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VW GDI Fuel In-Tank Pump Duty Cycle Testing

Using DrivePRO and PicoScope

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 Drive

Select 01 Engine Electronics

Select Dynamic Requests

Select Measuring Groups

Enter 106 Then, Select OK

Fuel Rail Pressure will be displayed

Fuel Rail Pressure is DME desired fuel rail pressure

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 34.5 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.

Attach the negative lead of your scope to battery negative.

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. 

select channel A and DUTY CYCLE under the SELECT THE TYPE OF MEASUREMENT tab.

You can choose to measure between 2 cursors or the whole trace. In this case we selected between (rulers) cursors.

In the blue highlighted area the measured duty cycle is displayed.

The duty cycle measured using Pico Scope confirms the faulty duty cycle reading displayed using DrivePRO diagnostic software.

This zoomed view is a known good duty cycle measurement. The blue highlighted area indicates the measured duty cycle.