Tune-Up - v8 GMC Pickup 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" Series, and on a plate at 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 catalytic converter, do not allow or create a condition of engine misfire in one or more cylinders for an extended period of time. Unburned fuel in exhaust system will cause system to overheat and/or damage to catalytic converter.

ENGINE COMPRESSION

When making compression checks, disconnect ignition battery voltage connector from ignition coil. Remove air cleaner. Hold throttle and choke wide open. Remove all spark plugs. Crank engine through at least 4 compression strokes.

COMPRESSION SPECIFICATIONS

HIGH TENSION WIRE RESISTANCE

Grasp spark plug wire rubber boot. With a twisting motion, break boot loose from spark plug. Remove end of wire from spark plug. Repeat procedure at distributor. Using an ohmmeter, check resistance while gently flexing wire. If resistance is not to specification, or fluctuates from any value to infinity, replace cable.

HIGH TENSION WIRE RESISTANCE (OHMS)

IGNITION

DISTRIBUTOR PICK-UP COIL RESISTANCE

Ignition Coil (Carbureted With Internal Coil)

1. Disconnect all electrical connections from coil. Connect an ohmmeter as shown in connection "A". (Scheme 4) Reading should be zero or very low. If reading is not zero or very low, replace coil.
2. Connect an ohmmeter as shown in connections "B" and "C". (Scheme 4) Using high scale, reading should show continuity. If reading is infinite, replace coil.

Scheme 4

Throttle Body With External Coil

1. Disconnect all electrical connections from coil. Connect an ohmmeter as shown in connection "A". (Scheme 5) Reading should be infinite. If reading is not infinite, replace coil.
2. Connect an ohmmeter as shown in connection "B". Using low scale, reading should be very low or zero. If reading is not low or zero, replace coil.
3. Connect an ohmmeter as shown in connection "C". Using high scale, meter should show continuity. If reading is infinite, replace coil. Connect all coil connections and ensure coil ground is clean.

Scheme 5

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 equipped with carburetors are equipped with Delco-Remy High Energy Ignition (HEI) systems which use an integral ignition coil and module, and vacuum and centrifugal advance mechanisms. No adjustments are required.

Engines equipped with Throttle Body Injection (TBI) are equipped with Delco-Remy High Energy Ignition (HEI) with Electronic Spark Timing (EST). The EST system consists of distributor module, EST portion of 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 6

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. Place automatic transmission in Park and manual transmission in Neutral. Check or adjust ignition timing with engine at normal operating temperature, and distributor vacuum line disconnected and plugged.

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) timing by-pass connector at distributor.
2. Place automatic transmission in Park and manual transmission in Neutral. Check and adjust ignition timing with engine at normal operating temperature. Reconnect by-pass connector. Clear trouble code by removing ECM power fuse for 10 seconds.

IGNITION TIMING SPECIFICATIONS (DEGREES BTDC @ RPM)

IDLE SPEED

Note. Idle speed on fuel injected engines is controlled by the Electronic Control Module (ECM) and is not adjustable. Carbureted engines may be equipped with 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. Place transmission in Park (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.

COLD FAST IDLE SPEED (RPM)

Carbureted Engine With Idle Speed Solenoid

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. (Scheme 7)
2. Connect idle speed solenoid wire and open throttle slightly to allow solenoid plunger to fully extend. Turn solenoid screw setting the curb idle speed to specification.

Scheme 7

Carbureted Engine Without Idle Speed Solenoid

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 7)

SLOW IDLE SPEED (RPM)

IDLE AIR CONTROL (IAC) MOTOR MINIMUM IDLE SPEED

Note. Minimum idle adjustment is necessary only when throttle body assembly has been replaced.

1. Remove air cleaner and gasket. Plug TBI vacuum port for THERMAC (Thermostatic Air Cleaner). With Idle Air Control (IAC) valve connected, ground diagnostic "test" terminal of ALDL connector (jumper terminal "A" to "B").
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 "test" terminal. Start engine and check minimum idle speed. Adjust to specification, if necessary.
4. Turn ignition off. Using an awl, remove plug for minimum idle speed adjustment by piercing, then applying leverage. Start engine. Adjust idle screw to specification. Turn ignition off. Reconnect IAC valve electrical connector. Install air cleaner and gasket.

THROTTLE BODY MINIMUM IDLE SPEED (RPM)

TPS OUTPUT VOLTAGE CHECK

Note. No throttle position sensor is used on carbureted engines. Throttle Position Sensor (TPS) on fuel injected models 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 1.25 volts. Replace TPS if voltage is more than 1.25 volts. Before replacing TPS, ensure minimum idle speed is set correctly.

IDLE MIXTURE

Note. Air fuel mixture on fuel injected engines is controlled through the Electronic Control Module (ECM) and is not adjustable.

Idle Mixture Plug Removal (Carbureted Engine)

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

Best Idle Procedure (Carbureted 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 or Park, adjust idle speed to specification. See SLOW IDLE SPEED (RPM) table. 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 screws with RTV sealant.

EMISSION CONTROL

See EMISSIONS section.

FUEL SYSTEMS

ROCHESTER CARBURETOR

THROTTLE BODY INJECTION

FUEL PUMP SPECIFICATIONS

BATTERY

BATTERY SPECIFICATIONS @ 0°F (-18°C)

SPARK PLUGS

SPARK PLUG TYPE

SPARK PLUG SPECIFICATIONS

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 (1)

CAPACITIES

FLUID CAPACITIES

SYSTEM REFRIGERANT CAPACITIES