Tune-Up - v8 GMC Cab & Chassis R2500

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 on the dash and toe panel on "P" models, and on a plate at the upper left corner of dash on all other models. Engine code numbers are located at front of block, at right cylinder head or at rear of block, at left cylinder head.

ENGINE CODES

TUNE-UP NOTES

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

Always refer to Emission Control Label in engine compartment before attempting tune-up. If manual and label differ, always use emission 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.

ENGINE COMPRESSION

When making compression checks, disconnect ignition switch connector Pink wire from high energy ignition system. Remove air cleaner. With throttle and choke wide open, and all spark plugs removed, 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 not to specifications, or fluctuates from infinity to any value, replace cable.

HIGH TENSION WIRE RESISTANCE (OHMS)

5.0L & 5.7L

E-I-I-E-E-I-I-E (Both banks, front-to-rear).

7.4L

1. E-I-E-I-E-I-E-I (Left bank, front-to-rear).
2. I-E-I-E-I-E-I-E (Right bank, front-to-rear).

VALVE CLEARANCE

All vehicles are equipped with hydraulic lifters. Lifters should be adjusted to zero lash.

DISTRIBUTOR

Engines with carburetors are equipped with Delco-Remy High Energy Ignition (HEI) systems which use an integral coil and module. No adjustments are required.

Engines with throttle body injection (TBI) are equipped with Delco-Remy High Energy Ignition with Electronic Spark Timing (EST). The EST system consists of distributor module, electronic control module (ECM), externally mounted ignition coil and connecting wires. No centrifugal or vacuum advance is used on engines equipped with EST.

Firing Order & Timing Mark (Magnetic Timing Pick-Up Is Located At 10 Degrees ATDC). Scheme 10

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.

Carbureted Engine

1. Connect an adapter between No. 1 spark plug wire or use an inductive pick-up. Do not puncture wires. Connect timing light according to manufacturer's instructions.
2. Check or adjust ignition timing with engine at normal operating temperature, and distributor vacuum line disconnected and plugged. Place automatic transmission in Park and manual transmission in Neutral.

Throttle Body Injection

1. 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. Disconnect 1-wire (Tan with Black stripe) connector at distributor.
2. Check and adjust ignition timing with engine at normal operating temperature and transmission in "P" or "N" (auto. trans.) or Neutral (man. trans.) Reconnect 1-wire connector. Clear trouble code by removing ECM power fuse for 10 seconds.

IGNITION TIMING SPECIFICATIONS (DEGREES BTDC@RPM)

HOT (SLOW) IDLE RPM

Note. Depending on engine size and application, on carbureted engines only, either a vacuum operated throttle kicker assembly or an idle speed solenoid is used to control idle under load situations and to reduce emissions.

1. With engine at operating temperature, choke wide open, ignition timing set and solenoid wire disconnected, adjust curb idle speed to specification by turning idle speed screw.
2. Connect idle speed solenoid wire and open throttle slightly to allow solenoid plunger to fully extend. Turn solenoid screw to set curb idle speed to specification. (Scheme 11)

Scheme 11

With engine at operating temperature, set ignition timing to specification. Remove air cleaner and plug disconnected hoses. Turn idle speed screw to set curb idle to specification. (Scheme 11)

IDLE SPEED (RPM)

1. With engine at operating temperature, choke wide open and ignition timing set, adjust idle speed screw to obtain specified curb idle speed. See Emission Control Label.
2. Apply 20 in. Hg vacuum using vacuum pump to fully extend throttle kicker plunger. Manually open throttle slightly and then release. With plunger held inward, turn plunger in or out to obtain 1600 RPM.

Note. Adjustment is necessary only when throttle body assembly has been replaced.

