What does it mean?
Here the module has found that the A fuel injection system is delivering more fuel than the calculation says it should need. The software tracks how much the fuel quantity or rail pressure correction has to be reduced to keep the commanded value, and when that correction runs into its limit in the subtracting direction the fault is stored.
Extra delivery normally means fuel is escaping somewhere inside the high-pressure circuit, so the pump has to move more than the engine actually burns, or that the rail pressure sensor is under-reporting and the system compensates by pumping harder. It can also be a metering valve that no longer controls the flow it is asked to control. Because unburnt or excess fuel can dilute the engine oil and damage the aftertreatment system, the fault should not be left unattended.
Symptoms
- Black or dark smoke under acceleration and a strong fuel smell
- Rough running and uneven idle
- Higher fuel consumption than usual with no change in driving style
- Engine oil level rising and smelling of fuel when leakage is severe
- MIL on, sometimes with rail pressure or particulate filter codes
- Reduced power once the module starts protecting the system
Likely causes
- Injector with excessive internal return leakage, so much of the delivered fuel never reaches the cylinder
- Leaking pressure limiter or pressure relief valve on the rail returning fuel to the tank
- Rail pressure sensor reading low, making the system deliver more than it needs
- Faulty fuel metering or volume control valve that opens further than commanded
- External high-pressure leak at a union, a pipe or an injector seal
- Leaking pressure control valve on the pump
- Worn injector nozzles that pass more fuel than their calibration assumes
- Incorrect injector calibration data programmed into the module
How to fix it
- Inspect the high-pressure circuit for external leaks at unions, pipes and injector seats, with the engine running and safely observed
- Perform an injector leak-off test and compare the return volume of each injector
- Check actual against desired rail pressure and verify the pressure sensor with a second data source or by comparing key-on values
- Monitor the metering valve duty cycle; an abnormally low duty with normal pressure suggests internal leakage elsewhere
- Check the pressure limiter and relief valve for excessive return flow
- Verify the injector calibration codes match the fitted injectors
- Check the engine oil for fuel dilution, which changes the urgency of the repair
- Replace the leaking component the tests identify, then confirm the correction values return to normal