P0068

SYMPTOM
P0068-MANIFOLD PRESSURE/THROTTLE POSITION CORRELATION

WHEN MONITORED
During all drive modes

SET CONDITION
If vacuum drops below 1.5"Hg with engine RPM greater than 2000 RPM and closed throttle. One Trip Fault. Three good trips to turn off the MIL.

POSSIBLE CAUSES
- VACUUM LEAK
- RESISTANCE IN (F856) 5-VOLT SUPPLY CIRCUIT
- (F856) 5-VOLT SUPPLY CIRCUIT SHORTED TO GROUND
- RESISTANCE IN THE (K1) MAP SIGNAL CIRCUIT
- (K1) MAP SIGNAL CIRCUIT SHORTED TO GROUND
- RESISTANCE IN THE (F855) 5-VOLT SUPPLY CIRCUIT
- (F855) 5-VOLT SUPPLY CIRCUIT SHORTED TO GROUND
- RESISTANCE IN THE (K22) TP SENSOR NO.1 SIGNAL CIRCUIT
- (K22) TP SENSOR NO.1 SIGNAL CIRCUIT SHORTED TO GROUND
- RESISTANCE IN THE (K900) SENSOR GROUND CIRCUIT
- MAP SENSOR
- THROTTLE POSITION SENSOR
- PCM

Always perform the Pre-Diagnostic Troubleshooting procedure before proceeding. Scan Tool Testing and Procedures

DIAGNOSTIC TEST
1. ACTIVE DTC

Note: Diagnose any TP Sensor or MAP Sensor component DTCs before continuing.

Note: If the P0501 - No Vehicle Speed Signal is set long with this DTC, refer to the P0501 diagnostics before continuing.

Note: The throttle plate and linkage should be free from binding and carbon build up.

Note: Make sure the throttle plate is at the idle position. Ignition on, engine not running.

With a scan tool, read DTCs.
Is the DTC active at this time?
Yes>> Go To 2
No>> Perform (NGC) POWERTRAIN VERIFICATION TEST VER-5. Powertrain Verification Test - Ver 5

2. VACUUM LEAK

Note: This code is enabled on engines with a plastic intake manifold and is intended to shut down the engine if a large crack occurs.

Note: A large vacuum leak is most likely the cause of this DTC.

Inspect the Intake Manifold for leaks and cracks.
Inspect the Power Brake Booster for any vacuum leaks.
Inspect the PCV system for proper operation and vacuum leaks.
Inspect the throttle plate to see if it is bent and will close entirely, if it is bent it may need to be replaced.
Inspect the MAP Sensor for proper installation.
Verify the engine is free from any mechanical failures.
Were any vacuum leaks found?
Yes>> Repair the vacuum leak as necessary. Perform (NGC) POWERTRAIN VERIFICATION TEST VER-5.
No>> Go To 3

3. MAP SENSOR OPERATION

Start the engine.
With a scan tool, monitor the MAP Sensor voltage.
Snap the throttle.
Does the MAP Sensor voltage vary from below 2.0 volts at idle to above 3.5 volts at wide open throttle?
Yes>> Go To 4
No>> Go To 11

4. TP SENSOR OPERATION

Ignition on, engine not running.
With a scan tool, monitor the TP Sensor voltage while slowly pressing the throttle pedal from closed to wide open throttle.
Does voltage start at approximately 0.8 of a volt and go above 3.5 volts with a smooth transition?
Yes>> Perform (NGC) POWERTRAIN VERIFICATION TEST VER-5.
No>> Go To 5

5. EXCESSIVE RESISTANCE IN THE (F855) 5-VOLT SUPPLY CIRCUIT

Turn the ignition off.
Disconnect the TP Sensor harness connector.
Disconnect the PCM harness connectors.

CAUTION: Do not probe the PCM harness connectors. Probing the PCM harness connectors will damage the PCM terminals resulting in poor terminal to pin connection. Install Miller Special Tool # 8815 to perform diagnosis.

Measure the resistance of the (F855) 5-volt Supply circuit from the TP Sensor harness connector to the appropriate terminal of special tool # 8815.
Is the resistance below 5.0 ohms?
Yes>> Go To 6
No>> Repair the excessive resistance in the (F855) 5-volt Sup ply circuit. Perform (NGC) POWERTRAIN VERIFICATION TEST VER-5.

6. (F855) 5-VOLT SUPPLY CIRCUIT SHORTED TO GROUND

Measure the resistance between ground and (F855) 5-volt Supply circuit in the TP Sensor harness connector.
Is the resistance above 100k ohms?
Yes>> Go To 7
No>> Repair the short to ground in the (F855) 5-volt Supply circuit. Perform (NGC) POWERTRAIN VERIFICATION TEST VER-5.

7. THROTTLE POSITION SENSOR

Connect the PCM harness connectors.
Ignition on, engine not running.
With a scan tool, monitor the TP Sensor voltage.
Connect a jumper wire between the (K22) TP Sensor No.1 Signal circuit and the (K900) Sensor ground circuit in the Sensor harness connector.
Does the TP Sensor voltage change from approximately 4.9 volts to below 0.5 of a volt with the jumper wire installed?
Yes>> Replace the Throttle Position Sensor. Perform (NGC) POWERTRAIN VERIFICATION TEST VER-5.
No>> Go To 8

Note: Remove the jumper wire before continuing.

