Tune-Up - 4-cyl GMC S15 Pickup

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 and visible through windshield.

VIN ENGINE CODE

TUNE-UP NOTES

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 catalytic 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

VALVE ARRANGEMENT

1. I-E-I-E-E-I-E-I (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 7 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

Engine is equipped with a Delco-Remy HEI solid state distributor. No adjustments are necessary.

Firing Order & Distributor Rotation. Scheme 2

Ignition Timing

1. Install timing light 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 by-pass mode to check ignition timing. Ground terminal "A" to "B" at Assembly Line Diagnostic Link (ALDL) connector. (Scheme 3)
3. Place transmission in Neutral (man. trans.) or Drive (auto. trans.). Check timing and adjust if necessary. See «IGNITION TIMING SPECIFICATIONS»(/gmc/s15-pickup/1987-1991/remont/adjustments/#tune-up-4-cyl) table.

IGNITION TIMING SPECIFICATIONS (DEGREES BTDC @ RPM)

Scheme 3

EMISSION CONTROL

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

FUEL SYSTEM

TBI FUEL INJECTION

FUEL PUMP

All engines are equipped with an in-tank, electric fuel pump. Relieve pressure on fuel system before disconnecting fuel lines.

Relieving Fuel Pressure

1. Place transmission in Park (Neutral for manual transmissions) and apply parking brake. Loosen fuel filler cap to relieve tank pressure.
2. Disconnect 3 terminal electrical connector at fuel tank. Start engine and let run until engine dies from lack of fuel. Crank engine for 3 seconds to relieve residual pressure.

Fuel Pressure Test

Relieve fuel pressure. Plug thermac vacuum port on TBI (if equipped). Disconnect flexible fuel hose and install pressure gauge. Start engine and observe fuel pressure. See FUEL PUMP SPECIFICATIONS table. Relieve fuel pressure. Remove pressure gauge and reconnect fuel line using new "O" ring.

FUEL PUMP SPECIFICATIONS

CONTROLLED IDLE SPEED CHECK

1. Ensure no codes are displayed, idle air control system has been checked, and ignition timing is correct. See CODE 35 IDLE SPEED ERROR (2.5L ENGINE) chart in GENERAL MOTORS COMPUTER COMMAND CONTROL article in COMPUTERIZED ENGINE CONTROLS section. NOTE: Diagnosis of idle air control system is incorporated into Code 35 chart for the 2.5L engine.
2. Set parking brake and block drive wheels. Connect "SCAN" tester to ALDL connector with engine in open loop. Start engine and warm to normal operating temperature. Place transmission in Drive (auto. trans.) or Neutral (man. trans.).
3. Check "SCAN" tester for correct position of Park/Neutral switch. Check controlled idle speed and IAC valve pintle position counts. If within specifications, idle speed is being correctly controlled by ECM. See «CONTROLLED IDLE SPEED (RPM)»(/gmc/s15-pickup/1987-1991/remont/adjustments/#tune-up-4-cyl) table.
4. If not within specifications, see "ROUGH, UNSTABLE OR INCORRECT IDLE; STALLING" in TROUBLE SHOOTING in GENERAL MOTORS COMPUTER COMMAND CONTROL article in COMPUTERIZED ENGINE CONTROLS section.

CONTROLLED IDLE SPEED (RPM)

HOT (SLOW) IDLE RPM

1. Check controlled idle speed and perform idle air control system check before adjusting hot idle. See CODE 35 IDLE SPEED ERROR (2.5L ENGINE) chart in GENERAL MOTORS COMPUTER COMMAND CONTROL article in COMPUTERIZED ENGINE CONTROLS section. NOTE: Diagnosis of idle air control system is incorporated into Code 35 chart for the 2.5L engine.
2. Remove air cleaner, adapter, and gasket. On "S" and "T" series trucks, leave thermac hose connected. On all models, ensure that throttle lever operates freely.
3. With Idle Air Control (IAC) valve connected, ground terminal "B" to "A" at ALDL connector. (Scheme 3) Turn ignition on, but DO NOT start engine.
4. Wait at least 10 seconds for IAC valve pintle to extend and seat in throttle body. With ignition on, disconnect IAC valve electrical connector. Remove ground at ALDL connector.
5. Using an awl, pierce and apply leverage to remove minimum idle speed adjustment screw plug. Install tachometer, start engine, and allow engine to reach normal operating temperature. Allow engine RPM to stabilize.
6. Adjust idle RPM to specification with transmission in Neutral. See HOT (SLOW) IDLE SPEED RPM table. Turn ignition off. Remove tachometer. Reconnect IAC valve electrical connector. Install air cleaner, adapter, and gasket.

HOT (SLOW) IDLE SPEED RPM (1)

THROTTLE POSITION SENSOR (TPS)

Make jumper wires using Terminals No. 1214836 and 12014837. Connect digital voltmeter between TPS Dark Blue wire terminal and Black wire terminal using jumper wires. With ignition on and engine off, TPS voltage should be less than 1.25 volts. If more than specified, replace TPS. Remove voltmeter and jumper wires. Reconnect TPS connector.

Note. "SCAN" tester may also be used to perform TPS adjustment.

IDLE MIXTURE ADJUSTMENT

Idle mixture is controlled by Electronic Control Module (ECM) and is not adjustable.

COLD (FAST) IDLE RPM

Fast idle is controlled by Electronic Control Module (ECM) and is not adjustable.

BATTERY

BATTERY SPECIFICATIONS

SERVICE INTERVALS

REPLACEMENT INTERVALS

BELT ADJUSTMENT

All models use a single serpentine belt. Proper belt tension is maintained by an automatic tensioner.

CAPACITIES

COOLING SYSTEM CAPACITIES

ENGINE OIL CAPACITIES

TRANSMISSION & TRANSFER CASE CAPACITIES

A/C SYSTEM REFRIGERANT CAPACITIES

See also:
• IGNITION TIMING SPECIFICATIONS