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I just discovered the C4 fuel guage quirk

Teeonethousand

Well-known member
Joined
10 Nov 2012
Messages
97
What a brilliant forum this is.

I popped by the petrol station as was getting low and just put 30 squids in - when i got back into the car the fuel guage had not moved and continued to go down/sound the warning.

Funny i thought - then i googled this forum and up came an old thread that explains everything...

4wd 996`s have a saddle bag type of fuel tank because of the front diff and is split into two sections around the bottom of the tank where it is shaped around the diff.the fuel sender float will only lift from above this section and give a reading therefore from empty you have to fill the tank with enough fuel to give a reading usually about �30 worth of fuel for the gauge to register.as the level drops the amount of fuel left in the tank when the level reaches the split section is calculated by the instrument cluster according to the load,speed distance travelled and the reading is sent to the gauge until it is empty .so the fuel level below the split section is not the sender level measurement but the calculated level but is very accurate.

Is this a brilliant forum or what :)
 
Teeonethousand said:
Is this a brilliant forum or what :)

It is, until I discovered it I didn't even realise that my engine was in the back. :grin:
 
4wd 996`s have a saddle bag type of fuel tank because of the front diff and is split into two sections around the bottom of the tank where it is shaped around the diff.the fuel sender float will only lift from above this section and give a reading therefore from empty you have to fill the tank with enough fuel to give a reading usually about £30 worth of fuel for the gauge to register.as the level drops the amount of fuel left in the tank when the level reaches the split section is calculated by the instrument cluster according to the load,speed distance travelled and the reading is sent to the gauge until it is empty .so the fuel level below the split section is not the sender level measurement but the calculated level but is very accurate.

That was Sid Spanner's description.

The tank isn't exactly "saddle-bag" shaped - which sounds like it sits either side of the forward prop shaft.

It has reduced depth over the front diff, and then drops deeper in front of the diff (ie in front of front axle).

Given the varying price of fuel, the critical value below which the remaining fuel volume is "guessed" is in fact 19 litres.

The AWD models (so C4, Turbo etc) have additional driveshafts to and at the front (obviously), but this poses problems for the original position/size of the fuel tank.

So AWD models have a reshaped tank, to fit around the driveline and new chassis strengthening structures and driveshaft mountings etc.

The fuel gauge technically only operates above the "flat" of the tank, any fuel level below this is "guessed" by a fuel tank algorithm and sent to the electronic analogue display on the dash.

This means if you are empty and add only enough to fill the lower section of the tank, the sensor is still dry and the gauge won't move (ie under-reads fuel volume)

This might sound stupid, but it is preferable to having the sensor in the base of the tank and over-reading and hence you thinking you have more fuel.

Once enough fuel is added to fill the lower section (ie up to the "flat") the sensor is "wet" and the gauge reads as normal (ie a measure of the entire tank).

This "flat" volume is usually about 1/4 tank (below 19 litres the system actually guesses your remaining fuel from fuelling data, only once it is refilled above this level again is the alogorithm reset and continuous reading handed back to the sensor).

Note: in the diagrams below the "2WD" and "AWD" refer to chassis type, this means as GT models use AWD chassis and hence use AWD tanks.

996_fuel_tanks_209.jpg


PS - we have a search function on here too!
 
There is the same issue in the 2WD too, although it is about £18, so maybe 14 or 15 litres.
 
Porsche had a recall on the fuel gauge calibration parameters (ie sensor output mapping to fuel gauge was wrong), and you can get a sticky capillary (sensor), but there is no calculated (estimated) fuel level on a 2WD Carrera.
 
Perhaps I will see if Mine has all the recalls done.

If I run it really low, sticking £15 in it will not get noticed by the gauge.

(Not that I usually put £15 in, it was an experiment I was doing. If I have to go to the effort of putting fuel in the car, then I fill up!)
 
Yeah, whatever.

Have you not set up the "PeterS Fuel for Sports Vehicles Benevolent Fund" yet?

I have have PayPal funds standing by :thumbs:
 
T8996C4 said:
Teeonethousand said:
Is this a brilliant forum or what :)

It is, until I discovered it I didn't even realise that my engine was in the back. :grin:

+1

:floor: :floor: :floor: :floor: :floor:

You wont believe the number ofpeople go.... how do you put things in when the engine is in the back?
 
It's called the "cheapskate" factory option.

Mrs whiz got 25 quid of fuel after convincing the local supermarket that the pump was faulty....she had no idea about this quirk.....maybe one day I will tell her :lol:
 
wizard993 said:
It's called the "cheapskate" factory option.

Mrs whiz got 25 quid of fuel after convincing the local supermarket that the pump was faulty....she had no idea about this quirk.....maybe one day I will tell her :lol:


TBH i wondered whether the petrol pump had pumped fresh air :)

Mr GT4 - i did search the forum and got Sids description - dunno why i wrote google - habit maybe. Thanks for the additional detail, your knowledge is outstanding. Respect.
 
I never knew this was a known issue. :eek:

I thought I had a faulty sensor in the summer when I had an emptyish tank and put £20 of fuel in only for there to be no response to the fuel gauge. It only responded when I eventually filled the tank. :oops:

I put it down to a sticky float as the car is left in the garage and not driven for long periods.

It all makes perfect sense :D :D

Now I know, thanks :thumb: :thumb: :thumb:
 
I was running low and with only a £15 on me had the same problem - the gauge didn't move. The garage weren't convinced by my allegations of pump inaccuracy and I realised there was a 911 quirk when I filled up to the brim later on and it came to £15 less than normal.

Thanks for explaining it though! :thumbs:
 
"...calculates the remaining fuel based on an algorithm..."

It guesses how much fuel is left and therefore estimates whether it can get to the next garage, not this one, on whats left when the light comes on. Sounds exactly the right thing to do, obviously designed by a man. :)
 
Presumably you are relatively happy with the notion of a Range or MPG calculation?
 

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