low mileage only when tank run close to empty
01-10-06, 05:59 PM
#1
low mileage only when tank run close to empty
Two questions.
QUESTION 1:
I bought an '87 FD and got 17-18mpg for a while. Every time I pushed it near empty I got 14-15mpg. Then I used 14.5 gallons of gas twice and got 13mpg both times (city). Then the gears started turning in my head, so I tried refilling after only burning half a tank. I got 19mpg freeway @ 80mph, then 22mpg twice on the mountain. Method for measuring is odometer miles traveled divided by gas refilled. I fill to full every time, without topping off.
My emissions are significantly above average (with an rx-7!) Temp gauge almost always at 1/4. Last owner put in (visibly) new spark plugs and (visibly) new hoses and belts. I checked the timing myself. It was dead on to the width of a human hair, so I didn't adjust it. Everything works fine except for a fuel gauge that's broken half the time, which I imagine is just a bad sender. And some minor dashboard electrical issues (bad buttons, etc.). And I might have the "3800rpm hesistation", but it's very minor so it might be the ports opening. Also probably unrelated?
Anybody know what would cause poor mileage when my tank is low? I already tried searching the forums. I'd bet it's in there somewhere, but how do you type something like that in the search engine? I got a bunch of general low-mileage tips.
My guess was crud at the bottom of the fuel tank. The owners manual strongly recommends cleaning the tank annually, but the mechanic I called says it's not necessary. I should use fuel system cleaner then?? Or is that not the cause?
QUESTION 2:
My FD has ~105,000 miles on it. A forum member recommended getting the fuel injectors cleaned, since it probably hasn't been done before. So I called a mechanic. It's an $80 job, which is a package deal that includes cleaning/replacing associated parts/hoses. Given my good mileage (under proper conditions), clean emissions and otherwise smooth running FD, is it necessary? Or should I dump some fuel injector cleaner in the tank (or do nothing) and forget about it?
QUESTION 1:
I bought an '87 FD and got 17-18mpg for a while. Every time I pushed it near empty I got 14-15mpg. Then I used 14.5 gallons of gas twice and got 13mpg both times (city). Then the gears started turning in my head, so I tried refilling after only burning half a tank. I got 19mpg freeway @ 80mph, then 22mpg twice on the mountain. Method for measuring is odometer miles traveled divided by gas refilled. I fill to full every time, without topping off.
My emissions are significantly above average (with an rx-7!) Temp gauge almost always at 1/4. Last owner put in (visibly) new spark plugs and (visibly) new hoses and belts. I checked the timing myself. It was dead on to the width of a human hair, so I didn't adjust it. Everything works fine except for a fuel gauge that's broken half the time, which I imagine is just a bad sender. And some minor dashboard electrical issues (bad buttons, etc.). And I might have the "3800rpm hesistation", but it's very minor so it might be the ports opening. Also probably unrelated?
Anybody know what would cause poor mileage when my tank is low? I already tried searching the forums. I'd bet it's in there somewhere, but how do you type something like that in the search engine? I got a bunch of general low-mileage tips.
My guess was crud at the bottom of the fuel tank. The owners manual strongly recommends cleaning the tank annually, but the mechanic I called says it's not necessary. I should use fuel system cleaner then?? Or is that not the cause?
QUESTION 2:
My FD has ~105,000 miles on it. A forum member recommended getting the fuel injectors cleaned, since it probably hasn't been done before. So I called a mechanic. It's an $80 job, which is a package deal that includes cleaning/replacing associated parts/hoses. Given my good mileage (under proper conditions), clean emissions and otherwise smooth running FD, is it necessary? Or should I dump some fuel injector cleaner in the tank (or do nothing) and forget about it?
01-10-06, 06:07 PM
#2
Passing life by
You have a non turbo FC not FD. Fd's are tiwn turbo 1993 to 1995 in USA and Canada.
The gas milage is obscure b/c your head is obscure. Gas dosent magicaly disapear more when the tank gets low.
