GMC 4000 Series Service Manual page 354

A IR IN JE C T IO N R E A C T O R ( A . I . R . ) T R O U B L E D I A G N O S IS
TROUBLE
PUM P N O ISY
Before trying to isolate the cause, it should be noted that the A .I.R . system is not completely noiseless. Under normal
conditions, noise rises in am plitude as engine speed increases. A ir pump noise can be confused with other engine noises.
Hose disconnected or leaking.
Belt loose.
Faulty relief valve (if mounted in pump).
A " chirping" noise may be prevalent on new pump.
A continuous "kn ockin g " noise is indicative of
bearing failure.
Im proper belt tension.
Seized or binding pump.
Incorrect or m issing pressure setting plug (if relief valve
is mounted on pump).
Bent or misaligned pulleys.
B A C K F IR E IN Rich Fuel M ixture Caused B y :
EX H A U S T S YS T EM
Inoperative choke— m isadjusted or sticking closed.
Inoperative vacuum break.
Use of manual ch o ke; generally overchoking.
A ir cleaner elem ent restricted.
Im proper crankcase vent m aintenance.
High fuel level.
Diverter valve stuck in open position.
Diverter valve and distributor tim ing vacuum
switched.
B A C K F IR E OR
Leaking inlet manifold.
PO PPIN G IN IN L E T
M AN IFO LD
Incorrect ignition timing.
G M C S E R V I C E M A N U A L
PROBABLE CAUSE
FUEL SYSTEM— GASOLINE ENGINES
REMEDY
Re-connect or replace.
Retighten.
Replace valve.
Allow break-in time.
rear Replace pump.
Readjust.
Replace.
Replace plug,
Inspect belt alignm ent— Replace pulleys.
Inspect choke operation— correct as necessary.
Replace vacuum break.
Closer control of choking period.
Replace element.
Inspect system . Replace PCV valve. Check fitting at
carburetor— may be plugged with crankcase deposits.
Check PCV filter. Replace if dirty.
A djust float level.
Check v a lve — replace if defective.
lines Correct hose routing,
Check manifold bolts for tightness.
Check tim ing and set to specs.
Sec. 6M
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