Fluctuating gas mileage - horrible to exceptional - Fixed
#1
11-26-2007, 12:23 PM
Fluctuating gas mileage - horrible to exceptional - Fixed
I own a 2000 ZR2 Blazer with about 76,000 miles on it. I have only had it for a couple months.
Thanks to this form I was able to replace my fuel filter and found my thermostat was broke as I was only hitting 154 degrees. I have replaced my thermostat with a Stat SuperStat 195. Now my temperature is normal.
I also ran two tanks of gas with Seafoam and one treatment through the oil as well. The oil was changed after about 100 miles.
My problem is I get great horrible gas mileage at times as bad as 11-14mpg yet other times I have got as good as 22.5mpg which I never expected to get with the Blazer. I just wish I would get that all of the time.
The drivng that seems to be killing me is the driving I do to work and back. This drive is 50mph and is ten miles long with only about 4 stops along the way. I have been accelerating very slow (under 2,000rpm). What I have found now is the faster I have been accelerating the better my mileage has been. I can not explain it? The engine is very very slow to warm when driving to work slower. I do not even get to full temp by the time I get to work. Yet driving hard I am up to temp about half way there. Could a sensor be stuck casuing this issue.
I have tried drving slow versus driving faster on a 80 mile trip this past week as well. This posted the same results. 22.5mpg driving faster. 13.89 driving slower.
I am no mechanic when is comes to cars but I can use a wrench fairly good!
Thanks to this form I was able to replace my fuel filter and found my thermostat was broke as I was only hitting 154 degrees. I have replaced my thermostat with a Stat SuperStat 195. Now my temperature is normal.
I also ran two tanks of gas with Seafoam and one treatment through the oil as well. The oil was changed after about 100 miles.
My problem is I get great horrible gas mileage at times as bad as 11-14mpg yet other times I have got as good as 22.5mpg which I never expected to get with the Blazer. I just wish I would get that all of the time.
The drivng that seems to be killing me is the driving I do to work and back. This drive is 50mph and is ten miles long with only about 4 stops along the way. I have been accelerating very slow (under 2,000rpm). What I have found now is the faster I have been accelerating the better my mileage has been. I can not explain it? The engine is very very slow to warm when driving to work slower. I do not even get to full temp by the time I get to work. Yet driving hard I am up to temp about half way there. Could a sensor be stuck casuing this issue.
I have tried drving slow versus driving faster on a 80 mile trip this past week as well. This posted the same results. 22.5mpg driving faster. 13.89 driving slower.
I am no mechanic when is comes to cars but I can use a wrench fairly good!
#2
11-26-2007, 12:42 PM
RE: Fluctuating gas mileage - horrible to exceptional
The short driving mileage is a killer. I have a 6mi commute to work and I'm lucky to get in the 15mpg range with my Rainier (was the same with my Bravada before).
The problem with short trips is that the fluids in the vehicle never warm up to full operating temperatures. The coolant may warm up, but the engine oil, transmission fluid, front/rear diff oil, and t-case fluid never do. This means that these fluids are thicker, creating a higher parasitic loss. But that is only half of the equation. The engine runs more fuel rich in an effort to warm itself up, burning more fuel. The transmission will slip the torque converter to warm it's fluid as well. Add in stops and you see the end result.
While I do think that 14mpg is pretty bad for a short mileage trip, that is dependent on so many things that it is difficult to pin a few things down as being the culprit. I too have found that babying my vehicles doesn't yeild better mileage so I drive it normally.
Your highway mileage seems exceptional. With my Bravada, the best I ever got was slightly north of 18mpg. With my Rainier, my last long trip had a net mileage of 25mpg so that is easier on the old pocket book! But are you saying that when you drive slow on a long trip your mileage drops from 22.5mpg driving normally to ~14mpg???
The problem with short trips is that the fluids in the vehicle never warm up to full operating temperatures. The coolant may warm up, but the engine oil, transmission fluid, front/rear diff oil, and t-case fluid never do. This means that these fluids are thicker, creating a higher parasitic loss. But that is only half of the equation. The engine runs more fuel rich in an effort to warm itself up, burning more fuel. The transmission will slip the torque converter to warm it's fluid as well. Add in stops and you see the end result.
While I do think that 14mpg is pretty bad for a short mileage trip, that is dependent on so many things that it is difficult to pin a few things down as being the culprit. I too have found that babying my vehicles doesn't yeild better mileage so I drive it normally.
