Ford VSG-411 Service Manual page 71

03A-7
Emission Control Svstem 03A-7
DIAGNOSIS AND TESTING
Symptoms of EGR System Malfunctions
Many problems of poor engine performance are incorrectly
diagnosed as being related to the EGR valve or EGR
system. Therefore, it is important to know what engine
symptoms
can
be EGR-related and how to test the
system.
CAUTION: Improper use of
leaded
gasoline can plug
up
an EGR valve with deposits that restrict flow.
Poor Fuel Economy
This is an EGR condition only if it is related to detonation or
some other symptom of restricted or zero EGR flow.
Rough Idle
This can be caused by an EGR valve stuck
open,
by dirt
on the valve seat, or by loose mounting bolts. Loose
mounting will cause a vacuum leak and a hissing noise. A
stuck valve can be diagnosed by a functional test and by
visual inspection.
Surge, Stall, or Won' t Start
Can be caused by the valve stuck
open.
Detonation (Spark Knock)
Any condition that prevents poor EGR gas flow can cause
detonation. This includes a valve stuck closed, leaking
valve diaphragm, restrictions in flow passages, EGR
disconnected, or a problem in the vacuum source. On
engines with high spark advance, detonation is serious
enough to destroy an engine. Diagnose the condition by
performing the tests in this section.
EGR System Gas Flow Test
The exhaust gas flow test must be made with the engine
idling and hot. It tells you if the gas flow passages are
open.
1. Remove the vacuum line from the valve diaphragm.
Plug the line to prevent a vacuum leak or dirt entry.
2. Attach a hand vacuum pump to the valve diaphragm.
3.
Apply 15 inches of vacuum to the valve, a little at a
time.
If the gas flow is okay, the engine will begin to idle roughly
at some point, or it may stall. If the idle doesn' t change,
there is a restricted passage in the valve or the spacer.
Remove the valve for inspection. If the valve diaphragm
fails to hold vacuum, install a new EGR valve.
HAND VACUUM
PUMP
Diaphragm Leak Test
The diaphragm leak test can be performed with the engine
Off.
1. Attach a hand vacuum pump to the vacuum nipple of
the EGR valve.
2. Apply eight inches of vacuum and trap (hold) it.
The diaphragm must hold 7-8 inches of vacuum for at least
30 seconds. If it holds vacuum, the diaphragm is okay.
Proceed to the Source Vacuum test. If the diaphragm won' t
hold vacuum, replace the EGR valve.
VACl
PI
APPLY 8 INCHES
VACUUM AND TRAP
SECOND HAND