MPG Drop??

Anyone have any ideas as to what could be the cause of my 97 corolla losing about 3mpg? I use to average about 31mpg all the time, but now for about the last 6 months or so I'm only getting about 28mpg. I've kept the oil changed every 5k miles or so since I've had it (bought it with only 10k miles on it in 2000). I recently changed the plugs, and now Im just curious as to what could cause such a major loss in mpg?

I use to get about 340miles/tank, and now Im getting about 300miles/tank.

Any ideas?

check out your egr system and make sure the egr valve is not leaking. you should also check o2 sensors, fuel and air filter, and for leaking vacuum lines. have you got gas from an unusal place or switched grades recently?

are there any other engine symptoms like loss of power, misfires, or a rough idle?

don't forget about suspension and wheel problems like low tire pressure or alignment problems.

check out your egr system and make sure the egr valve is not leaking. you should also check o2 sensors, fuel and air filter, and for leaking vacuum lines. have you got gas from an unusal place or switched grades recently?

are there any other engine symptoms like loss of power, misfires, or a rough idle?

don't forget about suspension and wheel problems like low tire pressure or alignment problems.


Alignment could definetly be a factor as it is out, and I've been meaning to get it corrected as soon as I can.

I've recently passed the 100k mark, and would like to know what are some things I should look at to make sure my baby is going to last another 100k or more? What goes into checking the egr system? Is it something I can do or should i have mech. look at it? Same for the O2, and vacuum lines. Also, should I have the timing belt replaced anytime soon, or how should I go about that? Its not giving me any probs, but Im just wanting a good list of prevenative maintainence. Any help/ideas would be great. Thanks

Alignment could definetly be a factor as it is out, and I've been meaning to get it corrected as soon as I can.

I've recently passed the 100k mark, and would like to know what are some things I should look at to make sure my baby is going to last another 100k or more? What goes into checking the egr system? Is it something I can do or should i have mech. look at it? Same for the O2, and vacuum lines. Also, should I have the timing belt replaced anytime soon, or how should I go about that? Its not giving me any probs, but Im just wanting a good list of prevenative maintainence. Any help/ideas would be great. Thanks

corollas are very reliable, even at 100,000+ miles, you should be fine even if you dont take care of it that much, follow the car owner's manual, simple maintenance is good

the timing belt of ANY vehicle should be replaced every 100,000 miles. if it snaps or skips time you could possibly blow your engine. at minimum you will need to have the car towed. i would go ahead and do that. all sensors and the egr valve should light your check engine light so i wouldnt worry about that. some forgotten tune ups though are the pcv valve, crank case breather filter, the cabin air filter, radiator flush, and transmission filter.

the timing belt of ANY vehicle should be replaced every 100,000 miles. if it snaps or skips time you could possibly blow your engine. at minimum you will need to have the car towed. i would go ahead and do that. all sensors and the egr valve should light your check engine light so i wouldnt worry about that. some forgotten tune ups though are the pcv valve, crank case breather filter, the cabin air filter, radiator flush, and transmission filter.

Any advice on how to check these things, or should I get someone else to do it? Im all for doing anything I can myself, but Im clueless on these things. A "how to" would be great.

Thx

Any advice on how to check these things, or should I get someone else to do it? Im all for doing anything I can myself, but Im clueless on these things. A "how to" would be great.

Thx

an easy step to check the belts for me is by looking at it, if it looks thin and the threads are wearing out, its a good idea to replace them, and from experience it would be wise to have your belts change by a shop, i think most shops do belt replacements fairly cheap

i would consider the timing belt a chalenge to the beginner mechanic. but if you want to try it yourself, i would encourage that because then you'll not be so "beginner" after you do it. the timing belt is located under the timing cover. this is the cover that is on the side of your engine under the pulleys and belts. remove all accessories from your engine >>> e.g. belts, pulleys, alternator, AC compressor, etc. then you can remove the cover. dont move the cam or the crankshaft after you remove the timing belt. you have to mark the belt so that you put the new one back on in the same way. put a mark on the belt and the crankshaft and the cam so that you can transfer the marks to the new belt in the same spots and then line up the marks on the new belt to the marks on the crankshaft and cam. put the belt on and tighten the belt tensioner. lastly, put the cover back on and re-install all of your engine accessories. if you loose the marks, put your crank at top dead center on #1 piston (or line the timing mark up to the zero mark on your timing cover bottom half) and line the hole up that is on the center of the cam so that it lines up with the mark that is on the head. if this is your first one, i would buy the manual for your specific model of car from autozone. and just a last note.... if you do try it yourself, it took me three days to get it right on my first one about 8 years ago so be prepared to not have your car for a few days. hope this helps and if you need more help, just post again!!!!!

