P1681

What Does Code P1681 Mean?

DTC P1681 signifies a detected malfunction within the Metering Oil Pump (MOP) system. This code is typically set by the Engine Control Module (ECM) or Powertrain Control Module (PCM) when it identifies an issue with the MOP’s electrical circuit, mechanical operation, or its ability to deliver the specified amount of oil. The MOP is a critical component in certain engine designs, most notably rotary engines (e.g., Mazda RX-8) and some specialized diesel applications, where it precisely injects oil for internal lubrication or into the combustion chamber to aid in sealing and cooling. The ECM/PCM monitors the MOP’s operation through various means, which may include monitoring the pump’s motor current, solenoid activation, position sensor feedback (for variable displacement pumps), or dedicated flow sensors. When the ECM/PCM detects a discrepancy—such as an open or short circuit in the MOP’s control wiring, a motor drawing excessive or insufficient current, or a failure to achieve a commanded oil delivery rate—it interprets this as a malfunction and illuminates the Malfunction Indicator Lamp (MIL), setting code P1681.

Common Symptoms

• Check Engine Light (MIL) Illumination: This is the most direct and common symptom.
• Rough Idle or Poor Engine Performance: Incorrect oil metering can lead to improper combustion or lubrication, affecting engine smoothness and power delivery.
• Reduced Engine Power: The ECM/PCM may enter a “limp mode” to protect the engine if it detects a critical lubrication issue, significantly reducing available power.
• Increased Exhaust Smoke: If the MOP is over-injecting oil, particularly in rotary engines, unburnt oil can lead to noticeable white or blue smoke from the exhaust.
• Higher Oil Consumption: An MOP malfunction could result in either over-injection (leading to consumption) or under-injection (leading to wear and tear that ultimately causes consumption).
• Engine Knocking or Unusual Noises: Insufficient oil delivery can lead to increased friction and wear on internal engine components, producing abnormal mechanical noises.
• Catalytic Converter Damage: Prolonged excessive oil injection can foul the catalytic converter, leading to reduced efficiency and potential failure.

What Causes the Code P1681?

• Faulty Metering Oil Pump (MOP): This is the most common cause, including internal mechanical failure (worn gears, seized pistons), electrical motor failure, or a failed control solenoid within the pump assembly.
• Wiring Harness Issues: Open circuits, short circuits, or high resistance in the MOP’s power supply, ground, or control signal wires. Damage can be caused by chafing, heat, or rodent activity.
• Corroded Electrical Connectors: Oxidation or damage to the pins within the MOP connector or its associated control module connector can lead to intermittent or complete loss of electrical connection.
• Faulty MOP Control Module: In some systems, the MOP has a dedicated control module separate from the ECM/PCM. A failure in this module can prevent proper pump operation.
• Clogged Oil Passages or Lines: Blockages in the oil supply lines to the MOP or the injection lines from the MOP to the engine can restrict oil flow, leading the ECM/PCM to detect an inability to meet demand.
• Low Oil Level or Incorrect Oil Type: While not a direct MOP component fault, extremely low engine oil levels or the use of an oil type not recommended for the MOP system can impair its function.
• ECM/PCM Failure: Though rare, a faulty ECM/PCM that cannot properly command the MOP or incorrectly interprets its feedback signals can set this code.

How to Diagnose and Troubleshoot

Diagnosis of P1681 requires a methodical approach, often involving a digital multimeter (DMM), an OBD-II scan tool with advanced functions, and thorough visual inspection.

