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Old April 14th, 2003, 13:08
Manuel Manuel is offline
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Location: Lomita, CA
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Unhappy Gas Gauge one 1999 XJ

In the last 2 months or so I've noticed that my fuel gauge goes down quite a bit quicker than it used to. From previous experience, I knew that when the needle was at the half full point, that I had 10 gallons of gas in it.(20 gallons for full tank)
Well, that's not true anymore. The needle now gets to the half way point when it only uses up 8 gallons or so.
I noticed this because I reset my trip odometer everytime I fill up. When I hit the half full mark, I could figure out what mileage I was getting right away.(i.e. 160 miles at half full point(10 gallons)= 16 MPG).
Well, now I'm only putting 130+ miles before I hit the half full mark.

Any ideas? I think it's possible that it may just be a bad connection.(higher resistance throwing off my gauge) I figure just reseating a connnector might do it.

Anyone have any experience with this?
Yes my XJ is properly tuned. I just passed a smog test about a month ago. I have verified that I have more fuel than the gauge shows.

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Old April 14th, 2003, 13:11
juicexj24 juicexj24 is offline
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Location: USA
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gas gauge

Mine is doing the same thing but burning the hell out of some gas. At 3/4 full I have 85 miles, at 1/4 I have 175 miles. I can't figure what it is but when the light comes on it's over, full up or shut up.
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Old April 14th, 2003, 23:52
djtucci djtucci is offline
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i have the same problem. i took it into a trustworthy shop and they informed me has to do with the fuel (relay?/pickup?). not sure exactly what part it is. it may or may not help but here is some info from alldata:::

The electric fuel pump is located inside of the fuel pump module.

The fuel pump used in this system has a permanent magnet electric motor. Fuel is drawn in through a filter at the bottom of the module and pushed through the electric motor gearset to the pump outlet.

Check Valve Operation: The pump outlet contains a one-way check valve to prevent fuel flow back into the tank and to maintain fuel supply line pressure (engine warm) when pump is not operational. It is also used to keep the fuel supply line full of gasoline when pump is not operational. After the vehicle has cooled down, fuel pressure may drop to 0 psi (cold fluid contracts), but liquid gasoline will remain in fuel supply line between the check valve and fuel injectors. Fuel pressure that has dropped to 0 psi on a cooled down vehicle (engine off) is a normal condition. Refer to the Fuel Pressure Leak Down Test for more information.

Voltage to operate the electric pump is supplied through the fuel pump relay.

The fuel gauge sending unit (fuel level sensor) is attached to the side of the fuel pump module. The sending unit consists of a float, an arm, and a variable resistor (track).

The resistor track is used to send electrical signals to the Powertrain Control Module (PCM) for fuel gauge operation and for OBD II emission requirements.

For fuel gauge operation: As fuel level increases, the float and arm move up. This decreases the sending unit resistance, causing the fuel gauge to read full. As fuel level decreases, the float and arm move down. This increases the sending unit resistance causing the fuel gauge to read empty.

After this fuel level signal is sent to the PCM, the PCM will transmit the data across the CCD bus circuits to the instrument panel. Here it is translated into the appropriate fuel gauge level reading.

For OBD II emission monitor requirements: A voltage signal is sent from the resistor track on the sending unit to the PCM to indicate fuel level. The purpose of this feature is to prevent the OBD II system from recording/setting false misfire and fuel system monitor trouble codes. The feature is activated if the fuel level in the tank is less than approximately 15 percent of its rated capacity. If equipped with a Leak Detection Pump (EVAP system monitor), this feature will also be activated if the fuel level in the tank is more than approximately 85 percent of its rated capacity.

A combination fuel filter and fuel pressure regulator is used on all engines. It is located on the top of fuel pump module. A separate frame mounted fuel filter is not used with any engine.

Fuel Pressure Regulator Operation: The pressure regulator is a mechanical device that is not controlled by engine vacuum or the Powertrain Control Module (PCM).

The regulator is calibrated to maintain fuel system operating pressure of approximately 339 kPa 34 kPa (49.2 psi 5 psi) at the fuel injectors. It contains a diaphragm, calibrated springs and a fuel return valve. The internal fuel filter is also part of the assembly.
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Old April 15th, 2003, 05:47
Judd W. VA Judd W. VA is offline
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Being the owner of a '98 all this applies to mine also. Not having any specific problems as of yet (knock wood) i am aware of a problem of cheap construction with the tank sending unit. All the other info is invaluable to diagnostics and i printed it up. Did you notice a comment at the end of the Alldata info about there being a FILTER integral to the fuel pressure regulator?? Thanks again.
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Old April 15th, 2003, 16:15
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RichP RichP is offline
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Location: Effort, Pa
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The resistor track I understand is 'painted on' and not a real wire wound resistor. Mine has been bad for years, reads full on fillup then drops to E randomly and backup to full. Never stays anwhere near whats in the tank. I drive by odometer/trip meter and have for years. If the fuel pump goes then replacing it should fix the problem but I'm not going to spend the money on it right now unless I have to. You also might want to check your connections on the back of the instrument cluster, supposedly they fixed that in 99 as it was a problem that plagued 97+ XJ's.
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