Technical Insight: Why EGR Flow Problems Can Mimic Diesel Injector Combustion Faults

Page Type: Technical Insight

Rough running, smoke and uneven combustion are often associated with diesel injector problems.

However, abnormal exhaust gas recirculation can change the amount of fresh air available for combustion and produce similar symptoms.

EGR Changes Intake Composition

The EGR system introduces controlled exhaust gas into the intake stream.

If actual EGR flow differs substantially from commanded flow, cylinder oxygen availability can change.

Similar Symptoms, Different Cause

Possible symptoms may include:

• unstable idle;
• smoke;
• poor response;
• combustion roughness.

These overlap with several injection faults.

Compare Air and Fuel Evidence

Diagnosis may include:

• EGR command;
• EGR position;
• airflow data;
• intake pressure;
• cylinder contribution;
• injector correction.

The objective is to determine whether the abnormal combustion begins with air control or fuel delivery.

Avoid Fuel-System Tunnel Vision

Replacing an injector does not correct an EGR valve that remains open or otherwise produces incorrect flow.

For a corresponding example, see EGR-versus-Injector Case Analysis: Why Rough Idle Does Not Automatically Prove Uneven Injector Delivery.

FAQ

Can EGR problems cause rough idle?

Yes.

Can they create smoke?

Potentially.

Should injector data still be checked?

Yes, but together with air-path information.