Tune-Up - 4-cyl GMC Cab & Chassis R2500

ENGINE IDENTIFICATION

The eighth character of the Vehicle Identification Number (VIN) identifies the engine. The VIN number is stamped on a plate attached to the left top side of dash.

ENGINE CODE

TUNE-UP NOTES

Note. When performing tune-up procedures described in this article, these notes and precautions must be followed.

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 an 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 (HEI) system. With air cleaner removed, throttle 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 cap. Using an ohmmeter, check resistance while gently twisting wire. If resistance is not within specifications, or fluctuates from infinity to any value, replace cable.

IGNITION COIL WIRE

Remove ignition coil wire from coil and distributor cap. Check terminals for corrosion and clean if necessary. Check coil wire resistance. Replace wire if resistance is excessive.

HIGH TENSION WIRE RESISTANCE

VALVE ARRANGEMENT

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

VALVE CLEARANCE

All engines are equipped with hydraulic lifters, adjustment is not required.

DISTRIBUTOR

All 2.5L engines use Delco-Remy HEI distributor with Electronic Spark Timing (EST). No adjustments other than timing are necessary.

2.5L 4-Cylinder Firing Order. Scheme 1

IGNITION TIMING

1. Install timing light with adapter between No. 1 spark plug and No. 1 spark plug wire, or use inductive pick-up. Do not puncture wire. Run engine until it reaches normal operating temperature.
2. Electronic Spark Timing (EST) must be in Bypass Mode to check ignition timing. Ground terminal "A" to "B" at Assembly Line Diagnostic Link (ALDL) connector.
3. Place transmission in Neutral (man. trans.) or Drive (auto. trans.). Set average timing of cylinders No. 1 and No. 4 to specification. Adjust if necessary.

IGNITION TIMING SPECIFICATIONS (DEGREES BTDC @ RPM)

HOT (SLOW) IDLE 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 with vehicle in Neutral. Turn ignition switch "OFF". Reconnect IAC valve electrical connector. Install air cleaner and gasket.

SLOW IDLE SPEED (RPM)

TPS OUTPUT VOLTAGE CHECK

Note. On 2.5L models, Throttle Position Sensor (TPS) is not adjustable.

1. Connect digital voltmeter between TPS connector terminals "B" and "C" using jumper wires. With ignition on and engine stopped, TPS voltage should be less than 1.25 volts. Replace TPS if voltage is 1.25 volts or more.
2. If TPS is not functioning properly, trouble code 21 may be given indicating a shorted TPS or trouble code 22 may be given indicating an open TPS circuit. See COMPUTERIZED ENGINE CONTROLS section for additional information.

IDLE MIXTURE

Note. On 2.5L engines, idle mixture is controlled by Electronic Control Module (ECM) and is nonadjustable.

COLD (FAST) IDLE RPM

Note. On 2.5L engines, fast idle is controlled by Electronic Control Module (ECM) and is nonadjustable.

All 2.5L engines are equipped with a Delco-Remy HEI solid state distributor. No adjustments are necessary.

DISTRIBUTOR PICK-UP COIL RESISTANCE

IGNITION COIL

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

Scheme 2

EMISSION CONTROL

See EMISSIONS section.

FUEL SYSTEMS

All 2.5L engines use General Motors Model 700 Throttle Body Injection (TBI).

FUEL PUMPS

All 2.5L engines are equipped with an in-tank, electric fuel pump.

FUEL PUMP SPECIFICATIONS

BATTERY

BATTERY SPECIFICATIONS

STARTER

All 2.5L engines use a Delco-Remy 5MT solenoid-actuated starter with overrunning clutch.

STARTER SPECIFICATIONS

ALTERNATOR

All models use a Delco-Remy alternator with integral voltage regulator.

ALTERNATOR SPECIFICATIONS

ALTERNATOR REGULATOR

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

REGULATOR OPERATING VOLTAGE

BELT ADJUSTMENT

All models use a serpentine belt for driving accessories.

SERVICE INTERVALS

REPLACEMENT INTERVALS

CAPACITIES

FLUID CAPACITIES

SYSTEM REFRIGERANT CAPACITIES