This will take a minute to provide info about everything leading up to decreased gas mileage, so please stay with me. I bought my '00 max a few months ago with 75,000 miles. It had some hesitation upon acceleration. I changed the plugs....no difference. I added fuel injection cleaner and octane booster at the same time to the fuel tank...no difference. I read that a dirty throttle body and/or idle air control valve may cause this. I bought some throttle body cleaner at advanced auto. I removed the maf sensor and intake duct and opened the butterfly to the throttle body and sprayed the cleaner. I used a rag to remove as much grime as I could. I removed the solenoid part of the idle air control valve and sprayed in there also. I pretty much used a whole can without removing the iacv or the tb, which probably would have been better if I did. A few days later, the maf sensor went out. I ordered a new one from Dave B. and finally the hesitation is gone. Now, after one month+ driving with the new maf, my gas mileage has gone to crap. Before changing the plugs and all of that other stuff, I could get 220 miles on 1/2 tank. Now, after all that has happened above, I am lucky if I get 150 miles on 1/2 tank. Is there anything at all that would cause this difference? Would it make a difference if I removed the iacv and tb and cleaned them properly? Is it possible to have a bad O2 sensor and no cel? If anyone has experienced this or can offer some advice, I would greatly appreciate it. Btw, I have no cel. Please help me if at all possible. Gas prices suck. Thanks in advance.

 

I forgot to mention. I didn't see any reason to have the ecu reprogrammed after the maf sensor since the car acts fine. Could this have anything to do with my crappy mileage?

 

Coilpacks...

 

I am definately no Mr. Goodwrench, but it seems odd that the gas mileage was fine before all that was done for the problem to be the coilpacks. If one of the coilpacks were bad, wouldn't there be some hesitation or rough idle or some indication? Not doubting you at all. Just checking. Thanks....Matt

 

Well I missed where you said that the hesitation is gone. Have you disconnected the battery overnight? If not try it so it starts fresh and see what happens. Check the fuel filter and make sure you have no gas leaks...

 

I unhooked the battery the night before changing the maf sensor. Is it possible for the fuel filter to cause a difference of 70 miles on 1/2 tank in a period of less than 2 weeks? I really think that the problem began when I sprayed the cleaner directly into the tb and iacv without removing them. Maybe I screwed up a sensor and it just isn't throwing a cel or perhaps the computer needs to be programmed. I have no clue to be honest. I appreciate your help very much.

 

I cleaned my TB in the same way you did, I did almost the same thing and I have had no problems. I would unhook the battery again with the new MAF and see if it helps. Its worth a shot, it only takes a few min, and if it does not work your not out anything.

 

I think I'm experiencing the same problem. I have a sleeper 2k max and get 160 to a half tank every time if I'm lucky just like you.. and I don't drive hard at all... I have new coils, new plugs, and a relatively new maf, but my mileage is still ****. I cleaned my tb about 3000 miles ago, but didn't touch the butterfly valve, and don't know of any way I could have messed it up. Also, I have not touched my iacv. I've tried the same methods you have (fuel injector cleaner etc) to try and raise mpg as well as seafoam, but no luck. Please let me know what you come up with as the diagnosis for your problem.

EDIT: here is my original poor gas mileage thread, of course with no answer other than to use Auto RX, seafoam, injector cleaners etc: http://forums.maxima.org/showthread.php?t=436870

 

Weather probably has something to do with it... My car always gets crappy mileage during winter time. Its because cold engine is less efficient then a hot engine. Most of the energy is wasted on heat trying to warm up the engine instead of being transfered to the wheels. Also, if you unplug the battery to try to reset the computer, the computer will have to re-learn everything which will give you crappy mileage. It will go back up once the computer learns your driving habit and adjusts its programs. This usually takes a couple of hundred miles for the computer to learn.

 

It may be the winter gas. Try resetting the ECU. That might help also.

 

damn, i usually only get 150-200 miles out of a hal of tank, and about 320ish total = approx 21-22 mpg. thought that was normal. still getting 27 highway.

 

Please tell me you are kidding with this.

 

Is it possible that reprogramming the ecu will restore previous mpg results? Anyone experience this? I am open to all suggestions. Thanks to all who have responded.

 

When you burn gasoline, not all the energy created from combustion is turned into kinetic energy. Some of the energy is loss through heat transfer. A car's engine is most efficient when it's at its operating temperature (ie better gas mileage), Not when its cold. So the colder the weather temperature is, the longer it takes for the engine to reach operating temperature.

