Case Studies 07

Vehicle Details

• Make / Model: Volkswagen Polo

• Engine: 1.2 TDI, 3-cylinder

• Engine Code: CFWA

• Fuel System: Bosch Common Rail

• ECU: Bosch EDC17

Customer Complaint

• Hard starting (intermittent)

• Rough idle / slight misfire

• No permanent injector DTC stored

• Engine otherwise runs, but refinement not satisfactory

Objective

To verify:

1. Electrical health of injectors

2. ECU control signals

3. Injector actuation consistency

4. Eliminate injectors as a root cause before deeper mechanical work

Injector Type – Technical Background

• Solenoid-type common rail injectors

• Two-wire injector (no polarity)

• ECU-controlled low-side switching

• Operating voltage: up to ~80 V peak (inductive kick)

• Injection events:

  • Pilot injection

  • Main injection

  • Post injection (if applicable)

Step 1: Visual & Preliminary Checks

Performed:

• Checked injector connectors for:

  • Oil ingress

  • Bent pins

  • Loose fit

• Checked engine harness near injectors for chafing

Result:

 No visible wiring damage

Step 2: Injector Resistance Test (Static)

Method:

• Ignition OFF

• Injector connector unplugged

• Digital multimeter in ohms mode

• Measured across injector terminals

Expected Value (CFWA):

• 0.4 – 0.6 Ω at ~20°C

Observed:

• All three injectors within tolerance

Conclusion:

 Injector coils electrically intact
 This test alone does NOT confirm injector operation

Step 3: Injector Pulse Verification (Without Oscilloscope)

Why:

A scanner may not log injector DTCs even if:

• Pulse is missing

• Pulse is distorted

• One injector is electrically silent

Tool Used:

• DIY LED Noid Light Probe

  • 5 mm LED

  • Series resistor (~1 kΩ)

  • Connected across injector pins

Method:

• Injector connector unplugged

• LED probe connected

• Engine cranked / idling

Expected Behaviour:

• LED flashes rapidly

• Equal brightness on all cylinders

• No missing pulses

Observed:

• All three injectors showed pulse

• No dead injector found

Conclusion:

 ECU is commanding all injectors

Step 4: Injector Pulse Analysis (Advanced – Oscilloscope)

(Optional but recommended)

Scope Settings:

• Voltage scale: 20–50 V

• Time base: 0.5–1 ms/div

• Probe across injector terminals

Expected Pattern:

• Sharp voltage rise (inductive)

• Short pilot pulse

• Longer main pulse

• Clean decay spike

Observed:

• Similar waveform shape on all injectors

• No shorted or open injector detected

Step 5: Cylinder Balance / Smooth Running Test

Tool:

• Launch X431 live data

PIDs Observed:

• Injection quantity correction (mg/stroke)

• Engine speed fluctuation

• Idle stability

Expected:

• Corrections within ±2.0 mg/stroke

• No single cylinder compensating excessively

Observed:

• Corrections within normal range

Conclusion:

 No injector over-fueling or under-fueling

Step 6: Leak-Off (Return Flow) Test

Purpose:

To detect internal injector leakage affecting:

• Rail pressure

• Hot starting

• Idle stability

Method:

• Common rail leak-off kit

• Engine idling for fixed time

Expected:

• Similar fuel quantity in all measuring tubes

Observed:

• Balanced return flow

Conclusion:

 Injectors hydraulically healthy

Final Conclusion – Injector System

 Injectors ruled out as root cause

Diagnostic Insight (Important Lesson)

Despite symptoms resembling injector failure:

• No injector replacement was required

• Root cause was turbo actuator position sensor (G581) mis-calibration

• Incorrect boost control caused:

  • Combustion instability

  • Apparent misfire symptoms

  • Rough idle perception

Key Takeaways for Technicians

• Injector problems are often suspected first, but not always guilty

• Always:

  • Verify ECU command

  • Verify injector response

  • Correlate injector data with boost & air system

• Do not replace injectors based on symptoms alone

Author’s Note

This case demonstrates the importance of:

• Structured diagnosis

• Electrical + mechanical reasoning

• Avoiding premature component replacement