Chevrolet 70 Series 1970, Service Manual

Page: 561 / 841

ENGINE ELECTRICAL 6Y-20
W orn and d irty plugs may give s a tisfa c to ry
o p eratio n at idling speed, but under o p erating con­
ditions they frequ ently fa il. Faulty plugs a r e evi­
dent in a num ber of w ays, such a s w asting gas,
pow er lo s s , lo ss of speed, h ard sta rtin g , and gen­
e r a l po o r engine p e rfo rm an c e.
S park plug fa ilu re , in addition to n orm al w ear,
may be due to dirty o r leadedplugs, ex c essiv e gap,
o r broken in su lato r.
D irty o r leaded plugs may be evident by black
carbon deposits, o r re d , brown, yellow, o r b lis te r ­
ed oxide deposits on the plugs. The black deposits
a r e usually the r e s u lt of slow speed driving and
sh o rt ru n s w here sufficient engine operating tem p ­
e r a tu re is seldom reach ed . Worn piston rin g s,
faulty ignition, o v e r -ric h fuel m ix tu re and sp a rk
plugs which a r e too "co ld ” w ill also re s u lt in carbon
d ep o sits. Red, brown, o r yellow oxide deposits, a
consequence of the com bustion of leaded fuel,
usu ally re s u lt in sp a rk plug fa ilu re un der s e v e re
o p eratin g conditions.
The oxides have no ad v e rse affect on plug
o p eratio n a s long a s they rem ain in a powdery
sta te . B ut, under high speed o r h ard pull, the pow­
d e r oxide deposits m elt and form a heavy glaze
coating on the in su la to r which, when hot, a c ts as
a good e le c tric a l conductor, allowing c u rre n t to
follow the d eposits and sh o rt out the plug.
E x ce ssiv e gap w e ar on plugs of low m ileage
usually indicates the engine is operating at speeds
o r loads that a r e consistently g re a te r than norm al
o r that a plug which is too "hot" is being used. In
addition, electro d e w e a r may be the re s u lt of plug
overheating, caused by com bustion gases leaking
p a s t the th re a d s and gask ets, due to insufficient
co m p re ssio n of the s p a r k p lu g gask ets, d irt under
g ask e t, o r u se of old g a sk e ts. Too lean a fuel m ix­
tu r e w ill also re s u lt in excessive electro d e w ear.
S park plug life w ill also be affected by in c o r­
r e c t tim ing of the engine which re s u lts in e x c e s s ­
ively high o perating te m p e ra tu re .
B roken in su la to rs a r e usually the r e s u lt of
im p ro p e r In stallation o r c a re le s s n e s s . B roken
up p er in su la to rs usually re s u lt fro m a poor fitting
w rench o r an outside blow. The crack ed in su lato r
m ay not m ake its e lf evident im m ediately, but will
a s soon a s oil o r m o istu re p e n e tra te s the fra c tu re .
The fr a c tu re is usually ju st below the crim ped p a rt
of the sh ell and may not be visible.
B roken low er in su la to rs usually re s u lt from
c a re le s s n e s s when regapping and g en erally a re
v isib le . T his type of a b re a k may r e s u lt fro m the
plug op eratin g too "Hot" such as encountered in
su sta in e d p e rio d s of high speed operation o r under
ex tre m ely heavy loads, especially if not in sta lle d
c o rre c tly . S park plugs with broken in su la to rs
should alw ays be re p la ced .
S p ark p lu gs, to give good p erfo rm an c e in a
p a r tic u la r engine, m ust o p erate within a c e rta in
Figure
7 —Wrench Socket Installed on Plug ITypical)
te m p e ra tu re range (neither too hot no r too cool).
If the s p a rk plug re m a in s too "C ool," oil, soot,
carbon, and lead com ponents w ill deposit on the
in su la to r, causing FOULING and MISSING. If the
plug ru n s too "H ot," the deposits accum ulated on
the in su lato r su rfa ce during continuous slow o r
stop-and -g o driving may becom e b lis te re d , e le c ­
tro d e s w ill w ear rapidly, and u n d er ex trem e con­
ditions, p re m a tu re ignition (preignition) of the fuel
m ix ture re s u lt. EITHER CONDITION WILL SER­
IOUSLY A FFEC T THE PERFORMANCE OF THE
ENGINE.
R efe r to "Specifications" at end of th is section,
a s th e u se of sp a rk plugs in the p ro p e r H eat Range
is of v ital im po rtance to good engine p erfo rm an c e.
F requently, the wrong type of s p a rk plug, one with
an im p ro p er Heat Range fo r the engine, may have
been in stalle d when replacing s p a rk plugs o rig in ­
ally fitted by the engine m an u factu rer and such
m isapplication may lead to p oor p erfo rm an c e.
NOTE: The Heat Range of s p a rk plugs u sed on
vehicles covered by th is m anual is designated by
the two digit num ber on th e p o rc elain of the plug.
The high er th e num ber the higher the H eat Range
of the plug.
IMPORTANT: Spark plugs having a higher
Heat Range should not be In stalled u n less constant
fouling due to light duty operation is c le a rly de­
term in ed .
ABNORMAL OPERATION
W here abnorm al operating conditions cause
chronic carbon o r oil fouling of the plugs, the u se
of a type with one o r two num bers hig her (a "Hot­
t e r " type) than recom m ended in "S pecifications,"
w ill g enerally rem edy the trouble; and by th e sam e
form ula, w here chronic preig n itio n o r rap id e le c ­
tro d e w ear is experienced, a type with one o r two
num b ers low er (a "C ooler" type) w ill generally be
found sa tisfa c to ry .
SPARK PLUG REMOVAL
IMPORTANT: B efore rem oving any sp ark p lu g ,
CHEVROLET SERIES 70-90 HEAVY DUTY TRUCK SHOP MANUAL