The gas milage is obscure b/c your head is obscure. Gas dosent magicaly disapear more when the tank gets low.
01-10-06, 06:38 PM
#3
Yeah, it's an FC. That's a typo. If I counted out FB, FC, FD I would have gotten it right. Also note that I'm posting in the 2nd generation specific forums. And likewise you said your blog is not "Of or characterized by a highly developed or wide-ranging skill or proficiency". 
I record my mileage rather carefully. And I speculated worse mileage when the tank is low because someone on Car Talk got poor mileage (and engine trouble) because the poor student always kept his tank low. The host of Car Talk said it was from the crud that collects at the bottom.
I record my mileage rather carefully. And I speculated worse mileage when the tank is low because someone on Car Talk got poor mileage (and engine trouble) because the poor student always kept his tank low. The host of Car Talk said it was from the crud that collects at the bottom.
Last edited by ericgrau; 01-10-06 at 06:47 PM.
01-10-06, 06:41 PM
#4
this makes no sense at all........when there is less gas in the car there is less wieght therefore you should get better mileage....even though I doubt that the difference in mileage between a full tank compared to an empty one is even truly measurable....
and the fact that you end up with a range of mileage that fluxuates by nearly 10 mpg I would question your math
and the fact that you end up with a range of mileage that fluxuates by nearly 10 mpg I would question your math
01-10-06, 06:43 PM
#5
Originally Posted by ericgrau
The host of Car Talk said it was from the crud that collects at the bottom.
is he a comedian?
seriously though - if there was that much crud in the bottom of the tank it would stop the pump from sucking fuel because the sock would clog - then you'd get wicked sweet mileage
01-10-06, 06:54 PM
#6
Passing life by
And wicked lean conditions lol.
No man the milage matters not. It is simple the flunctuations of temp elivations and your foot. The injectors inject the same amount of fule based on temp andair flow and load. So dont wori about it any more and fill up when on E and enjoy your REX.
No man the milage matters not. It is simple the flunctuations of temp elivations and your foot. The injectors inject the same amount of fule based on temp andair flow and load. So dont wori about it any more and fill up when on E and enjoy your REX.
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01-10-06, 07:01 PM
#8
Passing life by
Originally Posted by ericgrau
And likewise you said your blog is not "Of or characterized by a highly developed or wide-ranging skill or proficiency".
01-10-06, 07:02 PM
#9
www.dictionary.com: compleat
You mean completed, I presume?
Anywayz, does anybody have a clue what may cause this problem? Another possible variable may be AC, if that helps (i.e., maybe I had the AC on when I had poor mileage?). It's kinda noisy and weak. But it doesn't seem to affect mileage as far as I can tell. I believe I used it frequently right after I bought the car and I got 16-17mpg.
You mean completed, I presume?
Anywayz, does anybody have a clue what may cause this problem? Another possible variable may be AC, if that helps (i.e., maybe I had the AC on when I had poor mileage?). It's kinda noisy and weak. But it doesn't seem to affect mileage as far as I can tell. I believe I used it frequently right after I bought the car and I got 16-17mpg.
Last edited by ericgrau; 01-10-06 at 07:06 PM.
01-10-06, 07:06 PM
#10
Passing life by
Originally Posted by ericgrau
www.dictionary.com: compleat
You mean completed, I presume?
You mean completed, I presume?
What part of the car does not get worse MPG when lower on gas did you not understand? It’s kind of like what shade of green do you go on? An EFI system is not dependant upon amount of gas in tank.
Last edited by iceblue; 01-10-06 at 07:08 PM.
01-10-06, 07:17 PM
#11
I notice that to. When I have a full tank of gas I notice it takes a lot longer to get to half tank than it does half tank to empty. I believe that's because of the little bobber in the gas tank that tells the gas level guage in your dash how much fuel is in the tank. The bobber is not completely accurate so when at half tank or less it seems like you are getting worse gas mileage than you were from full-half tank.