8. EXCESSIVE RESISTANCE IN THE (K22) TP NO.1 SIGNAL CIRCUIT

Turn the ignition off.
Disconnect the PCM harness connectors.

CAUTION: Do not probe the PCM harness connectors. Probing the PCM harness connectors will damage the PCM terminals resulting in poor terminal to pin connection. Install Miller Special Tool # 8815 to perform diagnosis.

Measure the resistance of the (K22) TP Sensor No.1 Signal circuit from the TP Sensor harness connector to the appropriate terminal of special tool # 8815.
Is the resistance below 5.0 ohms?
Yes>> Go To 9
No>> Repair the excessive resistance in the (K22) TP Sensor No.1 Signal circuit. Perform (NGC) POWERTRAIN VERIFICATION TEST VER-5.

9. (K22) TP SENSOR NO.1 SIGNAL CIRCUIT SHORTED TO GROUND

Measure the resistance between ground and the (K22) TP Sensor No.1 Signal circuit in the TP Sensor harness connector.
Is the resistance below 100 ohms?
Yes>> Repair the short to ground in the (K22) TP Sensor No.1 Signal circuit. Perform (NGC) POWERTRAIN VERIFICATION TEST VER-5.
No>> Go To 10

10. EXCESSIVE RESISTANCE IN THE (K900) SENSOR GROUND CIRCUIT

Turn the ignition off.
Disconnect the MAP Sensor harness connector.
Disconnect the PCM harness connectors.

CAUTION: Do not probe the PCM harness connectors. Probing the PCM harness connectors will damage the PCM terminals resulting in poor terminal to pin connection. Install Miller Special Tool # 8815 to perform diagnosis.

Measure the resistance of the (F856) 5-volt Supply circuit from the MAP Sensor harness connector to the appropriate terminal of special tool # 8815.
Is the resistance below 5.0 ohms?
Yes>> Go To 12
No>> Repair the excessive resistance in the (F856) 5-volt Sup ply circuit. Perform (NGC) POWERTRAIN VERIFICATION TEST VER-5.

12. (F856) 5-VOLT SUPPLY CIRCUIT SHORTED TO GROUND

Measure the resistance between ground and the (F856) 5-volt Supply circuit in the MAP Sensor harness connector.
Is the resistance above 100k ohms?
Yes>> Go To 13
No>> Repair the short to ground in the (F856) 5-volt Supply circuit. Perform (NGC) POWERTRAIN VERIFICATION TEST VER-5.

13. MAP SENSOR

Turn the ignition off.
Connect the PCM harness connectors.
Ignition on, engine not running.
With a scan tool, monitor the MAP Sensor voltage.
Connect a jumper wire between the (K1) MAP Signal circuit and the (K900) Sensor ground circuit.
Does the scan tool display MAP voltage from approximately 4.9 volts to below 0.5 of a volt with the jumper wire installed?
Yes>> Replace the MAP Sensor. Perform (NGC) POWERTRAIN VERIFICATION TEST VER-5.
No>> Go To 14

Note: Remove the jumper wire before continuing.

14. EXCESSIVE RESISTANCE IN THE (K1) MAP SIGNAL CIRCUIT

Turn the ignition off. Disconnect the PCM harness connectors.

CAUTION: Do not probe the PCM harness connectors. Probing the PCM harness connectors will damage the PCM terminals resulting in poor terminal to pin connection. Install Miller Special Tool # 8815 to perform diagnosis.

Measure the resistance of the (K1) MAP Signal circuit from the MAP Sensor harness connector to the appropriate terminal of special tool # 8815.
Is the resistance below 5.0 ohms?
Yes>> Go To 15
No>> Repair the excessive resistance in the (K1) MAP Signal circuit Perform (NGC) POWERTRAIN VERIFICATION TEST VER-5.

15. (K1) MAP SIGNAL CIRCUIT SHORTED TO GROUND

Measure the resistance between ground and the (K1) MAP Signal circuit in the MAP Sensor harness connector.
Is the resistance below 100 ohms?
Yes>> Repair the short to ground in the (K1) MAP Signal circuit. Perform (NGC) POWERTRAIN VERIFICATION TEST VER-5.
No>> Go To 16

16. EXCESSIVE RESISTANCE IN THE (K900) SENSOR GROUND CIRCUIT

CAUTION: Do not probe the PCM harness connectors. Probing the PCM harness connectors will damage the PCM terminals resulting in poor terminal to pin connection. Install Miller Special Tool # 8815 to perform diagnosis.

Measure the resistance of the (K900) Sensor ground circuit from the MAP Sensor harness connector to the appropriate terminal of special tool # 8815.
Is the resistance below 5.0 ohms?
Yes>> Go To 17
No>> Repair the excessive resistance in the (K900) Sensor ground circuit. Perform (NGC) POWERTRAIN VERIFICATION TEST VER-5.

17. PCM

Note: Before continuing, check the PCM harness connector terminals for corrosion, damage, or terminal push out. Repair as necessary.

Using the schematics as a guide, inspect the wire harness and connectors. Pay particular attention to all Power and Ground circuits.
Were there any problems found?
Yes>> Repair as necessary. Perform (NGC) POWERTRAIN VERIFICATION TEST VER-5.
No>> Replace and program the Powertrain Control Module per Service Information. Perform (NGC) POWERTRAIN VERIFICATION TEST VER-5.