Tune-Up - 4-cyl GMC Cab & Chassis S15

ENGINE IDENTIFICATION

The 8th 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.

Due to late changes and corrections, always refer to Emission Control Label in engine compartment before attempting tune-up. If manual and label specifications differ, use 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 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 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.

VALVE ARRANGEMENT

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

VALVE CLEARANCE

Check rocker arm bracket nuts for looseness and retighten as necessary before adjusting valves. Adjust valves with engine cold.

VALVE ADJUSTMENT

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

DISTRIBUTOR

All 2.5L engines use Delco-Remy High Energy Ignition (HEI) distributor. Some models 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

1. Remove air cleaner and gasket. Plug TBI unit vacuum port for THERMAC (Thermostatic Air Cleaner). Remove T.V. cable from throttle control bracket. Connect tachometer to engine.
2. Disconnect Idle Air Control (IAC) connector. Start engine and warm to normal operating temperature with transmission in Park (auto. trans.) or Neutral (man. trans.).
3. Install Air Passage Plug (J-33047) into throttle body idle air passage. Ensure plug seats air tight. (Scheme 2)
4. If plug covers throttle stop screw hole, remove plug by drilling 5/32" hole through casting into plug. Using 1/16" punch, remove plug.
5. Using Torx Bit No. 20, turn throttle stop screw to adjust slow idle to specification. Turn off engine and remove air passage plug from throttle body air passage.
6. Install T.V. cable into throttle control bracket. Install IAC connector. Seal throttle stop screw with silicone sealant. Install air cleaner and gasket.

Installing Throttle Body Air Passage Plug. Scheme 2

IDLE SPEED (RPM)

TPS OUTPUT VOLTAGE CHECK

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

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. See COMPUTER 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.

FUEL PUMPS

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

FUEL PUMP SPECIFICATIONS

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

IGNITION COIL RESISTANCE - OHMS @ 75°F (24°C)

EMISSION CONTROL

For information on emission control systems, see EMISSION section.

FUEL SYSTEMS

All 2.5L engines use General Motors Model 300 Throttle Body Injection (TBI).

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

Using a tension gauge, measure belt tension midway between pulleys. If tension is not as specified in table, adjust tension or replace belt.

BELT ADJUSTMENT Tension in Lbs. (Kg) Using Tension Gauge

SERVICE INTERVALS

REPLACEMENT INTERVALS

CAPACITIES

FLUID CAPACITIES

SYSTEM REFRIGERANT CAPACITIES