1. Remove air cleaner and gasket. Plug TBI unit vacuum port for THERMAC (Thermostatic Air Cleaner). With Idle Air Control (IAC) valve connected, ground terminal "A" to "B" at ALDL connector.
2. Turn ignition switch to "ON" position, DO NOT start engine. Wait approximately 30 seconds for IAC valve pintle to extend and seat in throttle body.
3. With ignition on, disconnect IAC valve electrical connector. Remove ground at ALDL connector. Using an awl, remove plug for minimum idle speed adjustment by piercing, then applying leverage.
4. Start engine. Adjust idle screw to specification. Turn ignition switch "OFF". Reconnect IAC valve electrical connector. Install air cleaner and gasket.

SLOW IDLE SPEED (RPM)

TPS OUTPUT VOLTAGE CHECK

Note. Throttle Position Sensor (TPS) is not adjustable.

Connect digital voltmeter between TPS connector terminals "A" and "B" using jumper wires. With ignition on and engine off, TPS voltage should be less than one volt. Replace TPS if voltage is more than one volt. Before replacing TPS, ensure minimum idle speed is set correctly.

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. Distance between cuts depends on size of punch to be used. 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 degree 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 degree angle and drive the plug out of the casting.

Heavy Duty Emissions Only

1. Set parking brake and block drive wheels. Warm engine to normal operating temperature. Remove air cleaner. Place transmission in Neutral or Park and connect tachometer.
2. Turn idle mixture screws in lightly to seat them, then back out 3 turns. Do not turn screws tightly against seat or damage may result. With engine running, choke open, and transmission in Neutral, adjust idle speed to specification. Adjust mixture screws to obtain maximum RPM.
3. Readjust idle speed screw to specification, and readjust mixture screws to obtain highest RPM. Adjustment is correct when highest RPM can be achieved with minimum number of turns of mixture screws from seated position. Turn ignition off, and install air cleaner. Seal idle mixture screw with RTV sealant.

Note. Idle mixture is not adjustable on TBI models.

1. Place transmission in "P" (auto. trans.) or Neutral (man. trans.). Place cam follower on highest step of fast idle cam. Disconnect and plug vacuum hose to EGR valve, canister purge hose and purge signal hose at canister.
2. Start engine without touching throttle. Turn fast idle speed screw to adjust speed to specification.

FAST IDLE SPEED (RPM)

Note. Idle speed is not adjustable on TBI models.

IGNITION

DISTRIBUTOR PICK-UP COIL RESISTANCE

Ignition Coil - Internal Coil

1. Disconnect all electrical connections from coil. Connect an ohmmeter at connection "A". (Scheme 12) Reading should be zero or very low. If reading is not zero or very low, replace coil.
2. Connect an ohmmeter at connections "B" and "C". (Scheme 12) Using high scale, reading should show continuity. If reading is infinite, replace coil.

Scheme 12

Ignition Coil - External Coil

1. Disconnect all electrical connections from coil. Connect an ohmmeter at connection "A". (Scheme 13) Reading should be infinite. If reading is not infinite, replace coil.
2. Connect an ohmmeter at connection "B". (Scheme 13) Using low scale, reading should be very low or zero. If reading is not low or zero, replace coil.
3. Connect an ohmmeter at connection "C". (Scheme 13) Using high scale, meter should show continuity. If reading is infinite, replace coil. Connect all coil connections and ensure coil ground is clean.

Scheme 13

EMISSION CONTROL

See EMISSIONS section.

FUEL PUMP

FUEL PUMP SPECIFICATIONS

FUEL SYSTEMS

ROCHESTER CARBURETOR

THROTTLE BODY INJECTION

ELECTRICAL

BATTERY SPECIFICATIONS @ 0°F (-18°C)

STARTER

All models are equipped with Delco-Remy overrunning clutch starters.

STARTER SPECIFICATIONS

ALTERNATOR

All models are equipped with Delco-Remy alternators with integral voltage regulators. No adjustments are possible.

ALTERNATOR SPECIFICATIONS @ 80°F (27°C)

ALTERNATOR REGULATOR

Regulators are Delco nonadjustable, integral with alternator.

REGULATOR SPECIFICATIONS

SERVICE INTERVALS

REPLACEMENT INTERVALS

BELT ADJUSTMENT

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

CAPACITIES

FLUID CAPACITIES

SYSTEM REFRIGERANT CAPACITIES