 

Another consideration, with all the TB cleaner you used, you may have damaged an O2 sensor or two. This same thing happened to me. Do the O2 diags, and see what's up.

 

This is true my friend. The gases created by combustion is very hot. If your engine block is cold, it will cool down thos gases pretty fast lowering the pressure in the cylinder(heat transfer). The higher the pressure in the cylinder, the more engery is converted into motion (pressure created from combustion moving the piston)

So if youre engine is hot, less heat transfer loss and more pressure to push teh pistions creating mechanical movement. The colder the engine, the more the engine sucks up the heat from the combustion cooling down the heated gas causing a decrease in pressure. Remember, the hotter a gas is, the more they expand creating more pressure if they are in an enclosed space.

Colder engines decreases the thermal engery used to move the pistons. They absorb a lot of the energy using it to raise the temperature of the engine instead of converting it to kinetic.

My point to all of this is the closer the temperature of the engine is to the temperature of the burned gas, the less energy is lost through heat transfer which is a waste of gas.

 

There seems to be some debate on that - see the thread below.
http://forums.maxima.org/showthread.php?t=409686
I've also seen threads that indicated a drop in mpg after running a new MAF without programming, but I can't find it.

As far as gas mileage - 150 to 1/2 of a tank doesn't sound too bad - I usually get 170 - 200 in warm weather. When it gets cold, I start getting worse than 150 sometimes.

 

try turning on that little button that says O/D!

Seriously, though, is your engine running cool? Does the heater want to blow out heat as soon as the temp needle moves? If so, try changing your thermastat. Might help some.

 

I spoke with someone at AutoZone and they told me that their code scanner can't check to see if I have a bad sensor without a check engine light. Is he full of sh*t or is this true? If he is correct, will I have to go to the dealership and pay to have it checked out there? Also can you have a bad O2 sensor with no check engine light and can you have a bad O2 sensor and the car still run normal? Thanks again. Btw, anyone know how much dealerships usually charge for a scan and also to have the ecu reprogrammed?

 

your O2 is probably good. The guy wasn't BS'ing you either. Make sure your airfilter is clean, check your temp like I said, and disconnect your battery overnight. You can also run a little octane booster through your gas to try to clean your injectors and valves. One other thing: it's significantly colder out so check your tire pressure. Might be low!

 

On a more serious note, what is the weather like where you are? Here we are experiencing 10-15F weather now and my gas mileage has dropped 15-20% as a result...

 

Current temp is 37. Lately, on average, the temp reaches mid 40s during the day. 10-15.....bbbbuuurrrr. We sometimes have temps that cold in Jan. or Feb. during the night, but usually not for very long. I wouldn't think that mid 40 weather would cause a significant decrease in gas mileage.

 

With our weather I get 20-22 mpg most of the time now, whereas in summer I would get 26-30 depending on how hard I drive it... How does this match your mpg?

 

WINTER GAS...In the winter gas is less refined due to an increased need for oil products. Think home heating oil, less people walking. Gas is less refined to help keep up with demand. Some people can see up to a 30% decrease in gas mileage in the winter, though average is probably closer to 15%.

 

The car was purchased in September and up until cleaning the tb and iacv and replacing the maf, it got around 24mpg. Now, it is getting probably 17 +/- mpg. The maf was replaced in late October or early November and I have been getting crappy gas mileage since. Even if cold weather does affect gas mileage, I don't think the weather has gotten cold enough here to contribute to a loss of 7 mpg.

 

Btw, I also have a '97 max and have not seen any difference in gas mileage. Both cars are driven under the same conditions. It seems to me if cold weather were a factor, I would see the affects in my 97 also.

 

I get 220 to the tank. No codes, perfectly maintained, 54,000 miles. I have not changed the maf but I dont have any of the symptoms of a bad maf. I drive 11 miles round trip to work 5 days a week & on the weekends I am usally in my truck. I just chalked it up to hard driving & short trips. My car hits 85 to 100 MPH everyday on the way home for about a 1 mile stretch which cant bee good for gas mileage. I figure something has to be wrong but I dont know what it is & it certaintly is not a maintnance issue. Air Filter is clean & oiled. Fuel filter was changed by me at 50,000 miles as well as plugs & pcv valve. Mobil 1 full synthetic motoroil changed 4 times a year & Mobil 1 transmission fluid changed every 2 years. BG44K fuel injection cleaner once a year in june. Raditator flushed once a year & filled with toyota red & redline water wetter. TB cleaned. It is not a maintnance issue. I had given some thought to possibly running rich but my plugs were fine. My guess is as good as yours as to the bad mileage per gallon.

