TOYOTA PREVIA 91-97 REPAIR MANUAL

DRIVEABILITY AND EMISSIONS CONTROLS 4-5

EMISSION CONTROLS

I See Figure 1

Due to varying state, federal, and provincial regulations, specific emis-
sion control equipment may vary by area of sale. The US emission equip-
ment is divided into two categories: California and 49 State (or Federal). In
this section, the term "California" applies only to cars originally built to be
sold in California. Some California emissions equipment is not shared with
equipment installed on cars built to be sold in the other 49 states. Models
built to be sold in Canada also have specific emissions equipment,
although in many cases the 49 State and Canadian equipment is the same.

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CATALYST

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20010 FED &

Fig. 1 Example of an emission control system information label

A PCV valve is used in the line to prevent the gases in the crankcase

from being ignited in case of a backfire. The amount of blow-by gasses

entering the mixture is also regulated by the PCV valve, which is spring

loaded and has a variable orifice.

The important components of the PCV system are the following'

• PCV valve


• Valve cover


• Air intake chamber


• Ventilation case


• Hoses, connections and gaskets

Fig. 3 PCV system flow

Positive Crankcase Ventilation (PCV) System

OPERATION

» See Figures 2 and 3

Exhaust blow-by gasses are routed from the crankcase to the intake mani-
fold, where they are combined with the fuel/air mixture and burned during
combustion. This reduces the amount of hydrocarbons emitted by the exhaust.

••- PCV VALVE IS CLOSED.

<• PCV VALVE IS OPEN.

> VACUUM PASSAGE IS

LARGE.

. PCV VALVE IS OPEN.
-. VACUUM PASSAGE IS SMALL

' PCV VALVE IS FULLY

OPEN.

Fig. 2 The PCV valve functions differently according to the

engine operating conditions

TESTING

» See Figures 4, 5 and 6

Inspect the PCV system hoses and connections at each tune-up and

replace any deteriorated hoses. Check the PCV valve at every tune-up and

replace it at 30,000 mile (48,000 km) intervals.

The PCV valve is easily checked with the engine running at normal idle

speed (warmed up).

Cylinder Head Side

Clean Hose

Fig. 4 Blow air from the cylinder head side of the PCV valve, it

should flow easily