Tune-Up - 6-cyl 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

TUNE-UP NOTES

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

Note. 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 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)

4.8L

1. 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.

Firing Order & Timing Marks. Scheme 3

IGNITION TIMING

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" hex screw at rear of solenoid. (Scheme 4) Reconnect electrical lead at solenoid.

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. 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.

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)

Scheme 5

FAST IDLE SPEED (RPM)

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

DISTRIBUTOR PICK-UP COIL RESISTANCE

IGNITION COIL

1. Disconnect all electrical connections from coil. Connect an ohmmeter (Scheme 6), connection A. Reading should be zero or very low. If reading is not zero or very low, replace coil.
2. Connect an ohmmeter (Scheme 6), connections B and C. Using high scale, reading should show continuity. If reading is infinite, replace coil.

Scheme 6

EMISSION CONTROL

See EMISSIONS section.

FUEL PUMP

All models use mechanical fuel pumps.

FUEL PUMP SPECIFICATIONS

FUEL SYSTEMS

CARBURETORS

BATTERY

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 & TRANSMISSION CAPACITIES