 

220 to the tank.....If my math is correct, that is about 12mpg. There has got to be something wrong. Have you ever taken your car to a dealer? If you happen to find out wtf is going on, let me know. Sweet @$$ ride btw. I like the wheels and the paint.

 

Does anyone know if it is possible to have a bad sensor, but yet not throw a check engine light? If so, is there any way to diagnose this since the scanner used by Advanced Auto will only diagnose a problem that throws a cel. Sorry for all of the questions, but it is driving me crazy knowing that my max went from 24mpg to 16-18 in a matter of weeks.

 

Sorry there buddy but gas is not less refined just because its winter.. I work in the oil industry so i know.

 

As Mclee45 said, it's not a difference in refining. What is different about gasoline in the wintertime is the increased addition of oxygenates such as MTBE and ethanol, which are supposed to promote more complete combustion and thus reduce emissions. In reality in just about any modern car made later than the early 80's oxygenates don't make much of a difference in emissions. But they still put them in, and they do tend to decrease performance and fuel economy.

 

Bump.... Is there anyone who can provide more feedback on this situation. It would greatly be appreciated.

 

I just bought my 2000 2 weeks ago, and I am getting just over 300 miles before filling up again. I was hoping to get better gas mileage with this car, not worse
My previous daily driver get about 22 mpg and I drive them the same way. Thing is the previous car is an 88 Camaro with a 5.7L. And the car was/is running lousy, has some fuel tank leaking issues, and I haven't really fine tuned the programming.
Reading some of the post on this thread it seems 20 mpg mixed driving is somewhat normal.

 

Don't forget that in some states, the gasoline you buy changes once the weather starts getting cold. Additives like ethanol or MTBE are added to gas to oxygenate it. This is done to reduce emissions, but it also does a number of fuel efficiency.

My gas mileage always goes down in the winter. I've always attributed it to the additives, since I can't drive the car as hard in the cold (snows aren't nearly as sticky as the Toyo's ).

 

Here's another thread for you:

http://forums.maxima.org/showthread.php?t=398513&page=5

There's evidently some kind of difference between the 2000 and 2001 MAF, but I don't know what it is exactly since I have a 2001 and haven't seen a 2000

 

As previously stated, I also have a 97 maxima and I have yet to see a change in gas mileage. Same weather conditions....same driving conditions.....same gasoline used.

 

Whitemax, there's no way that the 02 sensor is bad if no codes have been thrown, assuming that your ecu is good. I'm looking into how to check and see if possibly our engines are running cool, and if that's the case for me, I'll post my fix.

 

It sounds crazy, but its actually true around 240 to the tank. Even though i have new coils, spark plugs, oil change, flushed the radiator, and also installed Injen CAI. All of these things should bring me better mpg, but its the opposition direction. Don't know what's wrong. Also, sometimes the gas meter just take a deep dip on the meter scale when driving or parking, usually dropped 1/2 of the quarter marks on the meter.

 

Thanks for the reply. Does this mean that if a code is thrown that the CEL will come on? I just remember from changing the ks in my 97. I was able to pull the ks code, but at the time, there was no cel. Also, if the engine is running cool, how does that affect gas mileage? Did you happen to have your ecu reprogrammed when you had your maf replaced? Do you think this could have something to do with our problem? Sorry for all of the questions. Just trying to narrow down the possibilities. Thanks again....Matt

 

According to the dealership, if the cel is not on, no codes have been thrown, so yes, if a code is thrown by the ECU at anytime, the CEL will come on. I don't know if that's a new thing from the 4th to the 5th gen, but that's what the local dealership here in indianapolis said.

My understanding is that if the engine is running cool, the energy created by combustion of gasoline is absorbed into the engine in heat form, where an engine running at normal temperature will get all the power from each explosion. The portion of energy is being absorbed into heat instead of torque is compensated for by higher consumption of gas. More consumption of gas for the same amount of energy = lower gas mileage.

I had my maf installed by the dealership because I was in a pinch for time and I wanted it done right. I'm assuming they did the reprogram, but you never know sometimes... so I don't think that's our problem.