Fuel Gauge/Pump Diagnostic Tests - kw 4.6 5.7 01-06-04-022 01-06-04-010A battery cluster display DTCgasinaccurate inoperative instrument intermittent IPC light LH2 LS1 LS6 P0461 P1431 P2636 P2066

When servicing the vehicle with the VIN you entered, the following diagnosis might be helpful if the vehicle exhibits the described symptoms.

There have been past concerns that may result in the PCM commanding the fuel gauge to empty or the vehicle running out of fuel without using all the fuel in the tanks. As solutions have been identified, they have been addressed in service bulletins. Service Manual diagnostics and Service Bulletins should be checked to ensure that the customers concern has been properly diagnosed before any repairs are performed. The vehicle may have a concern of the fuel gauge dropping to empty and DTC P1431 (Fuel Level Sensor 2 Performance) present in Corvette and P2636 in the XLR or the vehicle may run out of fuel while still indicating 1/2 tank on the gauge.

The fuel system is designed to use the fuel in the RH (secondary) tank first and then the fuel in the LH (primary) tank. Moving from gage Full to Empty, the RH sender voltage will go from 2.5 volts to 0.7 volts, then the LH sender will go from 2.5 V to 0.7 V. If at any time the RH sender voltage is higher than 0.8 V while the LH is lower than 2.3V, there is probably a jet pump transfer or stuck float issue. The lack of jet transfer could be due to the lack of system pressure or a clogged or inefficient jet pump. This should be determined prior to servicing to avoid replacing the incorrect part. When the fuel gauge is indicating approximately half full/empty or lower, the left hand tank will be full and the right hand tank will be empty under normal operating conditions. When reading the Tech 2 data in the PCM, the sensors must indicate the appropriate fuel level for the PCM to correctly display the actual fuel level accurately.

The following information can be used to diagnose the fuel transfer process which takes place within the fuel system on these two vehicles.

Note: The 1997 through 2002 and early built 2003 Corvettes (Starting on November 25, 2002 with vehicle identification number 35111542, all Corvettes and all 2004 XLR use the FFS fuel system) without the FFS system did not have a secondary regulator which prevents the siphoning of fuel (LH to RH) after the vehicle is turned off. The siphoning could balance the fuel such that, at start-up, the sender voltages could be similar. That is why there is a 40 minute timer on the P1431 DTC on these vehicles to allow the fuel to transfer to the LH tank after start-up.

Fuel Gauge/Clogged Jet Pump Diagnostic Tests

Diagnostic Test Option A:

1. Add enough fuel to ensure fuel gage level is at approximately three quarters (¾) full.
2. With Tech 2, check for and record Diagnostic Trouble Codes (DTCs), and monitor fuel level sensors for both tanks for remainder of procedure.
3. Start vehicle. Run vehicle long enough to confirm RH fuel level decreases while LH remains full (2.5 V).

Diagnostic Test Option B (Draining Fuel):

1. With Tech 2, check for and record Diagnostic Trouble Codes (DTCs) and monitor fuel level sensors for both tanks for remainder of procedure.
2. If RH level is greater than 0.8 V while LH level is less than 2.4 V, there may be a jet transfer issue - continue procedure to confirm issue.
3. Add enough fuel to ensure fuel gage level is approximately three quarters (¾) full.
4. Install Kent-Moore pressure gage/ drain hose to Schrader valve at fuel rail with drain valve closed.
5. Start car and verify pressure is 55-60 psi. If pressure is not at this level correct as necessary.
6. Open drain valve, and drain fuel from rail into empty 5-gallon container. The gage pressure should be @30 psi.
7. Drain fuel. Monitor both level sensors. Drain until RH sensor level drops 0.15V or more. Verify that LH stays at 2.5V while RH decreases. If RH sensor does not move and LH level sensor decreases, increase gage pressure to 55 psi by closing the drain valve. See if RH sensor starts to decrease. If not, there is a jet transfer issue that needs to be resolved.
8. With vehicle/fuel pump still running, close drain valve, and verify system rail pressure returns to 55-60 psi.
9. Shut off vehicle, remove Kent-Moore tool and verify Schrader valve has fully closed. (Key-on vehicle to verify). Replace Schrader cap.