1. Verify DTC and Gather Freeze Frame Data: Connect an OBD-II scanner and confirm P1681 is present. Crucially, review any associated freeze frame data. This data provides snapshots of engine conditions (RPM, engine load, coolant temperature, vehicle speed) at the moment the fault was detected, which can offer valuable clues.
2. Visual Inspection of MOP and Wiring:
   • Locate the MOP (often on the side of the engine block near the oil pan or eccentric shaft housing for rotary engines).
   • Inspect the wiring harness leading to the MOP for any signs of physical damage, chafing, cuts, or melting. Pay close attention to areas where the harness might rub against engine components.
   • Check the MOP’s electrical connector for corrosion, bent pins, or signs of looseness. Ensure it is securely latched.
   • Examine any oil lines connected to the MOP for leaks, kinks, or blockages. For rotary engines, ensure the plastic lines leading to the eccentric shaft ports are intact and clear.
   • Check the engine oil level and condition. For rotary engines with a separate MOP oil reservoir, check that level as well.
3. Electrical Circuit Testing with DMM:
   • Power Supply: With the ignition ON and the MOP connector disconnected, use a DMM to check for battery voltage at the MOP’s power supply pin(s) as per the wiring diagram. If no voltage is present, trace the circuit back through fuses and relays.
   • Ground Circuit: Check for a good ground connection at the MOP’s ground pin(s) by measuring resistance between the pin and a known good chassis ground. Resistance should be less than 0.5 ohms. Higher resistance indicates a poor ground.
   • Control/Signal Wires (Backprobing): Reconnect the MOP. Using a thin probe to backprobe the control signal wires at the MOP connector (or the ECM/PCM connector, if necessary), monitor voltage or duty cycle with the engine running or during an actuator test (if applicable with your scanner). Compare readings to manufacturer specifications. For a PWM (Pulse Width Modulation) controlled pump, you should see a varying duty cycle.
   • Continuity Test: Disconnect both the MOP and the ECM/PCM connectors (or MOP control module). Using the DMM, check for continuity in each wire between the MOP connector and its respective pin at the ECM/PCM connector. Also, check for shorts to ground or power in each wire.
4. MOP Actuator Test (Bidirectional Scan Tool): If available, use a scan tool with bidirectional control capabilities to command the MOP to operate. Listen for the pump working and observe any feedback parameters on the scan tool (e.g., commanded vs. actual oil pressure/flow, motor current).
5. MOP Internal Resistance Test: With the MOP connector disconnected, measure the resistance across the MOP’s terminals (motor windings or solenoid). Compare this reading to the manufacturer’s specifications. An open circuit (infinite resistance) or a short circuit (near zero resistance) indicates an internal MOP failure.

Recommended Repairs and Solutions

Once the diagnostic steps have identified the root cause of P1681, the following repairs are typically recommended:

• Repair or Replace Wiring Harness/Connectors: If the visual inspection or DMM tests reveal damaged wiring, corroded pins, or loose connections, repair or replace the affected section of the wiring harness or the connector itself. Use heat-shrink solder connectors for durable repairs.
• Replace Faulty Metering Oil Pump (MOP): If all electrical circuits leading to the MOP test good (power, ground, control signals) and internal MOP resistance tests indicate a fault, the pump itself is likely the culprit. This is a common failure, particularly in high-mileage vehicles where the MOP is frequently active.
  • Mechanic’s Tip: When replacing the MOP, it is crucial to follow manufacturer-specific procedures, especially regarding priming the pump and lines to ensure no air pockets are present. For rotary engines, it’s often wise to inspect and potentially replace the small plastic oil injection lines if they appear brittle or damaged, as they can also lead to issues.
• Replace MOP Control Module: If the diagnostic process points to a separate MOP control module failing to issue commands or process feedback correctly, replacement of this module will be necessary. Verify all inputs to the module before condemning it.
• Clear Clogged Oil Passages: If diagnostic steps (e.g., visual inspection, oil flow tests) suggest restricted oil lines or injection nozzles, these components must be cleaned or replaced to restore proper oil flow.
• ECM/PCM Replacement or Reprogramming: Only consider ECM/PCM replacement as a last resort, after meticulously ruling out all other potential causes. If the ECM/PCM is deemed faulty, it will require replacement and often dealer-level programming/coding to the vehicle.
• Address Low Oil Levels: If low oil level was a contributing factor, ensure the engine oil is at the correct level and the appropriate type is used for the vehicle’s specifications.