43 replies to this topic

[bergmjs]

Hello, i've been noticing recently that my gas mileage has been pretty bad. I dont have the info switch trip computer so I cant get an exact measurement and I havent run the tank down all the way either to get a measurement, but I've been having to fill up more often now than a while ago. This is on my '96 GLT and there are no codes set. I did a search and they all seemed to point to either front O2 sensor (but my OBDII scanner says front O2 OK) or the thermostat.

The car warms up in a decent amount of time for it being 10 degrees out (5-10 min). When I start it in the morning its a little groggy and you can feel a lot of vibration in the cabin until its warm(I have poly upper). Also I can't smell any outstanding gas odor from the exhaust, it just smells like exhaust to me. I just did the PCV less than 1k miles ago thinking that would help, but not much. I put OEM plugs in while I did the PCV too, I havent changed the wires since I've had the car and the cap and rotor are aftermarket.

I've been running Mobil1 10W-30 full synthetic and just put M1 5W-30 full synth in for the winter and have been putting 93 gas in since the start of summer.

I just tried to pull my front O2 sensor to test it, but couldn't get the damn thing out. Is there a way to test it while its still in the car?

Thanks

[Oreo931]

Gas mileage on average tends to be worse in the winter due to the additives put in gasoline and the lower temperatures.

Is this extremely bad mileage (like 10mpg), or just a few mpg difference compared to what you are used to? If it is minor, I would chalk it up to the season.

[bergmjs]

Well my gas gauge is a little inaccurate i've found (it says 6 gal, I put in 5, it says 14 gal), but its always consistently inaccurate. I drove about 60 miles yesterday and it went from right below the 12 mark to a little above the 6 mark. So if going by that it would be about 12-15 mpg.

[Ihatespeedbumps]

I have fixed a few cars with the same symptom by installing new 02 sensors. As they age they get a little slow to respond, but not bad enough to flag a DTC.

[bergmjs]

So I should go through with removing the O2 sensor to test it with a torch? or can I test it with it in the car?

[erikv11]

I have fixed a few cars with the same symptom by installing new O2 sensors. As they age they get a little slow to respond, but not bad enough to flag a DTC.

but you only have to change the front sensor though, is that correct?

Well my gas gauge is a little inaccurate i've found (it says 6 gal, I put in 5, it says 14 gal), but its always consistently inaccurate. I drove about 60 miles yesterday and it went from right below the 12 mark to a little above the 6 mark. So if going by that it would be about 12-15 mpg.

both gas gauge and trip computer are inaccuarate for checking mpg
just fill up, watch odometer, refill, miles/refill_gallons = exact mpg

[bergmjs]

Ok, well I couldn't get the sensor out so I decided to try to test it still in the car. I warmed the car up to operating temperature with the o2 sensor plugged in, then unplugged it and with my DMM on DC volts and the ground lead on the ground of the battery (chassis ground, correct?) probed each of the 4 pins. I got about 0 on 2 of them as expected, then a -.38v on one and a varying .01 to .05v on the last one. Nothing even close to .7v, I believe it is dead.

[KRUZNBY]

Get an accurate measure of your mileage before you do anything.

[bergmjs]

Yea, i'm going to run the tank to almost empty, then put like 5 gallons in and see how far I get.

[KRUZNBY]

Yea, i'm going to run the tank to almost empty, then put like 5 gallons in and see how far I get.

It will be much better to start with a full tank to calculate your MPG as suggested above. Running a tank too low can damage your fuel pump.

[TyConn]

i'm REALLY surprised no one has said it yet, but AIR FILTER. my mother had her oil changed, and complained to me that she had really bad gas mileage about 2 weeks later. i checked her air filter, not only had it not been replaced in almost 2 years, but there was a RAT'S NEST IN IT!!!!!!!!!!!!! anyway, check that, and i'd replace it regardless

[bergmjs]

I replaced my air filter I believe about 10k miles ago, but i'll still go look at it. I don't do that much driving, so if I filled it all the way up I probably wouldn't get it down to empty until it was time for me to go back to school, where I can't replace an O2 sensor. I know it's bad to run it near empty, but I would hope one or twice won't hurt too much.