check out your egr system and make sure the egr valve is not leaking. you should also check o2 sensors, fuel and air filter, and for leaking vacuum lines. have you got gas from an unusal place or switched grades recently?

are there any other engine symptoms like loss of power, misfires, or a rough idle?

don't forget about suspension and wheel problems like low tire pressure or alignment problems.

Bump, because I'd like to get another round of replys if I may. I changed the tires and fixed the alignment but still about the same only now Im getting about 270/tank which is about 70/tank less that what I use to get. I need to check the egr system still...anything else? Any good tutorials on how to do this? Thanks for the replys in advance.

It's probably just Winter gas. Wait 'til Spring. I bet your mileage will improve by then.

It's probably just Winter gas. Wait 'til Spring. I bet your mileage will improve by then.

Well I thought that at first, but this problem has been going on for far too long for that to be the problem. I mean I started to notice this last spring, so I've went through summer/winter gas already to see. Im thinking its some sort of maintance that I don't know of ( I hope anyway ), because the car seems to run fine. The mileage just sux, whereas it hasn't always done so. Something has to be the cause.

3 MPG over a full tank of gas is small -- increasing the tire pressure could make that difference, as could a new air filter -- The O2 sensor may need to be changed if it has never been changed, as could the spark plugs, and the battery -- Weather could also make that kind of difference, as could your weight, the passengers you carry, and the stuff in the trunk.

I woudn't worry about it too much.

I forgot to mention something really important -- if you're measuring the MPG by using odometer readings over gallons to fill up -- keep in mind that a tire change, even a rotation of tires that are the same spec size, but especially a change of tire brand will likely cause odometer and speedometer error, since tires are not all the same exact size. Larger [circumference] tires will read less miles than smaller ones for the exact same distance because they cover more ground with each rotation. As tires wear, they get "smaller." Inflation pressure also effects size.

I forgot to mention something really important -- if you're measuring the MPG by using odometer readings over gallons to fill up -- keep in mind that a tire change, even a rotation of tires that are the same spec size, but especially a change of tire brand will likely cause odometer and speedometer error, since tires are not all the same exact size. Larger [circumference] tires will read less miles than smaller ones for the exact same distance because they cover more ground with each rotation. As tires wear, they get "smaller." Inflation pressure also effects size.

I was gonna say something like that, you beat me to it lol +1

I get a good 28-30 MPG as well on the highway -- 1995 Corolla 1.6L 4AFE, K&N, a little "tuning", and 195/60R14s -- the stock steel 14" wheel tire sizes i've seen are 175/65R14, 165/70R14, and sometimes 185/65R14.

Some tire math (I'm bored, what can I say):

In inches: diameter [ht], *circumference, radius, (rotations/mi.):

175/65/14 - 22.7, 71.4, 11.37, (887.3)
165/70/14 - 22.9, 71.9, 11.45, (881.2)
195/60/14 - 23.0, 72.2, 11.50, (877.6)
185/65/14 - 23.2, 73.0, 11.62, (867.9)

Notice that as the wheel/tire height and circumference increase, the number of wheel rotations per mile (what the odometer and speedo read) decreases. That means that a larger tire will have a "slower" odometer and speedo than a smaller tire.

Moreover, I don't think the differences between these tires would make for the 70 or so mile difference over 300 miles unless you have tires that are vastly different in size or seriously under- or over-inflated.

These are spec-sizes -- if you actually measure your tires, you may find their real dimensions to be different.

Maybe just treat this as a Corolla tire FYI :)