3.3 T1 Emissions Control Systems

3.3 T1 Emissions Control Systems

Quick Answer: ASE T1 Area G is about 5 scored questions (~10%). Focus on PCV, EGR, secondary air injection (when equipped), catalytic converter diagnosis, and EVAP canister/lines/valves. Emissions components both control pollutants and create drivability faults when they fail stuck open, stuck closed, or electrically open/shorted.

Emissions Gear on Medium/Heavy Gasoline Platforms

Even large gasoline engines must meet OBD II emissions requirements when certified for on-highway use. Vocational trucks may idle for hours, run PTOs, and see short-trip duty that stresses EVAP and catalyst monitors. T1 expects you to know what each subsystem does, how the PCM monitors it, and how failures feel in the cab (idle quality, surge, fuel smell, ping, MIL).

Positive Crankcase Ventilation (PCV)

Purpose

PCV meters crankcase blow-by gases back into the intake to be burned, preventing pressure that would otherwise push oil past seals and vent raw hydrocarbons to atmosphere.

Failure modes and clues

On large V8 gasoline truck engines, PCV may integrate into valve-cover baffles or use electronic valves. Always use the correct OEM-calibrated valve—universal “fits-all” flow rates upset idle strategy. After catastrophic engine wear (high blow-by), replacing PCV alone will not stop oil consumption; quantify blow-by if pressure returns immediately.

Exhaust Gas Recirculation (EGR)

Purpose

EGR dilutes the intake charge with inert exhaust to lower peak combustion temperatures and reduce NOx. Gasoline systems may use vacuum-controlled pintle valves (legacy) or electric EGR valves with position feedback (common on later OBD II engines).

Scan data and DTCs

Critical PIDs: EGR commanded position, actual position, differential pressure (DPFE-style where used), and related temperature sensors. Codes cover circuit faults, insufficient flow, and excessive flow.

Component diagnosis

1. Stuck open → rough idle, stall in drive, misfire at idle that cleans up when EGR is forced closed with a scan tool.
2. Stuck closed / passages carboned → detonation/ping under load, elevated combustion temps, failed EGR monitor, possible power loss if knock retard engages heavily.
3. Control circuit → voltage supply, ground, and signal rationality; do not replace a valve that never receives a command because of a blown fuse or open harness at the frame rail.

Carbon service: remove the valve and clean passages with approved methods; avoid blasting debris into the intake. On engines with EGR coolers (more diesel-centric, but know the concept), leaks can introduce coolant—rare on simple gasoline pintle systems but worth noting if mixed-fleet questions appear.

Secondary Air Injection (AIR)

Where equipped, an air pump (electric or belt-driven) injects air into the exhaust manifolds or catalyst inlet during cold start/open-loop to oxidize HC/CO and help the catalyst light off quickly. One-way check valves stop hot exhaust from back-flowing into the pump.

Diagnosis pattern:

• Cold-start pump command present? Listen/feel for pump; check current and voltage.
• Check valves melted or stuck → exhaust in the pump, noise, or efficiency codes.
• Vacuum/electrical diverter valves misrouting air.
• DTCs for air system incorrect flow often set only when enable criteria (cold ECT, specific RPM/load) are met—road-test accordingly.

If a pump is seized from exhaust backflow, replace the check valves with the pump or the new pump fails again—classic ASE trap.

Catalytic Converter Diagnosis (Emissions View)

Converters oxidize HC/CO and reduce NOx (three-way catalysts on stoichiometric gasoline engines). OBD II monitors efficiency by comparing upstream vs. downstream O2 activity: a good cat stores oxygen and damps downstream switching; a failed cat lets downstream look like upstream.

Diagnostic cautions:

• P0420/P0430 are efficiency codes, not automatic “new cat” tickets—fix for exhaust leaks, lazy O2 sensors, and rich/misfire root causes first.
• A restricted cat is a performance fault (see Area F tests) and may or may not set efficiency codes early.
• After repairs that dump raw fuel (misfire, leaking injectors), catalysts can be damaged; confirm with temperature, backpressure, and monitor results.

Evaporative Emission Control (EVAP)

Architecture

Fuel tank vapors route to a charcoal canister. The PCM purges vapors into the intake when conditions allow and runs sealing tests using:

• Purge solenoid (normally closed; PWM open during purge)
• Vent solenoid (normally open; closed during leak test)
• Fuel tank pressure (FTP) sensor
• Hoses, grommets, filler neck, and gas cap

Diagnostic approach

1. Verify cap sealing and OEM-spec cap.
2. Check purge/vent solenoid resistance, power, ground, and commanded operation (bi-directional controls).
3. Smoke-test the system at recommended low pressure; find cracked plastic elbows on top of tanks, rotten rubber at canisters mounted on frame rails, and rusted lines on vocational trucks.
4. Interpret FTP: no vacuum build during OEM test may mean vent stuck open, purge path issues, or sensor fault—use service flowcharts.
5. Do not seal a tank and apply high shop air pressure—you can rupture components and create unsafe conditions.

Fuel smell without MIL can still be a liquid leak (not vapor). EVAP is vapor; do not confuse wet fuel leaks at sender O-rings with canister saturation.

Interplay With Fuel Trims and Monitors

Emissions parts change fuel control:

• Open PCV or purge valve stuck open → unmetered fuel vapors or air → idle/trim chaos.
• EGR open at idle → misfire that looks ignition-related.
• Failed cat after rich operation → efficiency codes that return until the fuel fault is fixed.

Always check monitor readiness after emissions repairs. A truck may drive fine with a cleared MIL but fail inspection if EVAP or catalyst monitors are incomplete. Complete the OEM drive cycle with the correct fuel level window for EVAP (many systems need a mid-tank level to run).

Safety and Environmental Notes

• Capture fuel vapor and liquid properly; no open-bucket draining near ignition sources.
• Support tanks and straps correctly—vocational tanks are heavy.
• After secondary air or cat work, verify no exhaust leaks into cab or sleeper (CO hazard).
• Use only emissions-legal replacement parts; defeat devices are illegal and outside ASE-correct practice.

Exam Strategy for Area G

Map symptoms to stuck-open vs. stuck-closed logic. Rough idle that improves when EGR is commanded closed is classic stuck-open EGR. Ping under load with EGR flow codes is stuck-closed or plugged passages. Fuel smell + EVAP large-leak code after fueling often starts at the cap or filler neck, not a new PCM. Secondary air codes after pump replacement without check valves mean you fixed the victim, not the cause. Catalyst efficiency codes demand upstream fault elimination before parts cannon.