Pontiac Fiero 1988 Service Manual page 558

DRIVEABILITY A N D E M I S S I O N S - F U E L INJECTION (TBI) 6 E 2 - C 7 - 1
SECTION C7
EXHAUST GAS RECIRCULATION (EGR) SYSTEM
CO NTENTS
C7-1
RESULTS OF INCORRECT OPERATION
C7-2
PURPOSE C7-1
C7-2
OPERATION
C7-1
ON-CAR SERVICE C7-2
NEGATIVE BACKPRESSURE VALVE
C7-1
EGR VALVE R/R
C7-2
EGR VALVE IDENTIFICATION C7-1
EGR M a n i f o l d Passage
C7-2
PARTS INFORMATION . . . . .
C7-3
GENERAL DESCRIPTION
PURPOSE
The EGR system is used to lower NOx (oxides of
nitrogen) emission levels, caused by high combustion
t e m p e r a t u r e s , by d e c r e a s i n g the c o m b u s t i o n
temperature.
The main element of the system is an EGR valve
operated by vacuum, and mounted on the intake
manifold.
The EGR valve feeds small amounts of exhaust
gas back into the combustion chamber as shown in
Figure C7-1.
O P E R A T I O N
The EGR valve is opened by ported manifold
vacuum to let exhaust gas flow into the intake
manifold. The exhaust gas then moves with the
air/fuel mixture into the combustion chamber. If too
much exhaust gas enters, combustion will not occur
For this reason, very little exhaust gas is allowed to
pass through the valve, and none at idle. The EGR
valve is usually open under the engine is above idle
speed.
[ T ]
EGR VALVE
[ T ]
INTAKE AIR
I D EXHAUST GAS
[ T )
EGR VACUUM PORT
4S 0001-6E
5 j D I A P H R A G M 4 S 0113-6E
Figure C7-2 - Negative Backpressure EGR Valve
NEGATIVE BACKPRESSURE VALVE
The valve used on this engine is a negative
backpressure valve. It varies the amount of exhaust
gas flow into the manifold depending on manifold
vacuum and variations in exhaust backpressure.
The diaphragm on this valve (shown in Figure
C7-2) has an internal vacuum bleed hole which is held
closed by a small spring when there is no exhaust
backpressure.
Engine vacuum opens the EGR valve against the
pressure of a large spring. When manifold vacuum
combines with negative exhaust backpressure, the
vacuum bleed hole opens and the EGR valve closes.
Figure C7-1-Exhaust Gas Recirculation