend everything be changed that does not look brand new. This can be costly and not add at all to your car’s health or longevity.

I do want to thank you for posting the report. It’s truly great to have that level of knowledge to work with. And the report itself is a thing of beauty. I’ve rarely seen a report that thorough and clear.

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Please, learn the simple process of accurately measuring your MPG. Your entire list of posts indicated that you do not know how to do this.

  1. Fill up the tank. Record mileage.

  2. when the tank is 1/2 empty or more, fill up again. Record number of gallons and mileage. Subtract the two mileage numbers to get miles driven. Divide that by the number of gallons. Result is MPG.

  3. Repeat step 2 above as needed, recording numbers as above.

edit: TSM and I were typing at the same time…


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I like this detail. Fuel tanks should not be run to their bottoms.