Operation - Buick LeSabre 1965 Service Manual

ENGINE FUEL AND EXHAUST SYSTEMS
TROUBLE DIAGNOSIS 3 - 1 7
3 - 5 IMPROPER ENGINE
PERFORMANCE
a. Engine Idles Too Fast
A cold engine should operate on
fast idle for two to five minutes
depending on air temperature.
At 32°F. the fast idle cam should
move to slow idle position in
approximately 1/2 to 3/4 mile of
driving. At higher temperatures
it should move to slow idle posi-
tion in a correspondingly shorter
distance.
If the engine operates too long on
the fast idle cam, check the choke
thermostat setting (par. 3-8) and
the fast idle adjustment. See
paragraph 3-17 (Rochester 2-
Bbl.), 3-21 (Rochester 4-Bbl.),
or 3-25 (Carter).
If engine idles faster than the
specified idle speed when off the
fast idle cam, check throttle link-
age for binding or weak return
spring and adjust throttle stop
screw (par. 3-8). This trouble
can also be caused by a sticking
choke or dash pot.
b. Improper Idle and Low
Speed Performance
Rough idling and tendency to stall
may be caused by idling speed set
below the specified speed. Idle
mixture may be wrong due to im-
proper needle valve adjustment
(par. 3-8).
Rough idling, poor performance,
and back firing at low speeds
frequently originates in improper
ignition. Check ignition system
(par. 10-33).
High fuel pump pressure will
cause rough idling and poor low
speed performance (par. 3-12).
An intake manifold air leak will
cause rough idling and poor low
speed performance. A manifold
air leak produces a low, erratic
reading on a vacuum gauge con-
nected to the intake manifold.
Check for leaks at all pipe con-
nections and check manifold joints
with gasoline.
When rough idling and poor low
speed performance cannot be cor-
rected by checks of carburetion
and ignition mentioned above,
check cylinder compression.
Improper performance which is
most noticeable at low speeds
may be caused by sticking valves.
Sticking valves may be caused by
the use of low grade fuel or fuel
that has been in storage too long.
When a car is stored for any
length of time, fuel should be
drained from the tank, feed hoses,
fuel pump, and carburetor in or-
der to avoid gum formation.
c. Improper High Speed
Operation
Roughness or poor performance
above 22 MPH indicates faulty
ignition (par. 10-33) or improper
settings in the high speed circuit
of carburetor. Surging at high
speed-may be caused by low fuel
pump pressure (par. 3-12).
With Rochester c a r b u r e t o r s ,
surging at 75 to 80 MPH constant
speed indicates that the power jet
is stopped up or the vacuum piston
is sticking.
If there is lack of power at top
speed, check throttle linkage to
insure full throttle valve opening
(par. 3-9).
d. Excessive Detonation
or Spark Knock
Light detonation may occur when
operating an automatic transmis-
sion car in Drive with full throttle
at low speed even when ignition
timing is correct and proper fuel
is used. This light detonation is
normal and no attempt should be
made to eliminate it by retarding
the ignition timing, which would
r e d u c e economy and over-all
performance.
Heavy detonation may be caused
by improper ignition timing (par.
10-35), improper grade of fuel,
or by an accumulation of carbon
in combustion chambers. Series
4400 cars with the low compres-
sion engine may use regular fuel;
all others require premium fuel
because of their higher compres-
sion ratios.
Heavy detonation is injurious to
any automotive engine. A car
driven continuously under condi-
tions and with fuels which produce
heavy detonation will overheat and
lose power, with the possibility of
damage to pistons and bearings.
3 - 6 EXCESSIVE FUEL
CONSUMPTION
Complaints of excessive fuel con-
sumption require a careful in-
vestigation of owner d r i v i n g
habits and operating conditions as
well as the mechanical conditions
of the engine and fuel system;
otherwise, much useless work
may be done in an attempt to in-
crease fuel economy.
Driving habits which seriously af-
fect fuel economy are: high speed
driving, frequent and rapid ac-
celeration, driving too long in a
low speed range when getting un-
der way, excessive idling while
standing.
Operating conditions which ad-
versely affect fuel economy are:
excessive acceleration, frequent
starts and stops, c o n g e s t e d
traffic, poor roads, hills and
mountains, high winds, low tire
pressures.
High speed is the greatest con-
tributor to low gas mileage. Air
resistance i n c r e a s e s as the
square of the speed. For instance,
a car going sixty miles an hour
must overcome air resistance
four times as great as when going
thirty miles an hour. At eighty
miles an hour the resistance is
over seven times as great as
when going thirty miles an hour.