Don't quote me on this because that's just my take on things.
Don't quote me on this because that's just my take on things.
01-10-06, 07:42 PM
#12
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Originally Posted by Sgt. Pepper
I notice that to. When I have a full tank of gas I notice it takes a lot longer to get to half tank than it does half tank to empty. I believe that's because of the little bobber in the gas tank that tells the gas level guage in your dash how much fuel is in the tank. The bobber is not completely accurate so when at half tank or less it seems like you are getting worse gas mileage than you were from full-half tank.
Don't quote me on this because that's just my take on things.
Don't quote me on this because that's just my take on things.
I also noticed that . full tank to half lasted a lot longer then half to empty . i just figured it was a combination of the gauge and fuel level sensor not being accurate .
Possibly the funniest thing I have ever read.
01-11-06, 10:19 AM
#15
This is odometer miles divided by refill gas. It doesn't use my fuel gauge. My fuel gauge doesn't even work. I filled up 12 gallons to get the tank full and got 17mpg city. I filled up 14.5 gallons to get the tank full and got 13mpg city. I drove short distances then filled up 4-7 gallons to get the tank full and got 19-22mpg freeway/mountain. Question is why and what to do about it?
01-11-06, 05:44 PM
#17
Passing life by
Originally Posted by iceblue
It is simple the flunctuations of temp elivations and your foot. The injectors inject the same amount of fule based on temp andair flow and load.
01-12-06, 04:17 AM
#18
It's because the tank doesn't have equal volume at the bottom as it does at the top half of the tank. The sending unit does not compensate for this. The sending unit shows half at the middle of the tank, not when you have used half of full capacity. The tank sloops in after the half way point so the lower the level the descrepency grows.
They did that so you wouldn't be scrubbing your tank over speed bumps and squirrels.
They did that so you wouldn't be scrubbing your tank over speed bumps and squirrels.
05-10-06, 11:46 AM
#19
Okay, this has NOTHING to do with the fuel gauge or sender. The fuel gauge is broken. I'm not using the fuel gauge. Get off the fuel gauge.
I fill up the tank, drive and fill up the tank again. I divide miles driven by gas filled up (using the number shown on the pump). I don't measure the gas using the fuel gauge. I don't measure the gas using the fuel gauge. My mileage is definately lower when I refill from empty to full compared to when I refill from half to full. So why is this, and what do I do about it?
I am NOT comparing city mileage to freeway mileage. I am comparing city mileage filling from half to full and city mileage filling from empty to full. I am SEPERATELY comparing freeway mileage filling from half to full and freeway mileage filling from empty to full.
I fill up the tank, drive and fill up the tank again. I divide miles driven by gas filled up (using the number shown on the pump). I don't measure the gas using the fuel gauge. I don't measure the gas using the fuel gauge. My mileage is definately lower when I refill from empty to full compared to when I refill from half to full. So why is this, and what do I do about it?
I am NOT comparing city mileage to freeway mileage. I am comparing city mileage filling from half to full and city mileage filling from empty to full. I am SEPERATELY comparing freeway mileage filling from half to full and freeway mileage filling from empty to full.
Last edited by ericgrau; 05-10-06 at 11:48 AM.
05-10-06, 03:00 PM
#21
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The math you are doing is skewed by the gas pump system. Every pump doesn't shut off the same. They shut off by a vapor sensor on most newer pumps. Not every pump is calibrated the same way. Do this, spread your math out over three full tanks of gas and the miles traveled with that gas. this will give you a less flawed average. If you are still not happy with your gas milage, sell me your RX and buy a freakin Corolla or a Civic.
05-10-06, 05:54 PM
#22
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my milage fluctuates a lot in my 7 compared to other cars I've owned because there is a big difference between highway and city milage or driving slowly and running it through the gears mileage. You can't base your milage on a half a tank to get a proper reading you really need to base it off more like a month's worth of gas.
