Safety Warning
DIY auto repair can cause serious injury, fire, or vehicle damage. These guides are for informational purposes only. Always follow OEM torque specs, wear PPE, and consult a certified mechanic if you are unsure. You are solely responsible for your safety.
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HOMESERVICE MANUALSVOLKSWAGEN1991JETTA L4-1984CC 2.0L DOHCREPAIR AND DIAGNOSISPOWERTRAIN MANAGEMENTEMISSION CONTROL SYSTEMSDESCRIPTION AND OPERATIONSYSTEM OVERVIEW
1991 Volkswagen Jetta L4-1984cc 2.0L DOHC
System Overview
1991 Volkswagen Jetta L4-1984cc 2.0L DOHCSECTION System Overview
The emission control system is used to reduce harmful gases (CO, HC, and NOx) in accordance with federal and state regulations.
The emission control system has the following four sub systems:
1. CRANKCASE EMISSION CONTROL SYSTEM.
The crankcase emission control system prevents blow-by gases from escaping into the atmosphere. The blow-by gases in the crankcase are routed, through various components, back into the intake manifold for combustion.
2. COMPUTERIZED ENGINE CONTROL SYSTEM.
The computerized engine control system is made up of an electronic control unit (ECU), various sensors and output devices. This system controls engine operation (i.e. A/F mixture, idle speed and ignition timing) to reduce pollutants while maintaining optimum driveability conditions and reducing fuel consumption.
3. EVAPORATIVE EMISSION CONTROL SYSTEM.
The evaporative control system prevents the escape of fuel vapors from the fuel tank and fuel system into the atmosphere. It consists of various components, which collect and direct the fuel vapors to the intake manifold for combustion.
4. EXHAUST EMISSION CONTROL SYSTEM.
The exhaust emission control system can (depending on model and engine) consist of three-way catalytic converters and an exhaust gas recirculation valve to reduce exhaust emission levels.
The emission control system has the following four sub systems:
1. CRANKCASE EMISSION CONTROL SYSTEM.
The crankcase emission control system prevents blow-by gases from escaping into the atmosphere. The blow-by gases in the crankcase are routed, through various components, back into the intake manifold for combustion.
2. COMPUTERIZED ENGINE CONTROL SYSTEM.
The computerized engine control system is made up of an electronic control unit (ECU), various sensors and output devices. This system controls engine operation (i.e. A/F mixture, idle speed and ignition timing) to reduce pollutants while maintaining optimum driveability conditions and reducing fuel consumption.
3. EVAPORATIVE EMISSION CONTROL SYSTEM.
The evaporative control system prevents the escape of fuel vapors from the fuel tank and fuel system into the atmosphere. It consists of various components, which collect and direct the fuel vapors to the intake manifold for combustion.
4. EXHAUST EMISSION CONTROL SYSTEM.
The exhaust emission control system can (depending on model and engine) consist of three-way catalytic converters and an exhaust gas recirculation valve to reduce exhaust emission levels.
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When to See a Mechanic
Stop DIY work and contact a certified mechanic immediately if any of the following apply:
- • You smell fuel, burning insulation, or see smoke.
- • Brakes feel soft, pull hard to one side, or make grinding noises.
- • The engine overheats, stalls repeatedly, or misfires under load.
- • You are missing required tools, torque specs, or safe lifting equipment.
- • You are not confident in the next step or safety outcome.