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Fuel sending unit 1994 Pontiac Firebird Formula LT1

idabomb00 idabomb00
New User | Posts: 34 | Joined: 06/12
Posted: 07/11/12
12:22 AM

Hello, I have a 1994 Pontiac Firebird Formula. Some of you may recognize me from previous threads, but if you don't know I have been restoring this car to be road worthy for about a month now. It had been sitting close to 12 years.

I replaced the fuel sending unit and fuel pump all at once when I first got this car. They were both bad and keeping the car from running (obviously). The new fuel sending unit worked fine.

I recently have been driving on the road and the fuel gauge jumps an awful lot while running and will even show all the way past the full line. From experience I know that this is caused by the fuel sending unit circuit not being completed. It always works however when I start the vehicle later.

The fuel pump still works when this happens.

Does anyone have an idea of what might be causing it? I don't know much about it so I thought maybe somebody had an idea of where I can start looking before I pull the fuel pump assembly back out.  

waynep71222 waynep71222
Guru | Posts: 823 | Joined: 03/12
Posted: 07/11/12
02:09 AM

i would wonder if running the exhaust open under the car has melted anything..

i have on 3rd gen firebirds had issues with the bulk head connector under the back seat.. were the wiring from inside the car passes through the floor pan to get to the tank...

wind buffeting .. heat from the exhaust melting some of the insulation...

i seem to recall the ground connection for the pump is down there also.. but i don't have first hand knowledge of what it looks like..  i have pulled tanks on 4th gen camaros.. but i did not pay a LOT of attention to how it was connected.. other than it was there...  i was mostly way upset about having to cut the exhaust off the car to get it apart.. somebody had welded the muffler to the pipe..

if the ground connection is loose or damaged...

the pump could be trying to get electrons from the sending unit connections... causing your fluctuation..

you could also have a bad body ground connection at the front of the car.. or behind the engine to the firewall..

i have a 2 minute test for ground issues..

engine running . headlights on.. digital volt meter set to 20 volts DC scale..

1, battery Negative post to the battery positive post.. 14.1 to 14.7 volts..

2.  battery negative post to the engine block.. 0.04 volts is expected.

3.  battery negative post to the body.. 0.02 volts is expected..

4.  engine block to  the body...  0.02 volts is expected..

0.00 is not a good test result..

0.04 or 0.02 is expected... as copper wire has some resistance.. when there is a load .. to do the fractional OHM of resistance in the wire.. there will be a tiny voltage drop do to the resistance..  this is how engineers verify the circuit wiring is the proper diameter.. looking for the 0.02 or 0.04 volts when the circuit is powered up and loads are applied..

if you get more than the 0.02 or 0.04.. you have a ground connection issue..

practice this test.. it will eventually save you hours if not days of work...  i do this test on almost every car i open the hood on..  and every car before i close the hood and send it out the door..

just think of this... a ground is HALF the circuit...   and Electrons flow from NEGATIVE TO POSITIVE.

bad ground means there are NOT enough electrons to power the circuit when its trying to work..  

idabomb00 idabomb00
New User | Posts: 34 | Joined: 06/12
Posted: 07/26/12
11:17 AM

Thanks for the help, It turned out to be a loose prong on the connector under the vehicle that goes into the floorboard. Fixed it with some aluminum foil.  

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