In this article & video, how to test and diagnose a pressure low pressure (in-tank) fuel delivery fault on VW GDI engines. The specs for low side fuel rail pressure are shown as well as the correct duty cycle control of the in-tank fuel pump.
Begin by checking in-tank fuel pressure at the engine
Connect an inline gauge and start the engine and allow it to idle The gauge should fluctuate between 60 – 80 PSI, averaging 70 PSI – This is a good specification.
When Fault is Present
Most times on in-tank pressure delivery faults, system pressure will be maxed and the gauge will read around 100 PSI. This indicates an issue with the duty cycle control of the in-tank fuel pump.
Identify the vehicle you are working on
Select 01 Engine Electronics
Select Dynamic Requests
Select Measuring Groups
Enter 106 Then, Select OK
Fuel Rail Pressure will be displayed
Electrical Fuel Pump 1 is actual fuel rail pressure
Electrical Fuel Pump 2 is in-tank fuel pump duty cycle
With the vehicle in Park and the engine at idle, fuel rail pressure should be about the same as Electrical Fuel Pump 1 value.
Electrical Fuel Pump 2 Value will display in-tank fuel pump duty cycle. At idle about 60% is normal.
If a fault is detected in the system, Electrical Fuel Pump 1 will not match Fuel Rail Pressure.
Our subject vehicle has a difference of 33.5 bar.
Electrical Fuel Pump 2 is at almost 80%, representing a maximum duty cycle for the in-tank fuel pump.
Testing Using Pico Scope
To test and confirm the duty cycle displayed is the actual duty cycle, connect a lab scope to the positive control circuit at the fuel pump.
Backprobe the positive control circuit at the fuel pump.
Set scope to + or - 20v DC and 10 ms per division
Start and idle the engine.
Measuring Duty Cycle Using Pico Scope
PicoScope can measure duty cycle automatically by selecting ADD MEASUREMENT from the MEASUREMENTS tab.