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Hi. After draining and refilling my tank, I realized I only got about 240 MPT, MAX. (I noticed this because I just filled up my car yesterday, and again about 2-3 days before that) Doing the math, thats like 19 mpg. Thats freeway and city miles, probably split 50/50; or close to that. Most people say they get 30mpg, easy? What can I do to improve this?? Ive already cleaned the TB, MAF tube thingie, but im stumped as to why this is so low.

I dont think its the o2 sensor (right?) because the guy before me 'said' he replaced it. I have checked the pressure in my tires, its 36-37 PSI on all 4 corners; so it cant be that. I dont spike the RPM's up high constantly, either (3-4k). Im completely out of ideas on this one, and I really dont like putting 30 bucks in my car every other day (id rather spend the money and get it FIXED). Any input is appreciated.
 

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Nissan_4.2.0. said:
Hi. After draining and refilling my tank, I realized I only got about 240 MPT, MAX. (I noticed this because I just filled up my car yesterday, and again about 2-3 days before that) Doing the math, thats like 19 mpg. Thats freeway and city miles, probably split 50/50; or close to that. Most people say they get 30mpg, easy? What can I do to improve this?? Ive already cleaned the TB, MAF tube thingie, but im stumped as to why this is so low.

I dont think its the o2 sensor (right?) because the guy before me 'said' he replaced it. I have checked the pressure in my tires, its 36-37 PSI on all 4 corners; so it cant be that. I dont spike the RPM's up high constantly, either (3-4k). Im completely out of ideas on this one, and I really dont like putting 30 bucks in my car every other day (id rather spend the money and get it FIXED). Any input is appreciated.
do you have a CAI or a aftermarket filter? i've noticed an improvement in gas mileage when i popped in an aftermarket filter. maybe the gas station you refuel at "watered-down" their product. :p other than that, i can't think of any other solution. try switchig gas stations.
 

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265k miles and counting..
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It could be a number of things. The O2 sensor is a possible culprit, but so are a plugged catalytic convertor and leaky fuel injectors that are stuck open. A bad O2 sensor or a leaky injector could easily cause the engine to run too rich and fry the convertor due to excess raw fuel entering the cat.My abusively driven (pizza delivery car) B14 Sentra with a dead stock,127k mile GA16DE/auto trans combo averages 26-27 city and 40-45 highway, so there is DEFINITELY something wrong! I would have a code scan run by an auto parts store, most of them where I live will do it for free and tell you what codes are popping up. If that checks clean, the best diagnostic is done with a genuine Nissan CONSULT tool which would be found at a dealer. What you need to see with that is if the o2 sensor is oscillating or not. A steady reading indicates a faulty unit. If you want to skip the dealer's prices, you can perform a similar check with a digital volt ohm meter if you probe the wires correctly. To check for a faulty cat, remove the o2 sensor while the engine is cold, and thread a compression tester into the hole. Start the engine and check how much backpressure there is. I am not sure of the exact figure you should see, but it should not be more than 5-10 PSI IIRC. If it is high, have the cat replaced and that will fix your problem. As for the bad injector possibility, that is something a shop should be able to check for and I am unsure how to do it at home.
 

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i was having the same issue with mine and all i did to take care of it was run a couple of bottles of fuel injector cleaner though it and changed the oil. I'm getting between 26-32mpg now, and i drive my car pretty hard.
 

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get new o2 sensor,put in fuel system cleaner,clean out the throttle body,change air filter,oil change(with a little bit of oil stabilizer),new plugs and wires,check tire pressure.
 

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TwiztidKidd
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himilefrontier said:
To check for a faulty cat, remove the o2 sensor while the engine is cold, and thread a compression tester into the hole. Start the engine and check how much backpressure there is. I am not sure of the exact figure you should see, but it should not be more than 5-10 PSI IIRC. If it is high, have the cat replaced and that will fix your problem.
Hey now that's really smart how come I never thought of it. Thanks man. I'll have to try that. I suspect the catalytic converter too on my car. I took it off the car so many times, cause my whole exhaust is bolt-on... no leaks tho. If you look inside the cat from the outlet side I see a chunk out of that material missing ... a portion about 2" x 1/2". I have to test the backpressure.
 

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265k miles and counting..
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Twiz said:
Hey now that's really smart how come I never thought of it. Thanks man. I'll have to try that. I suspect the catalytic converter too on my car. I took it off the car so many times, cause my whole exhaust is bolt-on... no leaks tho. If you look inside the cat from the outlet side I see a chunk out of that material missing ... a portion about 2" x 1/2". I have to test the backpressure.
If it has chunks missing, it's bad.That means it overheated and melted down.
 

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Discussion Starter #8
himilefrontier said:
It could be a number of things. The O2 sensor is a possible culprit, but so are a plugged catalytic convertor and leaky fuel injectors that are stuck open. A bad O2 sensor or a leaky injector could easily cause the engine to run too rich and fry the convertor due to excess raw fuel entering the cat.My abusively driven (pizza delivery car) B14 Sentra with a dead stock,127k mile GA16DE/auto trans combo averages 26-27 city and 40-45 highway, so there is DEFINITELY something wrong! I would have a code scan run by an auto parts store, most of them where I live will do it for free and tell you what codes are popping up. If that checks clean, the best diagnostic is done with a genuine Nissan CONSULT tool which would be found at a dealer. What you need to see with that is if the o2 sensor is oscillating or not. A steady reading indicates a faulty unit. If you want to skip the dealer's prices, you can perform a similar check with a digital volt ohm meter if you probe the wires correctly. To check for a faulty cat, remove the o2 sensor while the engine is cold, and thread a compression tester into the hole. Start the engine and check how much backpressure there is. I am not sure of the exact figure you should see, but it should not be more than 5-10 PSI IIRC. If it is high, have the cat replaced and that will fix your problem. As for the bad injector possibility, that is something a shop should be able to check for and I am unsure how to do it at home.
Well, heres an update. I replaced the old busted up 02 sensor and ran a bottle of injector cleaner through the engine. I filled up the tank with regular 87, and ive driven 200 miles on only half a tank! So, I wanted to thank everyone for their suggestions because they contributed to solving my problem. I also had Autozone check for codes AGAIN but nothing came up, so im guessing everythings 'ok' for now. Anyway, thanks again.
 

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Replaced my plugs, but it took a week for my gas mileage to return to 28mpg.. for the first week with the new plugs in it was still at my old 20 mpg.. then on a new tank I began to gain marvelous miles per gallon.

Also, Twiz, your melted cat gives you an excuse to buy an aftermarket one! Smells like 2 hp gains, baby!
 

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TwiztidKidd
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807 Posts
I'd say the chunk missing out of the material inside was because of a shock... . But I'm gonna go ahead and replace it. It does smell like the catalytic's been fried. Prolly that's what caused my header to rust sooner than usual. First rust spot was right where the downpipe meets the catalytic and I had no leaks. Any idea what the max temperature on the catalytic is supposed to be ? I can borrow a temperature reading gun from a friend.
 
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