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Old 07-05-2009, 10:24 PM   #1
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The Incredible Shrinking Gas Tank

I've searched for this both here and at Renntech, but can't find anything similar.

'99 Boxster S Tiptronic. MAF and AOS replaced in February.

I usually get 400kms from full tank of fuel to the yellow low fuel light. The fuel gauge reads about 70kms to go when the light comes on. Normal. I usually fill the tank very soon after the yellow light and it takes 57 - 58 litres. But after the last fill up, I only got 360kms. I checked the receipt and found that only 52 litres went in.

The next time, I drove until there was only an indicated 35kms on the fuel gauge. It took just 54 litres to fill it.

The tank holds 64 litres, so I reckon 5 litres is 'missing'. The fuel gauge looks full, but the needle seems to fall more quickly than normal. Also, my MPG seems to have gone up by 10%

Can anyone shed some light on this? Thanks in advance.

Ed


Last edited by Opposite Lock; 07-05-2009 at 10:27 PM. Reason: Spelling
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Old 07-06-2009, 05:14 AM   #2
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Could be your sending unit (resides in the fuel tank, accessed under the battery).

I also think, but I am not completely sure, that there is a vent tube of some sort for the fuel tank. If this becomes clogged, you can't fill the tank fully. Other forum members should chime in on that though to verify it.
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Old 07-06-2009, 11:06 AM   #3
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Your car has a fuel return system which means unused gas goes back to the tank. With MY02s the cars went to a sealed non-return system.

As Randall said, the sending unit and also the pump is in the fuel tank. Could be the sender, but I'd check/replace the (external) fuel filter and inspect the lines for kinks first. A clogged filter will impede fuel distribution. The external pressure regulator could also be a problem.

While it does sound like a vent symptom, I don't know if that is likely in a recirculating system.

Anyone?
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Old 07-06-2009, 06:23 PM   #4
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Many thanks for the replies, guys.

Is the situation likely to worsen, or could I leave it until the next service (due in six months)? The last time I got my hands oily was about 30 years ago.
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Old 07-06-2009, 09:12 PM   #5
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My car is of similar vintage (98) with a similar problem. After digging in deep at Renntech this is what I discovered:

Our cars have a fuel gauge sending unit that sometimes loosens up out of position or the float simply sinks a little over time. This indicates that the tank is empty when it may actually be 1/4 full. Others have repaired or replaced the gauge sending unit only to have it loosen up or sink again after a few years. I have not fixed mine yet (gauge misreading for four years) because it really doesn't bother me much. I have had many other cars/boats/airplanes with no fuel gauge at all. A fuel gauge that is overly pessimistic is more of a "feature" than a defect in my simple mind. I just have smaller main tank with an extra large reserve.

If I ever need to get into the tank to repair something else I will also deal with the sending unit. For now I can't be bothered. I just pay attention to trip log mileage.
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Old 07-06-2009, 09:34 PM   #6
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Thanks, Topless. That makes sense. It would also explain the 10% 'increase' in MPG.

Do you have any idea what happens if you try to use that "extra large reserve"? Theoretically, the 'kms to go' readout would be zero. I have read that the car goes into emergency mode when fuel is very low. Do you know if this is triggered by the float?

Sorry for the awkward questions.
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Old 07-08-2009, 06:35 AM   #7
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As I understand it the DME will only activate limp mode if you actually run the tank dry so it is not affected by your gauge readings. I generally refill shortly after my "low fuel" light comes on and still have 4-5 gallons left in the tank.
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Old 07-08-2009, 06:41 AM   #8
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As an FYI, my mechanic showed me the fuel pump inside the tank and said, "See that? It needs fuel all around it to remain cool and not overheat. That's why you should never drive below a 1/4 of a tank unless you're in an emergency situation, and that's why your empty tank dummy light kicks on at 1/4."

Just thought you guys might like to know this.

BTW, I get 250 miles per tank in town (fill up to 1/4 reading when the light turns on). I can get nearly 300 miles on the freeway at 70 mph.
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Old 07-08-2009, 11:04 AM   #9
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Quote:
Originally Posted by Topless
Our cars have a fuel gauge sending unit that sometimes loosens up out of position or the float simply sinks a little over time.
Wow, maybe it leaks over time like my Weber carb floats do.
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Old 07-08-2009, 10:54 PM   #10
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Thanks a lot, guys.

Ed

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