Horrible MPG (was getting 37 mpg, now 27)

Hi, I own a 2015 es auto Mirage. Bought it almost new at 5 k miles. until 46 k miles I always average above 37 mpg.
Now the best I can get is 27 mpg. I changed, oil every 5k with mobile one full Synthetic. New air filter, New brakes, " made sure they where not the the problem" use good quality gas, not the problem. Not hard on the car. nothing has changed since I owned it a long time, same style driving. When the issue started thought maybe spark plugs, changed them didn't change. change air filter didn't matter, checked tires brakes, gas no fuel leaks... asked Mitsubishi they have no clue, asked my uncle a retired master mechanic he has no clue. Seriously Im lost. Anyone have any deep insite. No engine lights shifts fine. I also check my mpg manual each fill up and it correlates to the cars readings, so that is not the problem. Any Ideas? seems to run great just mpg is horrible for this car!

Have Mitsubishi or your uncle inspected the car? If not, I think that it would be in your best interest. It may be your oxygen sensors that have gone bad. Have you done any modifications to your car?

Ignore my post on your other thread... looks like you did all the obvious checks. I can't say I've ran into that problem yet. My work has almost 40 mirage's and the worst mpg I've seen on the display is 39mpg shown on trip "B".

check compression in all 3 cylinders.
From experience I know these cars run quite well with little to no compression in one cylinder. They just get hard to start in cold weather.

Someone with a very good scan tool (not a $59 code reader) needs to take a look at some basic diagnostic data in your car's computer. I would want to know what your car's long and short term fuel trim looks like. Without going into a huge explanation, it's a simple way to tell if your computer is intentionally directing more fuel to be burned by the engine for some reason...like to compensate for a real or perceived lean condition caused by too much air entering the engine.

Here's a quick overview: Fuel trim can be a valuable diagnostic tool

Any chance running higher ethanol fuel content, the drop matches up with my experiments/experience with other cars, FFV and non. The LTFT will be +20-30 even with E50. 26-28 throws a CEL on my non-ffv cars (<55% ethanol).

My wife filled her fairly new to her 2015 rogue a while back with a full tank of E85 (previous 2 cars were FFV). Next day when I drove it I noticed it didn't start a quick as normal instantaneous. Little while later notice a small CEL, 10 miles later I figured out what she did, dropped from 33 with me driving it to 25. We were doing a little road trip that day, so I just kept topping off and the CEL cleared the next night. I've blended in all my car, up to E50 usually no symptoms at all(other than some loss of mpg), other than the start the rogue didn't mind E85, but I tried not to work it very hard.

I use the same gas stations as my 2013 scion Tc which gets 28 mpg, and i drive it hard. so I dont think that is the issue. Yes I agree maybe a better diagnostic tool should be used. My harbor freight one reads nothing as I have no codes anyways. I did call Mitsubishi in Plano and they had no Clue, anyone maybe have suggestions for someone in the Dallas area. Its worth it for me to pay a little to figure it out as currently my simple things have all been done and still with the horrible MPG. BTW I did try 91 for to tanks and no difference made. this has been going on since 46k miles im almost at 50k. done everything I know so far. I might try a independent shop, this Mitsubishi im plano just told me , plugs, air filter, gas, oil, tune up, tires, brakes.... ive done all those! My uncles tools are outdated been retired 5 years....sorry if my english is bad I try. I will have my uncle check compression also. thanks for the ideas. I will update tomorrow my uncle said he can check compression in the morning, and have another app at Mitsubishi in Dallas.

Even something simple like a bad coolant temperature sensor can lower your fuel mileage...

Symptoms of a Bad or Failing Coolant Temperature Sensor

Again...this is something that won't necessarily cause a check-engine light, but it will be obvious to someone with a good scan tool.

Hello ivsito and welcome to the forum.

Hopefully you are able to figure this out soon. The first thing I thought of was fouled valves, as in this thread: 3A90 Intake valves stuck open, carbonized, full of gunk and car won't start. I expect this would lead to poor compression as IchabodCrane suggested.

Whatever the cause, please be sure to share your findings.

Please do, I've been in a similar situation all year. I luckily still have to do a lot of highway driving where it still shines, but city mpg is deplorable.