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Old 08-05-2009, 03:57 PM   #1
FabFlatFour
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Default Missing 5 Litres?

I'm not sure if this is the exact forum, but here goes...

So I was a little ways from home and the fuel light came on one evening a couple weeks ago. I finished what I was doing and later that evening I went to fill up but the gas station I went to had no gas (there was a power outage or something like that that crippled a local refinery). I figured I could make it home as there were no gas stations along my route. I ended up making it to the gas station a few blocks from my house barely; I ran out of gas (I know not the best thing to do...) as I turned into the gas station, but was able to roll right up to the pump.

This brings me to my question. My owners manual says that my 07 Impreza 2.5i has a 60L tank, however, I filled it up all the way and I could only put in a hair more than 55L. Where did that missing 5L of fuel go? I can't imagine that all the ancillaries (fuel lines, pumps etc) would hold that much fuel would they?

I tried searching for answers to this supposed (or perhaps perceived) discrepancy but to no avail. If anyone knows of a thread, please post it as I'm curious to learn what this discrepancy is due to or if perhaps there is an underlying problem.
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Last edited by FabFlatFour; 08-05-2009 at 04:05 PM.
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Old 08-05-2009, 04:27 PM   #2
Kean
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....perhaps you sucked air and the car simply stalled with close to 5 liters (just over 1 gallon) still left in the tank? I have never run my WRX or Forester that low before.
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Old 08-05-2009, 11:22 PM   #3
rick-l
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There will always be some unusable fuel in the car.
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Old 08-06-2009, 12:04 AM   #4
Uncle Scotty
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ran my 06 wrx out once---it went 27miles, city driving after the light came on

it took US14.5 gallons to fill

almost ran my 02 wrx out once.....light was on for 60 miles on the highway--i stopped at the first place to buy gas

took US14.9 gallons to fill


go figure
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Old 08-06-2009, 02:13 AM   #5
Kean
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....one thing I did notice on both my WRX and Forester is that when I fill up shortly after the light illuminates, I will always have a little over two gallons left in the tank (takes a little over 13.5 gals to fill). IIRC the manual says ~2.3? The most I pushed my luck was about 40 miles past although I don't recall what it took to fill it up that time around. I always figure I have at least 30 miles from the time the light goes on. ....anything past that and I'm tempting fate.
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Old 08-06-2009, 08:28 AM   #6
z&cobb
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The gas tank has a peculiar shape. Which is one reason to fill up as soon after the low fuel light comes on. The most I have driven after the light came on is about 25 miles and it took 14.X gallons. The book says it holds 15.9 gallons but don't try for that.

In cross section, the tank looks like a catamaran boat. There is a jet pump (like a venturi) that is used to take fuel returning from the engine to produce negative pressure in the pump. That pump then transfers fuel from the sub-compartment of the tank to the main compartment where the "real" fuel pump is located. I'm old enough to remember "Rube Goldberg" and it reminds me of some of his "inventions".

Remember this before you try to go for 15.8 miles on a fill up. I have never had a problem aiming for a 13 to 14 gallon fillup. So, it works OK by my standards.
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Old 08-06-2009, 09:40 AM   #7
FabFlatFour
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Ahh...I didn't realize that there is a jet pump that transfers fuel from a sub-compartment. Depending on how the jet pump is arranged if the fuel sloshed in the tank it may cause a vapor pocket and choke the actual fuel pump causing the car to die as Kean mentions above. Hmm interesting...I think this is device is kind of like an "ejector"...it would make sense as the return fuel from the engine would be like the motive fluid flow and the fuel from the tank would be the entrained fluid flow.

I wonder why the tank would be set up like this...perhaps to reduce sloshing that a single compartment tank could possibly have?
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Old 08-06-2009, 01:09 PM   #8
z&cobb
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Quote:
Originally Posted by FabFlatFour View Post
Ahh...I didn't realize that there is a jet pump that transfers fuel from a sub-compartment. Depending on how the jet pump is arranged if the fuel sloshed in the tank it may cause a vapor pocket and choke the actual fuel pump causing the car to die as Kean mentions above. Hmm interesting...I think this is device is kind of like an "ejector"...it would make sense as the return fuel from the engine would be like the motive fluid flow and the fuel from the tank would be the entrained fluid flow.

I wonder why the tank would be set up like this...perhaps to reduce sloshing that a single compartment tank could possibly have?
I should add that what I wrote is definitely true for my 02 wrx. I cannot say for sure that it is that way for all Impreza models and years post 2002. My first mod was a full set of Service Manuals for the first year wrx in the USA, figuring that even the dealerships were not up to speed yet on the car.

There's lots of nice grease and dirt on some of those volumes now.
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Old 08-07-2009, 03:07 AM   #9
Charlie-III
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Quote:
Originally Posted by FabFlatFour View Post
I wonder why the tank would be set up like this...perhaps to reduce sloshing that a single compartment tank could possibly have?
1-to get enough volumne to go somewhere
2-to clear the driveshaft
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