So I'm about 2500 miles into my cross country trip. 500-700 miles per day gives you some time to notice and think about things.....especially in places like Montana and Wyoming where gas stations can be 100 miles apart. I've noticed that my gas gauge needle never shows completely full after fill up, but whats more interesting is that after maybe 200~ miles, it shows 50% full, but the last 50% is prob good for about 100 miles. in other words, 50% remaining is prob closer to 1/3 remaining. I have a 73 Mach 1 that shows 3/4 full until suddenly its 1/8 full and you better get to the gas station. so quirks in gas gauges are not unfamiliar to me. does anyone else notice this kind of behavior with their gas gauge? it doesn't really bother me, just more curious about other people's cars and if there is a way to adjust the gauge reading. thanks

 

Mine acts similarly in my '87. I checked and cleaned the sender, which appears to operate properly. Somewhere between the sender and gauge is an additional resistance, which hoses up the gauge reading - most likely in the IP flex circuit - something I'll get to troubleshoot eventually. Additionally, my low fuel light appears inoperable, so I rely on the trip odometer to fill up at ~350 miles. I almost ran out of fuel once as I was checking for the fuel light operation - coming home from a dinner with my wife and daughter, no less.

 

The fuel level sender is a linear meter and I don't think there's any smarts in the gauge, if the tank was a perfect cube shape it would be more progressive but it isn't.

 

To clarify, not reading full when full is why I mentioned the additional resistance.

 

my question was more to try and establish a baseline of what normal is. I don't expect a 34 car to be super accurate, I just wondered if what I am seeing is normal for these cars.

 

My '89 digi-dash car has a slightly different sender arrangement (no low fuel switch in the sender) but otherwise same tank dimensions and sender resistance. Over the last few days I swapped fuel pumps and filter, and when I refilled I gave consideration to having only 10 gals of fuel can capacity. Ran the tank to a pint or less, swapped pumps, put exactly 10 gallons back in after a gallon only to check for leaks. The 10 gallons caused the gauge to read just shy of an estimated 5/8 tank in the gauge. The listed tank capacity is north of 20 gallons, so I expected my 10 gallons to offer a gauge reading at about 1/2 or slightly less. Gauge is linear, tank has a sloped bottom, so maybe not truly unexpected. Point is that the linear sender will be progressively less accurate as the tank level falls, since it's hardly straight-sided or flat-bottomed. Like others, I watch the trip meter to make ballpark guesstimations of miles remaining.

Curiously, the miles-remaining function in the digi trip computer seems more accurate than the fuel gauge reading.

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In one of those interesting discovery moments, this week I happened to have contact with a car-guy cousin who does manufacturing-plant operations rescues. He's currently functioning as a general manager at a high-performance fuel pump factory. They make drop in pump-and-sender modules for retrofits in US performance applications, in addition to their OEM supply business. Need to brainstorm a drop-in module that fits into 928 tanks via the sender opening. Plug the bottom discharge nozzle, two hoses out the top plus the electrical connections for pump(s) and the sender. They use swinging-arm level senders,something that might or might not be interesting to configure for the 928 linear sender/gauge.

 

Like already stated, the tank shape changes with volume, a lot.

Attempt was made on the gauge to compensate for that, note how close 1/4 is to empty, compared to 3/4 from full??


And..its much like airplanes. The fuel quantity indicator must be calibrated to read "zero" during level flight when the quantity of fuel remaining in the tank is equal to the unusable fuel supply. Which is when your light comes on (separate switch), you're dang near out of usable fuel no matter what the needle says...there'e your "I know my car..." calibration.

 

yeah. I don't know where empty happens, and I haven't waited to see a low fuel light come on. when doing hundreds of miles at a time in the west, my thinking is that at 50%( cleverly noted by Porsche as 2/4's, instead of 1/2) I have 100 miles left and I start looking for gas stations, which could be 10 miles away or 75 miles. I would think differently driving around town.

 

My '80 is full at an indicated 3/4 tank, and when I fill up at 1/4 tank it only takes 8-9 gallons (so it's really over half full). I haven't driven a long distance, so filling up at 1/4 tank is "safe" for me for now,

 

This symptom is common when the vent line between the top tank sections is pinched. If your tank has even been removed, it's worth a look.


The sender itself has something like 80 Ohms IIRC at empty, based on a foggy memory of something Rob Edwards shared recently in a sender thread. It would seem that you could check at the sender, clean the rear grounds, etc., and see if the sender, wiring or the gauge is contributing to what you are seeing.

 

I had a similar issue on my Euro 82. The fuel gauge reading was inaccurate as was the temp gauge. I checked all the wiring from the applicable sensors and it all checked out fine. It turned out to be an issue with the flexi pcb board on the back of the console, I replaced the ground traces with 0.5mm wire and it solved the problem.

If you disconnect the connecter from the fuel sensor and ground the socket marked 'G' (violet/black wire) does the gauge peg to full? Also if you ground 'W' socket (black/yellow) does the reserve light come on?

Cheers
Ben

 

Digital dashes can be reset and programmed to be accurate again.

for older cars-88, the aging PCB is a problem. My 87 was showing anomalies in the fuel gauge and voltmeter. Changed PCB and all works like day 0.