robert1947

Since I bought it used 1,000 miles ago my 1996 SE automatic (102,000 miles) has been getting 15 to 17 mpg in both city driving and combined freeway/city driving on premium fuel with very conservative driving habits. I would have expected nearly 20 in city and high 20s in city/freeway. Any suggestions? Here's what I have checked:

Knock sensor tests ok for resistance at F121 connector (548 Kohms) and ECU yields no stored codes;

New platinum NGK plugs made no difference; old plugs showed normal condition;

Tires inflated to 33 psi cold;

Air filter (disposable type) only slightly dirty (paper element transmits lots of light when held up to sun);

Auto trans service performed recently, shifts are smooth, 4th gear is attained as appropriate or can be switched off;

Engine runs smoothly, has normal idle rpm, and seems to have plenty of power at all speeds.

Things I haven't checked:

Could there be obstruction in stock exhaust system? How could I test for that without removing exhaust components?

Could the trans be slipping? How would I detect slippage?

I'd appreciate your ideas.

myke321

Wired, cuz i get about 19.5mpg with my 96, half freeway half road, with semi-aggressiave driving. Maybe regular oil changes and fuel system cleaner will work.

Mr.Duck

iTrader: (8)

heh, im at like 160 miles, and i havnt hit my half way mark yet.. im like a lil past the 3/4 mark

waveridr85

iTrader: (4)

fuel filter?
knock sensor replacement?
maf sensor replacement?

MDeezy

iTrader: (27)

you ruled out a lot of options,

O2 sensors are another source of bad fuel economy, since there are no stored codes the assumption would be made that your o2 sensors are fine.

Proper oil (as I've just read) can decrease fuel economy *(I have 10-30w, when 5-30w is what is suppost to be in our cars (thanks Nissan for giving me wrong a** info)

something isnt right to be burning that much gas but getting such milage, maybe injectors need cleaning? or Maybe time for the Fuel Filter to be changed.


just some ideas.

Terran

iTrader: (6)

I believe O2's can be bad (or in the process of going) without throwing a code. There is a way to test them but I don't know it. That is really awful fuel economy though, so something is definitely wrong. Good luck.

nostrixoxide

Maybe the fuel pressure regulator is faulty. Too high of a fuel pressure would cause a rich mixture ='s bad fuel economy.

MDeezy

iTrader: (27)

if he has a rich mixture he should see black smoke comming out of the back.
Check and see if there is some black smoke comming from the back, it would be hard to see while your driving, assuming its a small amount.

nismology

iTrader: (8)

10W-30 and 5W-30 are both 30 weight oils at operating temperature so that's not it.

nostrixoxide

Mdeezy,

He can still be rich w/o seeing black smoke. The fuel can still be getting burned up in the combustion chamber. Hence the use of more fuel....

TheMax95

I have really bad gas mileage as well. I'm averaging 15mpg city driving with semi aggressive driving habits. but I have a cel with code 0446 evap solenoid so when I change that hopefully I'll get better gas mileage.

Oh and it doesn't hurt to change the two front o2 sensors. about $110 with the maxima.org %10 discount from www.oxygensenors.com
-Paul

XoPtOn

iTrader: (2)

i get 150 at half a tank =(

MDeezy

iTrader: (27)

didnt know that, (learn something new every day)

thats seems to be the problem then since no codes, new KS, air filter good, no 02 sensor code.

just need a scan tool to pull some numbers and see if he is running rich.

How could something like that happen? that is A/F gets altered without it being hooked up to a scan tool and changed?

nostrixoxide

Mdeezy, if the FPR gets restricted that will increase the fuel pressure. Which means the injectors will spray more gas into the combustion chamber. If you really want to learn more about fuel pressure then I suggest you read alittle bit about how dry nitrous systems work.

MDeezy

iTrader: (27)

I defiantely will, I enjoy learning more about our cars and car in general.

what is the FPR? not to familiar with that acronym?

Well what you describe sounds like it could be the culprite for his bad fuel economy. Also maybe if his tire tread is just about gone, that might cause some loss also.

nostrixoxide

Mdeezy,

There are many possibilities why he is having bad fuel economy. Bad tires included, thats why as a unit (Orgers) we post our opinions on what could be the problem. Hopefully all of our information can help him resolve the problem.

(FPR) means fuel pressure regulator. On EFI (electronic fuel injection) motors the FPR controls/maintains a certain amount of pressure @ the fuel injectors. When the pressure is too low then there is less fuel being delivered by the fuel injectors. When the pressure is too high there is too much fuel being delivered. So the goal is to find the pefect Stoichmetric (Air/Fuel Ratio). Usually 14.7:1 is common for economical driving & most car manufacturers like to be somewhere close to this range. Boosted engines (Turbo/ Supercharger) generally run a richer mixture for example (12.5:1). The reason being that it is safer to run the motor rich then it is to be lean. And when your forcing more air into any engine you need to make sure that it's also getting the proper amount of fuel also.

If you want to learn more about this you can check out these websites. Theres alot of knowledge waiting to be soaked up, if your willing to learn.
http://auto.howstuffworks.com/fuel-injection.htm
http://www.holley.com/nosnitrous/
http://www.aempower.com/truck/faq.asp?tid=4

f550maranello2

iTrader: (2)

i had a bad fuel pressure regulator in my old car and let me tell you guys it was the biggest biatch to figure out that it was the culprit. the car didnt want to start and black smoke poured out the exhaust (1993 T-bird) and when i said it didnt want to start i mean it really didnt wanna start i had to floor the accelerator every time the car stood for more than an hour. but to make the point my car right now dosent get real good mpg either if im luky i get +/-20 mpg in the city and the fuel filter is new new spark plugs clean IACV and TB as well as many other things i think its just how these cars are.

MDeezy

iTrader: (27)

Thanks nostrixoxide, very informative and thanks for the link will check them out.

thats why I love the org and check it out daily and throw in advice or potential solution, we all help out others that need help.

nostrixoxide

Yep.................

MaximusAureleas

I just got my Max thursday. I got about 200 miles out of half a tank driving home all highway. Is that a decent number? I dont even know how big our tanks are.

f550maranello2

iTrader: (2)

right now i have like 260 and have a little more than a quarter