Chevrolet 1977 light duty truck, Service Manual

Page: 654 / 861

ENGINE PERFORMANCE DIAGNOSIS CHARTS
INTRODUCTION
E ngine P erfo rm an ce D iagnosis procedures are
guides th a t will lead to the m ost p ro b ab le causes o f
en g in e p e rfo rm a n c e com plaints. T hey consider all o f the
p arts o f the fuel, ig nition, and m echanical system s th at
could cause a p a rtic u la r co m plaint, and then outline
re p a irs in a logical sequence.
Each S ym ptom is defined, an d it is vital th a t the
correct one be selected based on the com plaints rep o rted
or found.
Review the S ym ptom s and th e ir definition to be sure
th a t only the correct term s are used.
T he w ords used m ay n o t be w h at you are used to in
all cases, but because these term s have been used
in te rc h a n g e ab ly fo r so long, it w as necessary to decide
on the m ost com m on usage and then define them . If the
d efinition is not understood, an d the exact Sym ptom is
not used, the D iagnostic p rocedure will not work.
It is im p o rta n t to keep two facts in m ind:
1. T he procedures are w ritten to diagnose p ro b ­
lems on vehicles th a t have " ru n well at one tim e " and
th a t tim e and w ear have created the condition.
2. All possible causes can n o t be covered, p a rtic u ­
larly w ith re g a rd to em ission controls th a t affect vacuum
advance. If doing the w ork prescribed does not correct
the com plaint, then e ith e r the w rong Sym ptom was used,
or a m ore d etailed analysis will have to be m ade.
All o f the Sym ptom s can be caused by w orn out or
defective parts such as Spark Plugs, Ignition W iring,
etc. If tim e a n d /o r m ileage indicate th a t parts
should be replaced, it is recom m ended th a t it be
done.
SYMPTOM DEFINITIONS
Dieseling
- E ngine continues to run a fte r the switch
is tu rn ed off. It runs unevenly and m ay m ake knocking
noises.
Detonation - A m ild to severe ping, usually worse
u n d er acceleration. The engine m akes sh arp m etallic
knocks th at change w ith throttle opening.
Stalls
- The engine quits ru n n in g . It m ay be at idle
or w hile driving.
Loads - The engine misses due to excessively rich
f u e l/a ir m ixture. U sually occurs d u rin g cold engine
op eratio n and is characterized by black sm oke being
em itted from the tail pipe.
Rough Idle
- The engine ru n s unevenly at idle. If
bad enough, it m ay m ake the vehicle shake.
M iss - S teady pulsation or je rk in g th at follows
engine speed, usually more p ronounced as engine load
increases. N o t norm ally felt above 1500 RPM o r 30
M PH. The exhaust has a steady spitting sound at idle or
low speed.
H esitation
- M om entary lack o f response as the
accelerator is depressed. C an occur at all vehicle speeds.
U sually m ost severe w hen first trying to m ake the vehicle
move, as fro m a stop sign. M ay cause the engine to stall
if severe enough.
Sag
- T he engine responds initially, then flattens out
or slows dow n, an d then recovers. M ay cause the engine
to stall if severe enough.
Surges
- E ngine Power v a ria tio n u nder steady
throttle o r cruise. Feels like the vehicle speeds up a id
slows down w ith no change in the accelerator pedal
position. C an occur at any speed.
Sluggish - E ngine delivers lim ited pow er u nder load
or at high speed. W o n ’t accelerate as fast as norm al;
loses too m uch speed going up hills; o r has less top speed
th an norm al
Poor Gas M ileag e
- G as m ileage significantly less
th an the average fo r the vehicle an d d riv etrain
com b in atio n in question.
Cuts Out
- T em p o rary com plete loss o f power. The
engine quits at sharp, irreg u la r intervals. M ay occur
repeatedly, o r in term ittently. U sually worse u n d er heavy
acceleration.
Backfire
- Explosion type noise e m a n a tin g from the
exhaust system. U sually occurs d u rin g deceleration.