ASE A1 Engine RepairGeneral Engine DiagnosisMedium
A technician is checking the exhaust gas recirculation (EGR) system. With the engine at operating temperature and idle, the EGR valve is commanded open using a scan tool. The engine begins to run very rough and may even stall. What does this indicate about the EGR system?
- AThe EGR valve is functioning correctly.
- BThe EGR passages are clogged.
- CThe EGR valve is stuck open.
- DThe EGR solenoid is faulty.
Show answer & explanationAnswer & explanation
Correct answer: A. The EGR valve is functioning correctly.
When the EGR valve is commanded open at idle, exhaust gases are introduced into the intake manifold, which is not desired at idle. A healthy engine will run rough or stall due to the dilution of the air/fuel mixture, indicating the EGR valve is functioning as expected.
Why the other options are wrong
- B. If the EGR passages were clogged, commanding the valve open would have little to no effect on engine idle, as exhaust gases would not reach the intake.
- C. If the EGR valve were stuck open, the engine would likely run rough or stall at idle without commanding it open.
- D. A faulty EGR solenoid would prevent the valve from opening when commanded, resulting in no change to engine idle.
EGR Valve Functional Test
Testing the EGR valve involves commanding it open. A healthy valve, when opened at idle, will cause the engine to stumble or stall due to exhaust gas entering the intake.
- Reduces NOx emissions
- Not active at idle or WOT
- Stumbling/stalling when opened at idle indicates proper function
Memory trick: Open EGR at idle, if it chokes, it's a good poke!