I have a 1994 legacy with 8000 miles and mpg is very poor . have changed o2 sensor plugs and fuel filter so far and check engine light keeps flashing also. can anyone help. John

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Wow, that's bad!
Someone stealing your fuel? Hole in the fuel tank or lines?

The stats say
Regular Gasoline EPA Fuel Economy 17 MPG combined city/highway (15 City, 22 Highway) 5.9 gal/100mi 359 miles Total Range https://www.fueleconomy.gov/feg/Find.do?action=sbs&id=25378

You need am auto electrician with a pro grade scan tool computer to check long term and short term fuel trim, check MAF and o2 efficiency. With that consumption your spark plugs should be black, if the engine management system is dumping fuel. The other possibility is that you are 'lead footed', racing from traffic lights, revving through gears, using 'kick down' all the time, and stomping on the brakes at the last minute. That would easily shave 50% off normal consumption.

https://www.google.com/search?q=2009+dodge+journey+bad+gas+economy

For the passenger heating, that one is easy, blend doors stuck, blend door actuator failed, or a linkage has come off. Videos show how to get there and check.

Has it ever been tuned up ie. Plugs, wires or boots, filters, and injection cleaning? Check the tire pressure, under inflated tires are the #1 cause of poor mileage. Also consider fuel quality and additives that don't contribute to combustion. I hope this helps, Take care.

Lead foot, tire pressures, stop /go driving. AC/poor compression, Trunk full of sandbags, transmission slipping, Wheel bearing friction, Brake shoe always on, etc.

I'm not sure what the problem is specifically, but I can assure you this--you're basically throwing money way (and possibly exacerbating the issue) by using 93 octane fuel. Your car is timed and tuned to run most economically and with best performance with 87 octane fuel. The higher the octane, the slower the fuel burns. Longer burn time can equate to more power, but ONLY if your engine is designed and tuned for this application.
Couple of suggestions..checkto make sure your exhaust is "free". A plugged catalytic converter can really rob mileage. Tire pressure and air filter are worth mentioning as well.

The biggest fluctuation on fuel economy is caused by the driver... The listed MPG that they show is under lab conditions. If you want to increase your fuel economy drive the speed limit, dont over accelerate from stop light to stop light. You can also try replacing your stock air filter with an aftermarket such as K&N filter.

When you do a tune up you should replace the cap, rotor, plugs, wires,airfilter, and fuel filter, this will insure that you get the best mileage. Also have the coil checked, for proper resistance. If it starts to get weak it will throw a lower than normal voltage spark. You can not look at spark plug wires and tell if they are good, this can only be done with an ohm meter. The resistance should be 3000 to 6000 ohms per foot of wire. One other note is if your car feels sluggish or lacks power is to check the catalitic converter. These clog up and stop air flow thru the engine. There are many many things on cars that will result in poor gas mileage. A good mechanic doesn't go by just the codes a car spits out, they have to read the numbers and diagnose what is wrong.

old car, when rings begin to wear oil leaks into the combustion chamber burning more oil and reducing mpg try a product called restore and use it at every oil change also make sure you dont have black oil and that you are changing the oil at regular itervels also plugs wires roter cap and fuel filter and air filter are changed any abuse to these areas will reduce engine life i change my 350000 mile car every 500 miles reusing my filter everyother time

have your EGR system checked . sounds to me like its not working or clogged

Most likely you need motor mounts for the shaking and clunking.
The check engine light being on could cause the accelerating problem. It could be something as simple as the fuel filter being old and plugged up. Have you ever changed the fuel filter? This could cause the engine to run lean making the check engine light come on. And when accelerating the engine will be "starving" for fuel causing poor acceleration.

Jen,

Some other easy items to check. Remove and clean the MAF/MAP Sensor and the inlet to the thottle body with some brake cleaner/electronics cleaner. Also may want to try some Fuel Injector cleaner as well. The O2 Sensors usually require a special socket which you can buy at the local auto parts store, and you can also use a "Crowfoot" Wrench. Usually if the O2 Sensors are bad they will set off the Check Engine Light. Some other items to consider is a Rotor and Rotor Cap if you have a distributer instead of a coil.

Sincerely,

JC