deteriorating gas mileage

jasonk50

Is it normal for a vehicle's gas mileage to
deteriorate as the car gets older and a lot of
miles put on it? When new, my 90 nissan pathfinder
used to go almost 300 miles on a tank of gas (15
gal. tank, so about 20 MPG, mostly freeway
driving), but now it has 135K miles, and I'm lucky
if I get even 250 miles (approx. 16.6 mpg) on a
tank. A few days ago I was very surprised when I
had driven only 100 freeway miles and the tank was
already half empty! My driving conditions, tire
inflation, maintenance patterns have stayed the
same. Is notable gas mileage deterioration just
part of the natural wear on the engine, or is there
something wrong?

bobs5

I have noticed a slight decrease over the years. Probably by about 2-3 mpg.
86 ply. rel. with 141k miles.
I got worse mileage when they brought out the oxygenated gas. The mileage will vary between different brands of gas as well.

A full tune-up might help.
Was the oxygen sensor ever replaced? I did this and got better mileage again.

bobs5

A clogged exhaust system would degrade gas mileage as well. like a bad catalytic converter.

btroy

I agree that mileage will fall off, but proper maintenance should keep it minimal- to the point that you probably wouldn't notice it until your engine is really worn out and burning oil, etc. You talk about miles per tankful and that makes me wonder about your method for estimating mileage. It's common for gas gauges to work differently as they age, and give you a lower-than-true reading as the floats deteriorate over time. This can get you refueling more often and getting less mileage per "tank". If you keep track of the actual number of gallons that you are pumping into the tank, you can do a more precise calculation of gas mileage and this may help.

burdawg

I would recommend a compression test for both of you. If the compression is low, the engine will develop less power for a given volume of fuel. The oil consumption may or may not be an indicator of cylinder or ring wear. Of course the valves, if never replaced or reground, can also be the source of leakage. With the type of mileage your both getting, however, I would probably just keep driving it until time for something new. The cost of repairs will probably overshadow the cost of the increase in fuel use.

spnx

I'm willing to bet that your O2 sensor is responsible. When I replaced the sensor on my Nissan, I went through a lot less fuel.

wsommariva

Check the tire pressure. My 13 year old SAAB never lost mileage. I kept it well maintained.

mazda323

Don't forget to check the air filter and the PCV valve. They can make a difference in the mileage.

sempai

I have a 97 honda civic with 50000 miles on it. I always got about 33 miles to the gallon. The 6 months the mileage dropped to about 20 miles to the gallon. I gave it a tune up but the mileage hasn't returned. Can it be the fuel filter or something else that would cause the car to get such low fuel economy? The car also runs great there is not a noticeable problem with it.

ccotenj

you dropped 13 mpg in one shot? overnight? or over a period of 6 months?

1) have your driving habits drastically changed? (i.e. new job location, etc.)
2) change your fuel filter
3) clean your fuel injectors
4) what do you mean "you gave it a tuneup"? changed plugs/wires, etc., but didn't change fuel filter?

more info needed...

-Chris

ccotenj

o2 sensor as well could be responsible (as stated by someone in an earlier post), although you should be getting a "check engine" light if it is completely shot.

-Chris

guitarzan

Anone mention the fuel pump? Perhaps that should be checked, I'm not sure.

kristine9

I just bought a used 95 Saturn SW1 Wagon. Those cars when new got amazing milage (38-40 mpg). And while I didn't expect it to be above 35 when I got it, I also didn't expect it to be 22-25 mpg. Where do I start? I''m feeling that if I can't get 30 mpg that there must be something really wrong with this car. Any thoughts about how to prioritize the maintainence would be appreciated. Thanks--kristine

ccotenj

i think it's safe to say that something is probably going on if you've gone from 40 mpg to 22 mpg.

1) when was the last time you had a tune-up?
2) when was the last time you changed the air filter and fuel filter?
3) if 1 and 2 are fairly recently, take it to the dealer (or otherwise competent mechanic) and have them read the dme codes and see if something has failed.

you shouldn't lose 50% of your fuel mileage.

there's another possibility here too. how do you drive? does the right foot go all the way to the floor every time you leave a light or stop sign? merely changing the way you accelerate can have a huge difference in your mpg.

-Chris

oldharry

Kristine9 and sempai:
A sudden drop as sempai notes, or low milage on a recent purchase as krintine9 notes (why it was traded?) may be the result of a worn timing belt jumping one tooth. Have your mechanic remove the cover and check the marks. A worn belt on some cars may cause major engine damage if it jumps too many teeth.

Harry

64chev

I have a 93 saturn that only got 190 per tank till i replaced the coolant temperature sensor now i get 355 per tank of 11 gallons have it checked its worth $95.

jonnytight

i have a 99 toy 4runner sr5 and the suv takes up
too much gas! i get around 12 - 13 mpg. is there
anything i can do to fix this? are there parts to
service? or is it the timimg? the car has only 15k
miles. what should i do

ccotenj

what did the sticker say it would get for mileage? what did you expect it to get?

-Chris

kkollwitz

I keep written records of my cars' mileage on all long trips, and informally monitor every tank of gas. When mileage drops and stays dropped, something is wrong. In 11 years of owning my Firebird Formula, mileage drop indicates problems (bad mass airflow sensor, old plugs, cracked distributor cap, gasket leaks...stuff like that) that are identified and fixed. After each fix, mileage goes back up to normal. This month, e.g., the car was in for some work, and is currently setting a mileage record....not bad for an old car.