Tune-Up - 6-cyl GMC Cab & Chassis S15

ENGINE IDENTIFICATION

Engines can be identified by the eighth character of the Vehicle Identification Number (VIN). The VIN number is stamped on a plate located at the base of the steering column on "P" models, and on a plate at the upper left corner of the dash on all other models.

ENGINE CODE

TUNE-UP NOTES

Note. When performing tune-up procedures described in this article, the following notes and precautions must be observed

Due to late changes and corrections, always refer to Emission Control Label in engine compartment before attempting tune-up. In the event of a conflict between specifications given in this manual and label specifications, follow label specifications.

When performing tune-up on vehicles equipped with a catalytic converter, do not allow or create a condition of engine misfire in one or more cylinders for an extended period of time. Damage to converter from overheating may occur due to loading with unburned fuel.

For tune-up purposes, "Light Duty" refers to vehicles up to 8500 lbs. GVW. "Heavy Duty" refers to vehicles exceeding 8500 lbs. GVW.

ENGINE COMPRESSION

When making compression checks, disconnect ignition switch Pink wire from High Energy Ignition (HEI) system. Remove air cleaner. Set throttle and choke in wide open position. Remove all spark plugs and crank engine through at least 4 compression strokes.

COMPRESSION SPECIFICATIONS

SPARK PLUGS

SPARK PLUG TYPE

SPARK PLUG SPECIFICATIONS

HIGH TENSION WIRE RESISTANCE

Carefully remove ends of wire from spark plug and distributor. Using an ohmmeter, check resistance while gently twisting wire. If resistance is incorrect, or fluctuates from infinity to any value, replace cable.

HIGH TENSION WIRE RESISTANCE (OHMS)

VALVE ARRANGEMENT

E-I-I-E-E-I-I-E-E-I-I-E (Front-to-rear).

VALVE CLEARANCE

All engines have hydraulic lifters which should be adjusted to one turn down from zero lash.

DISTRIBUTOR

All models are equipped with High Energy Ignition (HEI) systems and no adjustments are required. Some engines use an HEI/EST distributor. All spark timing changes in the HEI/EST distributor are done electronically by the Electronic Control Module (ECM) which monitors information from various engine sensors and computes correct timing and signals the distributor accordingly. No centrifugal or vacuum advance is used on engines equipped with EST.

Firing Order & Timing Marks (Magnetic Probe Receptacle Is Located At 10 Degrees ATDC). Scheme 3

IGNITION TIMING

Note. Engines are equipped with a receptacle for magnetic probe timing, located 10° ATDC. Do not use this location for timing with a conventional light.

Install timing light with an adapter between No. 1 spark plug and No. 1 spark plug wire, or use an inductive pick-up. Do not puncture wire. Engine should be at normal operating temperature, with choke fully open, air cleaner installed and A/C off (if equipped). Disconnect and plug vacuum hose at distributor. Set ignition timing at specified engine speed.

IGNITION TIMING SPECIFICATIONS (Degrees BTDC@RPM)

HOT (SLOW) IDLE RPM

1. Engine must be warm and choke fully open. Fast idle cam follower must be off fast idle cam. Set curb idle speed with solenoid energized, by rotating solenoid assembly.
2. Disconnect electrical lead at solenoid. Set base idle by adjusting 1/8" set screw at rear of solenoid. (Scheme 4) Reconnect electrical lead at solenoid.

Base Idle RPM Adjustment. Scheme 4

IDLE SPEED (RPM)

IDLE MIXTURE

Note. Mixture adjustment is not a normal tune-up procedure. DO NOT remove idle mixture plug unless vehicle fails emissions testing or carburetor has been disassembled.

1. Set parking brake and block drive wheels. Remove air cleaner for access to carburetor, but keep vacuum hoses connected. Disconnect and plug other hoses, as directed by Emission Control Label. Remove idle mixture needle plug (if equipped).
2. Place transmission in "P" (auto. trans.) or Neutral (man. trans.). Warm engine to normal operating temperature. Ensure choke is open and A/C is off (if equipped). Connect a tachometer and a vacuum gauge to engine. Check ignition timing and adjust if necessary.
3. Lightly seat mixture screw and back out 3 turns as a preliminary adjustment. Start engine and adjust idle speed screw to specified idle speed on Emission Control Label. Then adjust idle mixture screw to obtain highest RPM. Repeat both procedures to obtain best idle.
4. Reset curb idle speed to specification. Remove gauges, unplug and reconnect vacuum hoses and install air cleaner

IDLE MIXTURE PLUG REMOVAL

1. Remove carburetor from engine. Invert carburetor and drain fuel. Place carburetor in holding fixture with manifold side facing up.
2. Use a hacksaw to cut 2 parallel cuts in the throttle body, one on each side of the locator point below idle mixture screw plug. Cut down to the steel plug and no more. Place a flat punch at a point near the ends of the saw marks.
3. Hold punch at a 45° angle and drive into the throttle body until casting breaks away, exposing the steel plug. Hold a center punch vertically and drive it into the plug. Hold the punch at a 45° angle and drive the plug out of the casting.

COLD (FAST) IDLE RPM

Disconnect and plug canister purge hose and canister purge signal hose at canister. Place cam follower on high step of fast idle cam. Support lever with pliers and bend tang in or out to adjust fast idle RPM. (Scheme 5)

Note. On models with manual choke, rotate fast idle cam clockwise to farthest "UP" position.

Cold (Fast) Idle RPM Adjustment. Scheme 5

FAST IDLE SPEED (RPM)

AUTOMATIC CHOKE

All choke caps are retained with rivets and are nonadjustable.

FUEL PUMP

All models use mechanical fuel pumps.

FUEL PUMP SPECIFICATIONS

All models are equipped with High Energy Ignition (HEI) systems. No adjustments are required.

DISTRIBUTOR PICK-UP COIL RESISTANCE

IGNITION COIL

IGNITION COIL RESISTANCE - OHMS @ 75°F (24°C)

EMISSION CONTROL

For information on emission control systems, see EMISSION section.

FUEL SYSTEMS

CARBURETORS

BATTERY SPECIFICATIONS

BATTERY SPECIFICATIONS @ 0°F (-18°C)

STARTER

All models are equipped with Delco-Remy 10MT starter with overrunning clutch.

STARTER SPECIFICATIONS

ALTERNATOR

All models are equipped with Delco-Remy integral regulator alternators.

ALTERNATOR SPECIFICATIONS @ 80°F (27°C)

ALTERNATOR REGULATOR

All models are equipped with Delco-Remy nonadjustable regulator, integral with alternator.

REGULATOR SPECIFICATIONS @ 85°F (29°C)

BELT ADJUSTMENTS

BELT ADJUSTMENT - TENSION IN LBS. (KG) USING STRAND TENSION GAUGE

SERVICE INTERVALS

REPLACEMENT INTERVALS

CAPACITIES

COOLING SYSTEM CAPACITIES

ENGINE OIL & FUEL CAPACITIES

DIFFERENTIAL, TRANSFER CASE & TRANSMISSION CAPACITIES

SYSTEM REFRIGERANT CAPACITIES