[erikv11]

1. you can't get an accurate mpg the way you are describing.
2. you don't need to run it near empty, ever, to get an accurate mpg!

how to calculate mpg
a. fill it up. yep. all the way up, until the pump shuts off.
b. before you drive away, check your mileage on the odometer. write it down somewhere. or reset the trip odometer to zero.
c. drive a little bit, like 100 miles. more is better, but if you don't drive much then about 100 miles is a good start.
d. go back to the gas station, fill it up, all the way full just like before. write down how many gallons you added.
e. before you drive away, check your mileage on the odometer. subtract and figure out how many miles you drove since the last fill-up.
e. calculate: (miles driven between fill-ups)/(gallons added) = accurate mpg for that bit of driving.

[bergmjs]

1. you can't get an accurate mpg the way you are describing.
2. you don't need to run it near empty, ever, to get an accurate mpg!

how to calculate mpg
a. fill it up. yep. all the way up, until the pump shuts off.
b. before you drive away, check your mileage on the odometer. write it down somewhere. or reset the trip odometer to zero.
c. drive a little bit, like 100 miles. more is better, but if you don't drive much then about 100 miles is a good start.
d. go back to the gas station, fill it up, all the way full just like before. write down how many gallons you added.
e. before you drive away, check your mileage on the odometer. subtract and figure out how many miles you drove since the last fill-up.
e. calculate: (miles driven between fill-ups)/(gallons added) = accurate mpg for that bit of driving.

Clever.

[bergmjs]

Ok, I filled it up and drove 149.5 miles mostly on the highway, then filled it up again and it took 8.323 gallons to fill. Meaning I got approx 18 mpg. I also let the car warm up for probably about 5-10 minutes on most mornings so that would skew the measurement downward. Is this mileage low enough to warrant replacing an O2 sensor? $22 for 150 miles kinda sucks in my opinion.

[dimka18]

Ok, I filled it up and drove 149.5 miles mostly on the highway, then filled it up again and it took 8.323 gallons to fill. Meaning I got approx 18 mpg. I also let the car warm up for probably about 5-10 minutes on most mornings so that would skew the measurement downward. Is this mileage low enough to warrant replacing an O2 sensor? $22 for 150 miles kinda sucks in my opinion.

i have the same problem! i got a 95 t5r and on a FULL tank i get out about 220 miles! according to my AVG MPG it says 13.9! i have no idea y its this bad! i got brand new spark plugs! i guess ima have to try changing the o2 sensor!

[DonutsDemise]

check your rear brakes for seized pistons, that can go pretty unnoticed but affect your gas mileage. that or heavily dragging parking brakes. i had a seized piston that i didn't notice for several weeks (i don't drive that much) but after i fixed it my MPG went up 3mpg per tank from about 22 to 25 (mixed city/hwy) making me think i had it for a while. maybe you've just been driving into headwinds lately

[bergmjs]

check your rear brakes for seized pistons, that can go pretty unnoticed but affect your gas mileage. that or heavily dragging parking brakes. i had a seized piston that i didn't notice for several weeks (i don't drive that much) but after i fixed it my MPG went up 3mpg per tank from about 22 to 25 (mixed city/hwy) making me think i had it for a while. maybe you've just been driving into headwinds lately

Well I replaced the rear pads and hardware in the fall and the pistons slid easily and the boots looked intact. Also theres no audible noise from any wheel when driving with no brakes applied, tested from both inside and having someone drive by me at different speeds.

I adjusted the parking brake myself when I put new shoes in and did it the way the repair manual says to of tightening until the rotor locks then backing it off like 7 turns or whatever the number is, although the lever went limp the other day and the micro-adjust in the console was so loose that it was almost falling out.

I also just checked my tire pressure to make sure and they were all at 36, save the front right was at 32, but that wouldn't make that big of a difference.

I'll probably go see if my mechanic can break the sensor loose so I can test it sometime. I'm guessing its a combination of the winter gas, warming the car up in the morning, and the O2 sensor just getting old.

[7 VII 7]

Ok, I filled it up and drove 149.5 miles mostly on the highway, then filled it up again and it took 8.323 gallons to fill. Meaning I got approx 18 mpg. I also let the car warm up for probably about 5-10 minutes on most mornings so that would skew the measurement downward. Is this mileage low enough to warrant replacing an O2 sensor? $22 for 150 miles kinda sucks in my opinion.

This

basically you are dumping more fuel into the car and getting an average of 0 MPG

[Dave 54]

This

basically you are dumping more fuel into the car and getting an average of 0 MPG

+1, you should really only need to let it idle for about 30sec. put it in drive and creep like an old lady keeping it around 2k til it's warmed up to operating temp.