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1997 Pontiac Grand Prix GT
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Discussion Starter · #1 ·
Ever since I’ve owned my 97 GP GT the fuel gauge has been weird. Before I replaced the fuel pump the gauge would sometimes shoot up to full after starting, a quick restart would always fix it. It’s never done that with the new one but it feels like it’s off, not only is it impossible for me to get it to full (gauge will never go over about a quarter to full) but it appears to be using more fuel than it should. Going based off the gauge I calculated around 12 MPG which is incredibly hard to believe seeing as the car runs completely fine. One time I had the fuel light come on and I shut the car off and turned it back on later and the light was off and the gauge went up a little bit from last start. What could cause a gas gauge to “work” but be inaccurate?
 

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The wiring in the dash or pump basket/level sender may be having a connection issue. You may need to try getting in the trunk with a multimeter and measuring the leads to/from the pump basket. It may be easier to do outside the car, and with help if needed.

For now, I would base your fill-ups on the trip odometer, and limit yourself to about 250mi or so the first time after a fill-up, and do some gas pump math next fill-up for mpg estimation (trip miles ÷ pump gallons = mpg). That can let you go a bit further for routine driving when you know how much further a ~16-17gal tank may take you, but don't cut it too close.
 

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Discussion Starter · #3 ·
The wiring in the dash or pump basket/level sender may be having a connection issue. You may need to try getting in the trunk with a multimeter and measuring the leads to/from the pump basket. It may be easier to do outside the car, and with help if needed.

For now, I would base your fill-ups on the trip odometer, and limit yourself to about 250mi or so the first time after a fill-up, and do some gas pump math next fill-up for mpg estimation (trip miles ÷ pump gallons = mpg). That can let you go a bit further for routine driving when you know how much further a ~16-17gal tank may take you, but don't cut it too close.
I’ll have to check the wiring this weekend, what should I be looking for? I’m not too knowledgable on electrical work.
 

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I'd guess the sending unit may be no longer giving a clean signal of where the float arm is during use. I'll have to look up the wiring diagram for the plug pins when I get home later, unless someone chimes in sooner. A full replacement might be in order, if that's the case, to have a fresh pump and level sensor/sender. That's TBD, though, based on what happens when you do some digging. (Thankfully this generation of W-body's came with an access panel, so it's not a huge tank drop pain like 99% of vehicles.)
 

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1997 Pontiac Grand Prix GT
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Discussion Starter · #5 ·
I'd guess the sending unit may be no longer giving a clean signal of where the float arm is during use. I'll have to look up the wiring diagram for the plug pins when I get home later, unless someone chimes in sooner. A full replacement might be in order, if that's the case, to have a fresh pump and level sensor/sender. That's TBD, though, based on what happens when you do some digging. (Thankfully this generation of W-body's came with an access panel, so it's not a huge tank drop pain like 99% of vehicles.)
I’d hate to have to buy another pump so hopefully that’s not the deal. Thankfully they put the panel in saves a world of pain. That rubber seal is still a pain in the ass though lol I had to use vasoline haha.
 

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If your using an aftermarket fuel pump/sending unit assembly it is very possible. The calibrations on the aftermarket sending units can be a bit off. Being you've had issues both current & prior it could possibly be an issue with the fuel gauge motor itself. These 90's GM gauge clusters can sometimes be a bit fickle especially as they age.
 

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True. My 1st car from my parents, a 1989 GP base model 2.8L V6, had an all digital (spaceship) dash display that would randomly go blank after ~15yrs old. My dad had to resolder some grounds on the circuit board to try helping this, but electrical components in cars of the past 30yrs are not built to last forever, even if we try to make them.
 

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1997 Pontiac Grand Prix GT
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Discussion Starter · #8 ·
True. My 1st car from my parents, a 1989 GP base model 2.8L V6, had an all digital (spaceship) dash display that would randomly go blank after ~15yrs old. My dad had to resolder some grounds on the circuit board to try helping this, but electrical components in cars of the past 30yrs are not built to last forever, even if we try to make them.
There’s a couple GPs out at the junkyard, maybe I could screw around with some other gauge clusters.
 

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Ever since I’ve owned my 97 GP GT the fuel gauge has been weird. Before I replaced the fuel pump the gauge would sometimes shoot up to full after starting, a quick restart would always fix it. It’s never done that with the new one but it feels like it’s off, not only is it impossible for me to get it to full (gauge will never go over about a quarter to full) but it appears to be using more fuel than it should. Going based off the gauge I calculated around 12 MPG which is incredibly hard to believe seeing as the car runs completely fine. One time I had the fuel light come on and I shut the car off and turned it back on later and the light was off and the gauge went up a little bit from last start. What could cause a gas gauge to “work” but be inaccurate?
I have seen other fuel tank sending units that can get worn to the point where the sender unit develops play and looses the fuel level position. Most will replace the sending unit/pump assembly. One thing that can happen is the connection from float arm gets too worn which results in POOR signal to guage. This is somewhat common on an older model. Quickest fix is a new pump assembly. I suppose one could bend the signal pick up for good contact with the pump unit. Personally, I would replace the pump assembly. This is your best bet rather that trying to repair a loose connection on the sender wiring. Hope this helps, Jake(mr goodwrench).....
 
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