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Old 07-02-2016, 02:02 PM   #1
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Fuel Guage Showed 1/4--only took 8.9 to fill?

I am planning a 600 mile road trip soon and installed the Road Trip app on my iphone so I'll be able to plan my fuel stops (trip is through the Rocky Mtns) and monitor fuel consumption.

I've been suspicious of my fuel guage for a while but because most of my trips have been in the city with lots of fuel options I haven't worried too much and just topped off the fuel regularly. Well today, my guage was showing 1/4 tank so I decided to use my Road Trip app and start logging. I pumped slowly until it shut off and the quantity pumped showed 33.6 liters / 8.9 US gallons and the fuel guage now showed full. So it seems that my fuel guage is not accounting for approximately half a tank of fuel even though it travels from showing 'full' to '1/4' tank.

Next step was the search function here which revealed a couple of possibilities. Because the needle appears to travel properly, I'm kinda ruling out a sensor that's getting fouled by an obstruction in the tank. It seems as though I likely need to have the guage recalibrated which I read is a "at the dealer only" fix.

My battery did go dead on me a couple of years ago while in winter hibernation (tender got unplugged somehow) and I replaced it. So I'm guessing I lost the fuel guage sending settings.

Any thoughts are appreciated. Am I stuck paying the Porsche 'Premium' at the dealership to re-calibrate? If so I guess I'll have to get out and burn off this tank of gas as low as I dare to.

Holiday here but I'll be checking with my indy on Monday to see if he has the PIWIS but I don't think he does.
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Old 07-02-2016, 08:38 PM   #2
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Well, the tank volume is nominal 64 litres, but i've never got more than 60 litres in.

So half should be around 30 litres and 1/4 around 45 litres to fill. Don't know if the gauge really works linear, because most of the time there is still fuel in the tank even if the gauge says empty.

In general a tank sensor only needs adjustment if somebody had worked on it.

Ideas: does your tank overflow valve work properly. If the overflow doesn't work correctly maybe you won't get enough fuel in your tank.

But before doing some work on the car i would do some more tests, drive the car until the tank is empty and recheck how much fuel you get in and what the gauge says.

Regards, Markus
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Old 07-03-2016, 04:36 AM   #3
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Thanks Markus, maybe I'm over thinking this. Driving it to near empty is probably the first thing to do. At least I can see if the 'low fuel warning' light comes on and do a fill-up to see how much it will take.

I didn't make a note of how much fuel was in the car before the battery died and thought I might have lost the fuel level sensor settings when that happened. I have read that some guys have tried filling the tank and then disconnecting the battery over night and re-connecting to see if that would reset the sensor. I'm not sure if that has confirmed to be a fix or not.

At the moment the gauge needle does seem to travel towards empty too fast though. Thanks for the reply.
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Old 07-03-2016, 05:57 AM   #4
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My experience in trying to use my fuel gauge to accurately determine how much fuel is in the tank for racing was a failure; I found the gauge to be horribly inaccurate!

So inaccurate, that I have mostly concluded that its more of a guideline than a measurement. In my case, 3/4, 1/2, and 1/4 certainly do not match those amounts in gallons; and worse, the gauge isn't consistent - 1/2 tank one day will not equal 1/2 tank (gallons remaining) another day. At least its not accurate enough for me to use to manage the fuel load for races.

I have never re-calibrated the fuel gauge or done anything to try to improve it. I'd certainly be interested in the accuracy that you or others are finding.
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Old 07-03-2016, 06:07 AM   #5
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My experience in trying to use my fuel gauge to accurately determine how much fuel is in the tank for racing, I have found it to be horribly inaccurate!

So inaccurate, that I have mostly concluded that its more of a guideline than a measurement. In my case, 3/4, 1/2, and 1/4 certainly do not match those amounts in gallons; and worse, the gauge isn't consistent - 1/2 tank one day will not equal 1/2 tank (gallons remaining) another day. At least its not accurate enough for me to use to manage the fuel load for races.

I'd certainly be interested in the accuracy that you are finding since I worry that its just my car that does this.
Thank you! That is sort of comforting....in a crazy kind of way, at least I'm not alone on this issue.

I'll go with plan A and run this tank down till I (hopefully) get the low-fuel light and re-fill.
I'll post my results.
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Old 07-03-2016, 06:34 AM   #6
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I'm not sure this will help your confidence any but the low fuel light on mine is off as well. When my light comes on I can only fill 12.5 to 13 gallons. My light comes on when the gauge is about half way between E and 1/4. Car specs say it a 16+ gal tank. Once the pump kicks off I've stop trying to pump any more, because it will just puke back out.
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Old 07-03-2016, 07:03 AM   #7
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Originally Posted by jsceash View Post
I'm not sure this will help your confidence any but the low fuel light on mine is off as well. When my light comes on I can only fill 12.5 to 13 gallons. My light comes on when the gauge is about half way between E and 1/4. Car specs say it a 16+ gal tank. Once the pump kicks off I've stop trying to pump any more, because it will just puke back out.
Hmm...the gauges on these cars leave a lot to be desired. I'm starting to think that the only way to be safe is to use the trip meter at each fill to get a good handle on fuel consumption / mileage and do my own approximation of distance-to-empty. Assuming, of course that you are starting with a true fill of 16.9 US gal. (64 liters).