Note: The right side tank sending unit on known good vehicles has shown to be as low as .65 volts while problem vehicles have shown to be above 0.8 volts with the fuel level below a half a tank of fuel (measured).

Replace/repair components as required. Possible causes:

SI should be followed to determine the root cause. If the fuel gage is intermittent or dropped to empty due to a fuel system issue, there should be a DTC set. Follow SI symptom diagnostics for DTC.

1997 - 2003 without the FFS system -

1. If the fuel pump does not generate the required pressure (about 52 psi) to run the RH jet pump, the system should be evaluated for internal leaks or faulty regulator(s). The source of an internal leak could be a missing or cut o-ring, a loose connection, or a displaced regulator retaining clip. The fuel feed hose that is installed on the left tank (center hose on the left sending unit) feeds the fuel filter and the orifice hose that goes to the right tank. During assembly the Brown fuel feed hose O-ring may have been cut (this is the 2nd O-ring just inside the hose) and the pieces may have plugged the 0.030 orifice. If this occurs there will be no feed into the right tank and therefore the jet pump will not work and fuel will not return to the left tank.
2. The right side tank fuel sending unit is hanging up for some reason. The float and/or arm may be in contact with something within the tank. This is much less likely on the FFS system.
3. The newest concern relates to the use of reformulated fuels. Many of these fuels have been formulated to reduce their sulfur content. Occasionally, some fuels may contain a small amount of reactive sulfur that attacks the silver tipped sender wiper arm contacts. The silver is being transferred to the card itself resulting in irregular/erratic voltages. While both senders are affected by this concern, it is most likely the right hand sender that sends the incorrect voltage. This occurs because the PCM is expecting to see the voltage signal from the right hand sender remain below 0.7 volts in the empty position. Signals greater than 0.7 volts are not what the PCM expects to see, causing the PCM to command the fuel gauge to the empty position. Replacing the right hand sender will correct this condition until the reactive sulfur attacks the new sender wiper. The time that it takes for this to reoccur can vary anywhere from several days to much longer. The time frame depends on the amount of reactive sulfur in the fuel along with how long the particular tank full of fuel is in the tank. Changing to a different brand of fuel may provide a solution.
4. For the 2001 - 2002 model year there is a revised service calibration released for this concern. See bulletin 02-06-04-010a for more information.
5. For vehicles with the FFS system - you may find a connection problem at either the left or the right tank, "O" rings missing or cut at the left or right tank fuel connections of the crossover tube. This will also cause a lower than expected fuel pressure reading under normal operating conditions. The other possibility is that the jet pump itself is plugged. Check the Julian date on the RH fuel tank module. If the Julian date is 25532 or lower and no other cause is found, then the RH module will need to be replaced. The Julian date is printed on top of the sender next to the part number.

Once repairs are completed, especially a vehicle with the FFS system, prior to reassembling the vehicle (installing the 3rd member and torque tube), you may want to verify the system is functioning correctly. This may be accomplished by using a Tech 2 to monitor both sending unit readings. With approximately 4 gallons of fuel in both tanks. Use wiring schematic information from SI to install a fused jumper at the fuel pump relay with the key on and monitor fuel transfer from the right to the left tank. Voltage should go down in the right tank as voltage is going up in the right tank. Caution: With the torque tube and rear axle removed, the engine should not be started.

Both fuel systems -

6. The left side sender has a concern that is causing the voltage to remain above the specified value for the required time to set the DTC.
7. The right side sender is sending erroneous voltage readings to the PCM for some other reason.

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Please follow this diagnosis process thoroughly and complete each step. If the condition exhibited is resolved without completing every step, the remaining steps do not need to be performed. If these steps do not resolve the condition, please contact GM TAC for further diagnostic assistance. This diagnostic approach was developed for the vehicle with the VIN you entered and should not be automatically be used for other vehicles with similar symptoms.

(1997 - 2004 Chevrolet Corvette) and (2004 Cadillac XLR)

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