Tune-Up - 6-cyl GMC Cab & Chassis R3500

VALVE ARRANGEMENT

Front-to-rear.

VALVE CLEARANCE

All engines are equipped with hydraulic lifters. Valve adjustment is not normally required during tune-up. Valve lash may be adjusted if needed. See appropriate article in ENGINES section.

IGNITION SYSTEM

SPARK PLUG TYPE

SPARK PLUG SPECIFICATIONS

High Tension Wire Resistance

The spark plug wires are carbon impregnated conductors encased in 8 mm diameter rubber jackets. High tension wire resistance specifications are not available from manufacturer.

Ignition Coil

1. Disconnect all electrical leads from coil. Connect ohmmeter in position "A". (Scheme 1) Reading should be infinite. If not infinite, replace coil.
2. Connect ohmmeter in position "B". Using low scale, reading should be very low or zero. If not, replace coil. Now connect ohmmeter in position "C". Using high scale, meter reading should not be infinite. If it is, replace coil. Reconnect coil.

Scheme 1

Distributor

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

Firing Order & Distributor Rotation. Scheme 2

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. See IGNITION TIMING SPECIFICATIONS table.

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. Disconnect and plug canister purge hose and canister purge signal hose at canister.
2. Set curb idle speed by rotating solenoid assembly. See «HOT (SLOW) IDLE SPEED RPM»(/gmc/cab-chassis-r3500/1987-1991/remont/adjustments/#tune-up-6-cyl) table. Disconnect electrical lead at solenoid. Set base idle by adjusting 1/8" hex screw at rear of solenoid. (Scheme 3) Reconnect lead to solenoid. Unplug and reconnect hoses to canister.

Scheme 3

HOT (SLOW) IDLE SPEED RPM

THROTTLE POSITION SENSOR (TPS)

No throttle position sensor is used on carbureted engines.

IDLE MIXTURE ADJUSTMENT

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.

Idle Mixture Plug Removal

1. Remove carburetor from engine. Invert carburetor and drain fuel. Place carburetor in holding fixture with manifold side facing up. (Scheme 4)
2. Use a 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 punch at a 45 degree angle and drive plug out of casting.

Scheme 4

Idle Mixture Adjustment Procedure

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 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 tachometer to engine.
3. Check ignition timing and adjust if necessary. Turn idle mixture screw in or out to obtain highest RPM. Adjust idle speed solenoid to obtain curb idle speed specified on Emission Control Label.
4. Readjust idle mixture and curb idle to obtain highest RPM. Check and adjust base idle and fast idle. Turn off engine, remove tachometer, and install air cleaner. Remove block from drive wheels.

COLD (FAST) IDLE RPM

Engine must be warm and choke fully open. Disconnect and plug canister purge hose and canister purge signal hose at canister. With fast idle cam follower off fast idle cam, adjust curb idle and base idle. Place cam follower on high step of fast idle cam and adjust cold (fast) idle RPM. See COLD (FAST) IDLE RPM table. Unplug and reconnect hoses to canister.

COLD (FAST) IDLE RPM

EMISSION CONTROL

For information on emission control systems, see EMISSION CONTROL SYSTEMS section.

FUEL SYSTEM

CARBURETOR

FUEL PUMP SPECIFICATIONS

BATTERY

BATTERY SPECIFICATIONS

SERVICE INTERVALS

REPLACEMENT INTERVALS

BELT ADJUSTMENTS

Note. Tension in Lbs. (Kg) Using Strand Tension Gauge.

BELT ADJUSTMENT

CAPACITIES

COOLING SYSTEM CAPACITIES

ENGINE OIL CAPACITIES

TRANSMISSION CAPACITIES

A/C SYSTEM REFRIGERANT CAPACITIES

See also:
• IGNITION TIMING SPECIFICATIONS