Good info, thanks!
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Old 07-03-2016, 10:00 AM   #8
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Honestly, I could care less about having a calibrated fuel gauge. Mine travels from 3/4 to 1/4 only. It's not a sensor issue as my indy determined it travels normally, needs the magic porsche calibration. I use the trip odometer and I pretty much know I need to fill up at about 250-270 miles...usually get 13 in at that mileage. Not worth the free money for the dealership to me.
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Old 07-03-2016, 10:43 AM   #9
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Originally Posted by jdraupp View Post
Honestly, I could care less about having a calibrated fuel gauge. Mine travels from 3/4 to 1/4 only. It's not a sensor issue as my indy determined it travels normally, needs the magic porsche calibration. I use the trip odometer and I pretty much know I need to fill up at about 250-270 miles...usually get 13 in at that mileage. Not worth the free money for the dealership to me.
I agree, can't imagine how much that calibration would cost at the dealership. Then with my luck I'd leave my lights on, wake up to a dead battery and a fuel gauge that needed recalibrating
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Old 07-04-2016, 12:06 AM   #10
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I agree, can't imagine how much that calibration would cost at the dealership. Then with my luck I'd leave my lights on, wake up to a dead battery and a fuel gauge that needed recalibrating
Luckily, this shouldn't happen. It really is only if you've had work done. Mine is the result of a fuel sender replaced by po at an indy shop and the gauge hasn't been calibrated since. Never been to a dealer and afraid to see their pricing policies firsthand.
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Old 07-04-2016, 04:44 AM   #11
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I have a short-ish trip coming up in the next couple of weeks, 360 mi. (return). Should be the perfect opportunity to run the tank down and see what's really in there. I'll bring a jug of insurance fuel just in case.
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Old 07-08-2016, 08:02 AM   #12
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My fuelgauge show 1/1
But fueltank nearly empty.

Easy to fix, Clean the copper from oxid on the fuel-level stick in fueltank (on fuelpump )
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Old 07-12-2016, 08:22 AM   #13
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Originally Posted by Smallblock454 View Post
Well, the tank volume is nominal 64 litres, but i've never got more than 60 litres in.

So half should be around 30 litres and 1/4 around 45 litres to fill. Don't know if the gauge really works linear, because most of the time there is still fuel in the tank even if the gauge says empty.

Regards, Markus
The gauge is very non-liner Below is the calibration I did for my race data system. As you can see when full, a small change in volume creates a large change in voltage, so it is relatively accurate. However when you get to half way, the voltage change is far less, so the accuracy drops considerably. There is a 3v change for the first 30L but only a 0.7v for the last 30L, and only a 0.2v for the last 20L





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Old 07-12-2016, 10:38 AM   #14
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Hello specboxCO,

thanks for the diagram and clarification.

Regards, Markus
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Old 07-13-2016, 09:40 AM   #15
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Have you guys noticed that the fuel tank fills differently depending on the position of the car? Like if its slighly down or up hill, leaning to the left or the right? The gas stations in my area aren't close to being level. Last time I filled up, pump kept stopping, so I stopped filling even though i knew there was more space for fuel. Sure enough, it had only filled it 3/4. Anyone else experience this as well?
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Old 07-13-2016, 10:26 AM   #16
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Have you guys noticed that the fuel tank fills differently depending on the position of the car? Like if its slighly down or up hill, leaning to the left or the right? The gas stations in my area aren't close to being level. Last time I filled up, pump kept stopping, so I stopped filling even though i knew there was more space for fuel. Sure enough, it had only filled it 3/4. Anyone else experience this as well?
Try turning the nozzle 45 or a full 180 degrees and filling. This is the old 911 trick to get it full.

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Old 07-14-2016, 06:44 AM   #17
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Try turning the nozzle 45 or a full 180 degrees and filling. This is the old 911 trick to get it full.
i will try that thanks!
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Old 07-15-2016, 11:18 AM   #18
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Try turning the nozzle 45 or a full 180 degrees and filling. This is the old 911 trick to get it full.
Just tried this trick today and it worked like a charm! Filled it up how i normally do, waited until the pump clicked off, then checked my fuel gauge. It was only 3/4 full. So I turned the nozzle 180 degrees and continued to fill. Almost 2 more gallons went in! Tank full after that! Thanks for the tip again!
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Old 08-09-2016, 05:26 AM   #19
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I have a short-ish trip coming up in the next couple of weeks, 360 mi. (return). Should be the perfect opportunity to run the tank down and see what's really in there. I'll bring a jug of insurance fuel just in case.
and an extra fuel pump? In general, fuel is what cools and lubricates the pump. In my experience it has not been a good thing to run it low. Not sure if 986 is different from other vehicles.

BTW my fuel gauge takes 10-20 minutes to return to full after I've filled it. I assume the sender is just slow?
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Old 08-09-2016, 07:54 AM   #20
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and an extra fuel pump? In general, fuel is what cools and lubricates the pump. In my experience it has not been a good thing to run it low. Not sure if 986 is different from other vehicles.

BTW my fuel gauge takes 10-20 minutes to return to full after I've filled it. I assume the sender is just slow?

That did concern me when planning this test, it's good advice...thanks!

I did make the trip this past weekend. I filled the tank (slowly) before leaving town and when the pump shut off I thought....what the heck, I'll turn the nozzle 180 degrees and see if it takes any more. Sure enough, I was able to pump in another 11.45 liters (just over 3 US gal.)

There were thunderstorms forecast for the return trip so I decided against running the tank down and fueled up normally.

The interesting thing is I let the pump shut off again and turned the nozzle 180...it wouldn't take any more fuel this time and I was pumping slowly.

I have a 600 mile (one way) trip next week and I will continue to turn the nozzle to try to top it up a bit....maybe the issue is resolved, maybe not.

We'll see...thanks to all for the comments and suggestions
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