falling mpg

hi guys, my fuel milage has dropped from 28mpg, down to 23-24 mpg. and it happened in about a weeks time. what could have caused it. oh i have a 2005 altima 2.5 with 70,000 miles on it. i just put some fuel system cleaner in it yesterday, and going to put royal purple in it today or tommorow. i read somewhere the plugs dont haft to be changed until 100,000 miles. it is not missing when ideling, it just has a slight jump or stutter in it when driving. it is something you have to be feeling for to notice. what could have caused it to drop like that. oh and do they have fuel filters, if so where are they.

thanks larry

Plugs need to be changed after 60,000miles(100,000Kms)

Yes, they have a fuel filter but not sure where it is on your model. They do have a special clamp on them from memory so might be worth having it professionally changed.

Also, replace the exhaust O2 sensor. Common for them to fail :(

well i changed the oil first, went with royal purple. my MPG went up 6 MPG so far. and then i put new plugs in it yesterday, and it is running like a dream. i didnt put nkg (is that right) plugs in it i put platinum 2 plugs in it they were on sale, and im poor lol. well i must say that royal purple is just amazing, my car runs so quiet, and smooth, and has a little more umph. now a question, i did have a k&n filter but i was afraid it would let some dirt into the engine. i have the filter still, and i have some fram oil that it used with their simular filters, can i use that on the k&n filter, or just go buy some k&n oil.

If the K&N is brand new, install it as it is as they come pre-oiled. If it has been cleaned, then a light mist of filter oil is fine. Dont use too much, as too much will clog up the filter and suck the filter oil into the engine.

No problems with spark plugs, as long as they are new and are the correct plug for your engine they will make a difference :) NGK is the better brand but the difference is not much.

Search for NTK Exhaust O2 sensors if you want to replace them, good brand and cheaper than Bosch. Nissan actually buy their O2 sensors off NTK, so buying one from a dealer it is the same product without the dealer pricing ;)

I would buy a can of electrical contact cleaner (made by CRC, $5 at Autozone). Sometimes dust and fine dirt particles can collect onto the mass airflow (MAF) filament inside your MAF housing. All you have to do is disconnect your MAF housing from your air filter housing and spray a little bit (it doesn't take much!) of the cleaner onto the filament (look for the little metal wire inside the MAF housing). Even a little bit of dust can skew the voltage readings that get transmitted to your ECU, which causes your fuel trim to run more rich. It's a quick and easy fix, so it's worth a shot! :biggrin: