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newSchoolWorks.com › VoTechWorks › Transportation › Automotive Technology › Engine Systems: Theory, Diagnosis, and Overhaul › Lesson 11
TRN-AUTO19-02 · Lesson 11 · 22 clock hours · theory

Engine Emission Controls: PCV, EGR, and Evaporative Systems

Objective. Students will describe the function, components, and failure modes of the PCV, EGR, and evaporative emission control systems, perform a basic functional test on each, and correctly interpret a diagnostic trouble code related to each system using a scan tool.

⚠ Pending professional review

This lesson, its worked examples and its practice questions has not yet been verified by a certified examiner or master technician in the trade or by a certification or approval body. It is a study draft and may contain errors. Do not use it as instructions for work on a job site or on live equipment. Always follow your instructor, your supervising professional, and current official references and regulations.

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Why Emission Controls Matter

Internal combustion engines produce several categories of harmful emissions. Three onboard systems address the most significant sources: the Positive Crankcase Ventilation (PCV) system, the Exhaust Gas Recirculation (EGR) system, and the Evaporative Emission Control (EVAP) system. Understanding how each works, how each can fail, and how to test each is a core competency on the ASE A1 (Engine Repair) and A8 (Engine Performance) exams and in daily shop work.

Positive Crankcase Ventilation (PCV)

What it controls: Hydrocarbons (HC) from crankcase blow-by. Combustion gases leak past piston rings into the crankcase. Without ventilation those gases would build pressure, contaminate oil, and vent raw HC to the atmosphere.

How it works: Manifold vacuum draws crankcase vapors through the PCV valve into the intake manifold, where they are burned in the next combustion cycle. Fresh air enters the crankcase through a breather filter (usually on the valve cover or air cleaner). The PCV valve itself is a spring-loaded check valve that meters flow: it allows high flow at light throttle (high vacuum) and restricts flow at wide-open throttle to prevent oil pull-over.

Common failures:

Basic test: Remove the PCV valve and shake it — a good valve rattles. With the engine idling, place your thumb over the valve inlet; you should feel strong vacuum. A vacuum gauge at the PCV port should show manifold vacuum within the specification range listed in the service information.

Exhaust Gas Recirculation (EGR)

What it controls: Oxides of nitrogen (NOx). NOx forms when combustion temperatures exceed roughly 2,500 °F. EGR dilutes the air-fuel charge with inert exhaust gas, lowering peak temperatures and NOx output.

How it works: A metered amount of exhaust gas is routed back into the intake manifold. Older systems use a vacuum-operated EGR valve; most modern vehicles use an electronically controlled (stepper motor or solenoid) valve managed by the PCM. The PCM commands EGR only under moderate load at operating temperature — not at idle, not at wide-open throttle.

Common failures:

Basic test: With the engine at operating temperature and in gear (or at a fast idle), command the EGR valve open using a scan tool bi-directional control or apply vacuum with a hand vacuum pump to a vacuum-operated valve. A functional EGR system will cause a noticeable RPM drop (typically 100–200 RPM or more) because exhaust gas displaces combustible mixture. No RPM drop indicates no flow — suspect carbon plugging or a stuck valve.

Evaporative Emission Control (EVAP)

What it controls: Fuel vapor hydrocarbons (HC) that would otherwise evaporate from the fuel tank and fuel system into the atmosphere.

How it works: Fuel vapors are captured in a charcoal (activated carbon) canister. When the engine is warm and operating conditions are met, the PCM opens the purge valve (canister purge solenoid), and manifold vacuum draws stored vapors into the intake to be burned. A vent valve (canister vent solenoid) controls fresh air flow into the canister. The OBD-II system runs EVAP monitor tests: it seals the system and checks for pressure or vacuum decay to detect leaks as small as 0.020 inch in diameter (on enhanced EVAP systems).

Common failures:

Basic tests: Verify the fuel cap is properly installed and seals (pressure-test cap if a cap tester is available). Run the OBD EVAP monitor to completion on a scan tool and check readiness status. If a smoke machine is available, introduce smoke into the EVAP system through the service port and look for smoke escaping at the leak point. Always follow shop safety rules when using smoke machines around fuel systems.

Safety Notes

Connecting to DTCs

A scan tool is your first diagnostic step. Read and record all stored and pending codes before clearing anything. Cross-reference the code to the affected system, then perform functional tests to confirm the fault. For example, a P0401 tells you EGR flow is low — but the test drive and carbon inspection tell you why.

Key terms10

Blow-byCombustion gases that leak past piston rings into the crankcase; the source of HC emissions the PCV system captures.
PCV ValveA spring-loaded, one-way metering valve that controls the flow of crankcase vapors into the intake manifold under the influence of manifold vacuum.
Oxides of Nitrogen (NOx)A family of harmful air pollutants formed when peak combustion temperatures are very high; the primary emission the EGR system reduces.
EGR (Exhaust Gas Recirculation)A system that routes a metered portion of exhaust gas back into the intake manifold to dilute the charge, lower combustion temperature, and reduce NOx formation.
Charcoal CanisterAn activated-carbon canister in the EVAP system that stores fuel vapor from the tank until operating conditions allow the PCM to purge the vapors into the engine.
Purge Valve (Canister Purge Solenoid)A PCM-controlled solenoid that opens to allow manifold vacuum to draw stored fuel vapors from the charcoal canister into the intake manifold for combustion.
EVAP MonitorAn OBD-II self-test routine in which the PCM seals the evaporative emission system and checks for pressure or vacuum decay to detect fuel vapor leaks.
Diagnostic Trouble Code (DTC)A standardized alphanumeric code stored by the PCM when a monitored system or sensor operates outside its expected parameters; retrieved with a scan tool.
Bi-directional ControlA scan tool function that allows the technician to command a component (such as the EGR valve or purge solenoid) to activate, used to verify component operation during diagnosis.
Readiness MonitorAn OBD-II indicator showing whether each self-test (monitor) has run to completion since the last code clear; a scan tool displays ready or not-ready status for each monitor.
In class: Classroom lecture and diagram exercise on all three emission systems: students trace vapor flow on a system diagram, label components, and explain how each system reduces a specific pollutant (HC from crankcase blow-by, NOx from combustion, HC from fuel tank). · PCV and EVAP functional checks on a shop vehicle: students remove and inspect the PCV valve (shake test and visual), check for vacuum at the PCV port with a vacuum gauge, then perform a basic EVAP smoke test or OBD EVAP monitor run-to-complete and read results on a scan tool. · EGR system inspection: students locate the EGR valve (vacuum-operated or electronic), inspect passages for carbon buildup, command EGR open using a scan tool or vacuum pump, observe RPM drop as evidence of flow, and document whether the system is functional.
Materials: Shop vehicle with OBD-II port, scan tool, vacuum gauge, vacuum pump and gauge kit, EVAP smoke machine (if available) or OBD EVAP monitor procedure, PCV valve inspection checklist, system diagrams, ASE A1 task list (engine performance emission section)
Check: Short written quiz on system function and failure modes (10 questions, 80% passing); plus instructor sign-off on scan tool DTC interpretation exercise confirming student correctly identified the system indicated by an assigned code.

Practice check8

Answer each question, then check it. Nothing is recorded; this is your own practice.

ASE A8 Engine Performance — Emissions
1. What is the primary pollutant reduced by the PCV system?
ASE A8 Engine Performance — Emissions
2. A technician removes the PCV valve and shakes it. The valve does not rattle. What does this indicate?
ASE A8 Engine Performance — Emissions
3. During an EGR functional test at operating temperature, the technician commands the EGR valve fully open using the scan tool's bi-directional control. Engine RPM does not change. The MOST likely cause is:
ASE A8 Engine Performance — Emissions
4. Code P0401 is retrieved from a vehicle with a no-idle complaint. Which statement BEST describes the relationship between this code and the symptom?
ASE A8 Engine Performance — Emissions
5. A vehicle sets a P0456 code (EVAP system small leak detected). The FIRST step the technician should take is:
ASE A8 Engine Performance — Emissions
6. The EVAP monitor status on the scan tool shows 'Not Ready' after the technician cleared all codes. What does this mean?
ASE A8 Engine Performance — Safety
7. Which PPE and safety precaution is MOST important when performing an EVAP smoke test on a shop vehicle?
ASE A8 Engine Performance — Emissions
8. A technician places a thumb over the inlet of a PCV valve with the engine idling and feels no vacuum pull. After confirming the hose and port are clear, the MOST likely conclusion is:
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