Your highway mileage seems exceptional. With my Bravada, the best I ever got was slightly north of 18mpg. With my Rainier, my last long trip had a net mileage of 25mpg so that is easier on the old pocket book! But are you saying that when you drive slow on a long trip your mileage drops from 22.5mpg driving normally to ~14mpg???
#3
11-26-2007, 12:58 PM
RE: Fluctuating gas mileage - horrible to exceptional
Yep. On the last trip I drove about 20 miles (my normal work trip) then 80 or so miles country/highway. I accelerated faster and averaged 22.5mpg. On the way back I drove about 20-30 miles city and then the same highway/country roads back and averaged just shy of 14 while accelerating slower. I do not know if the acceleration has anything to do with it but is just something I have found so far that is in common with the mileage fluctuation.
I just need something to fix the 11-14 issue as I drive 100 miles a week for work and that would kill me!
I talked to my mechanic and he suggested an o2 sensor possibly being faulty. I was planning on taking it to him to test the sensors in a couple weeks unless someone has any ideas for me to try...
I just need something to fix the 11-14 issue as I drive 100 miles a week for work and that would kill me!
I talked to my mechanic and he suggested an o2 sensor possibly being faulty. I was planning on taking it to him to test the sensors in a couple weeks unless someone has any ideas for me to try...
#4
11-26-2007, 07:19 PM
RE: Fluctuating gas mileage - horrible to exceptional
You how the governemnt gives a City MPG, Hi-way MPG and Combined MPG rating? Well....there you go......try it.......you'll notice something.
BTW......it could also be in your technique......when doing the calculations......
Aloha, Mark
BTW......it could also be in your technique......when doing the calculations......
FYI, try calculating your MPG the "old fashion way."
There is going to be some variance, though I did say it was the "old fashion way." Start by filling up your gas tank, fully.......on level ground........first click off (second click off or whatever) or until you see the gas is it at a certain level in the filler neck (some stations will let you, some won't).........sometimes you'll have to wait a little for the gas to settle in the tank........or the tank to "burrrp." You don't need to record how many gallons were needed (this time). Though you must record your odometer reading (or set the trip odometer) and remember this fill up procedure.
Any way..........now, drive it...........fill it when needed (don't have to be exact about filling it), log the number of gallons that went in.............drive it some more...............fill it when needed…………………………always logging the amount of gallons that went in............repeat the procedure several times.
After a couple of cycles: of driving, and filling, and logging the number of gallons...........this last time, you're gonna go to the same gas station and follow the same procedure that you used to fill your gas tank the first time that you started your quest for truth. Record this last odometer reading.
So, all done filling her up..............how many gallons was it this time? OK, now add up all of the gallons since you first recorded the odometer. So, how many gallons was the total?
Now, take the beginning odometer reading and minus it from the ending odometer reading………………this number is the total number of miles that you've traveled.
One more calculation left..............now, divide the number of miles by the number of gallons you used..................that number, is your MPG. Use a calculator, if you have trouble.
Now, like I said it's the "old fashion way"............and this method doesn't take into account for an odometer that is not functioning correctly.
As for computers...............well, perhaps one day................I'll have more faith in them.................and, that includes the GPS method. I've been off roading, when I was actually on a paved street.............at least that is what the GPS map said, I was doing........LOL.
There is going to be some variance, though I did say it was the "old fashion way." Start by filling up your gas tank, fully.......on level ground........first click off (second click off or whatever) or until you see the gas is it at a certain level in the filler neck (some stations will let you, some won't).........sometimes you'll have to wait a little for the gas to settle in the tank........or the tank to "burrrp." You don't need to record how many gallons were needed (this time). Though you must record your odometer reading (or set the trip odometer) and remember this fill up procedure.
Any way..........now, drive it...........fill it when needed (don't have to be exact about filling it), log the number of gallons that went in.............drive it some more...............fill it when needed…………………………always logging the amount of gallons that went in............repeat the procedure several times.
After a couple of cycles: of driving, and filling, and logging the number of gallons...........this last time, you're gonna go to the same gas station and follow the same procedure that you used to fill your gas tank the first time that you started your quest for truth. Record this last odometer reading.
So, all done filling her up..............how many gallons was it this time? OK, now add up all of the gallons since you first recorded the odometer. So, how many gallons was the total?
Now, take the beginning odometer reading and minus it from the ending odometer reading………………this number is the total number of miles that you've traveled.
One more calculation left..............now, divide the number of miles by the number of gallons you used..................that number, is your MPG. Use a calculator, if you have trouble.
