What could cause bad gas mileage?

Just looking for some ideas on the types of parts that contribute to (really) poor gas mileage (200 to a full tank). I have replaced the plugs and the Fuel Regulator already and there are no check engine lights at the moment, and the car runs perfect (other than costing me a small fortune on gas). Just looking for some ideas.

maybe the gas return line is clogged

knock sensor, vehicle speed sensor, heavy foot

ECU, bad gas (or winter fuel w/ alcohol additives), O2 sensors malfunctioning, MAFS, long warm ups during winter months, tire pressures...

There are about a million things that could possibly contribute to poor gas mileage.

Those are all things I have been told could be the culprits. It is just really hard to know when the car is not giving any codes. Today I changed the fuel filter and cleaned the throttle body, doubt that will help much but at least it is a starting point. I just can't stop thinking it is the O2 sensors, even though they are not sending codes.

I also had complained about poor fuel mileage on Maxima.org, the replies mostly consisted of running BG 44K cleaner through it. I have not done it yet, but I plan to. I have been getting about 21 mpg on the highway.

Yesterday my check engine light came on and I went to a garage and had it scanned: O2 sensor Bank 1 Number 2. I don't have a service manual, and neither did the garage, but I am assuming that is the sensor closest to the catalytic converter on the side closest to the firewall. If anyone could confirm that, I would appreciate it.

So it is quite possible, I think, that the O2 sensors run at the 'edge' of their tolerance for a while before dying, therefore not triggering the check engine light. Next question is, how do you know which one to replace without a code? Isn't there a software product out there that will show voltages and conditions of all sensors, I'm pretty sure I have seen it....If I find it again, I will post it.

Scott

Scott, There is a sensor that tests them, my friend is having the same problem as I, and Nissan put the sensor on his front two O2 sensors the the other day. Personally, I don't think it tells you much because it says his are fine and he is getting very poor mileage as well.


But if yours are shot and the light is on you should just replace them (not by Nissan-too expensive) and see how your mileage improves if at all, and then let me know. I don't have a light on at the moment, but I think I am going replace them anyway because I have exhausted all other ideas, and if I can't resolve this then I have to sell the car. I went 139 miles today on almost a half tank of gas! I am averaging between 14-16 MPG which is unacceptable for a max.


PS- Don't count on the cleaner making much of a difference, I did it and I am still getting poor mileage. You can count of this being something mechanical.

Everyone says not to buy my O2 sensor at the dealer because of the price. What is a good price on these? I checked at the dealer, they quoted me $77 . I stopped at Pep Boys last night, they wanted $136 and 5 days to get it.

Scott

I got my sensors through the mail the other day from http://www.oxygensensorwholesale.com, and the shipping was free. They are both OE Bosch sensors and they were $70 each. I would not buy them from the dealer either, they mark up the sh*t out of everything. I have not put mine in yet and frankly I am not too optimistic about it actually solving this issue since I don't have a light or any stored codes, but I will give it a shot. If this does not solve it I am getting rid of this car and I will NEVER buy another Nissan product as long as I live. It seems like this is a common problem with this VQ motor-alot of people complain about it, and although some owners are willing to live with bad mileage I find it simply unacceptable that Nissan can produce such inconsistent results with a motor. I am keeping my fingers crossed in hopes the problem will be solved, I just want to break 300 miles for a full tank, I am currently lucky if I get over 250 miles for a full tank, which translates to about 14-16 MPG. That is just pathetic for a small 6 Cyl.

id go to Auto zone, they do free OBD testing "check engine light" prices are compeditive, or go to the junk yard and find a O2 sensor if they dont have a junk of your car find out what kind of engin it is and find out what other cars have that engine, there usualy pretty universal. good luck. im havin same problem with my pt cruiser, its not throwin an O2 signal but half tank gets me 50 miles and thats it