Now, like I said it's the "old fashion way"............and this method doesn't take into account for an odometer that is not functioning correctly.
As for computers...............well, perhaps one day................I'll have more faith in them.................and, that includes the GPS method. I've been off roading, when I was actually on a paved street.............at least that is what the GPS map said, I was doing........LOL.
#6
11-27-2007, 11:53 AM
RE: Fluctuating gas mileage - horrible to exceptional
Other ideas (actually, more questions than answers):
Is it warmer outside when you do the harder-acceleration test? Perhaps try a few days of one type of driving (all day), then a few days of the other to confirm the mileage readings.
Same thought applies with altitude. If you're always doing the soft-accel test when you're going to a destination that's at higher altitude, and on the way home doing the hard-accel test to a destination that's lower in altitude, you'd see this result.
Are you stopped longer at the 4 stops using one method than the other (perhaps traffic light timing)?
Does the torque converter lock when you are doing the slow-accel driving? Is it locked when you do the high-accel driving? Does it lock sooner?
Also this statement is concerning: "The engine is very very slow to warm when driving to work slower. I do not even get to full temp by the time I get to work." On a ten mile trip you should certainly be up to temperature unless it is extremely cold outside. I know in mine, I'm at operating temp in about 2-3 miles of driving at ~40MPH when the outside temp is 40-50F. Could be the thermostat is still not operating correctly. A cold engine will definitely burn more gas.
Is it warmer outside when you do the harder-acceleration test? Perhaps try a few days of one type of driving (all day), then a few days of the other to confirm the mileage readings.
Same thought applies with altitude. If you're always doing the soft-accel test when you're going to a destination that's at higher altitude, and on the way home doing the hard-accel test to a destination that's lower in altitude, you'd see this result.
Are you stopped longer at the 4 stops using one method than the other (perhaps traffic light timing)?
Does the torque converter lock when you are doing the slow-accel driving? Is it locked when you do the high-accel driving? Does it lock sooner?
Also this statement is concerning: "The engine is very very slow to warm when driving to work slower. I do not even get to full temp by the time I get to work." On a ten mile trip you should certainly be up to temperature unless it is extremely cold outside. I know in mine, I'm at operating temp in about 2-3 miles of driving at ~40MPH when the outside temp is 40-50F. Could be the thermostat is still not operating correctly. A cold engine will definitely burn more gas.
#7
11-27-2007, 12:59 PM
RE: Fluctuating gas mileage - horrible to exceptional
Temps have been about the same in the30's.I have been doing the test per quarter of a tank or so.One quarter I accelerate hard the next I drive slow but filling up in between and calculating the mileage at fill up.The driving is the same 100 miles every week so there is no difference in the terrain.
Traffic lights are very similar as far as average stop time per trip.
As far as the torque converter goes is it easiest tell if it is locked by getting up to speed and tapping to brake to see if the rpm's rise?
When acclerating harder (by harder I mean I am usually around 3,000 when shifting) my engine is at temp in about 5 miles.
Traffic lights are very similar as far as average stop time per trip.
As far as the torque converter goes is it easiest tell if it is locked by getting up to speed and tapping to brake to see if the rpm's rise?
When acclerating harder (by harder I mean I am usually around 3,000 when shifting) my engine is at temp in about 5 miles.
#8
11-27-2007, 02:03 PM
RE: Fluctuating gas mileage - horrible to exceptional
I drive a Vortech V6 4.3L. I normally get around 12-13 mpg (using theformula above to calculate)when I am driving in town. I normally shift at 2,000 rpms. I've found driving it any harder in town results in even worse gas milage. I never thought it was a problem though. I always just thought it was because it was a V6 suv. I may be wrong though. As for highway I can't really speak. I've never really been on any road trips with my blazer yet.
#9
11-27-2007, 02:15 PM
RE: Fluctuating gas mileage - horrible to exceptional
I have found if i accelerate faster (about 3,000 rpm-ish when it shifts)until about 50 mph then start to let off since im up to speed, I get better mileage too.
i own a '97 4x4x4dr, 143,000miles
i own a '97 4x4x4dr, 143,000miles
#10
11-27-2007, 02:54 PM
RE: Fluctuating gas mileage - horrible to exceptional
i was getting better than that when I first got mine. I was pushing the 30 mpg. Check your plugs, wires, cap, and rotor. I didn't realize that until after I got rid of the blazer. If I would have known that I wouldn't have gotten rid of the blazer. But My milage started suffering severly. Also another thing to watch is possibly your 02 sensors