Tune-Up - v8 GMC Magnavan G3500

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" series, and on a plate at upper left corner of dash on all other models.

VIN ENGINE CODES

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

5.0L & 5.7L

1. E-I-I-E-E-I-I-E (Both Banks, Front-to-Rear).

7.4L

1. E-I-E-I-E-I-E-I (Left Bank, Front-to-Rear).
2. I-E-I-E-I-E-I-E (Right Bank, 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 not available from manufacturer.

Ignition Coil - Carbureted Engine

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

Scheme 1

Ignition Coil - Throttle Body Injected Engines

1. Disconnect all electrical leads from coil. Connect ohmmeter in position "A". (Scheme 2) 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 should not be infinite. If it is, replace coil. Reconnect coil.

Scheme 2

Distributor

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

Firing Order & Distributor Rotation. Scheme 3

Carbureted Engine

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.

Throttle Body Injected Engines

1. 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. Ensure "SERVICE ENGINE SOON" light is off.
2. Disconnect EST by-pass Tan/Black stripe wire connector. On 5.0L and 5.7L engines, this connector is located near distributor. On 7.4L engine, this connector is located behind electrical junction box cover or inboard of bulkhead wiring connector.
3. Check and adjust ignition timing with engine at normal operating temperature, transmission in Drive (auto. trans.) or Neutral (man. trans.) and A/C off (if equipped). See «IGNITION TIMING SPECIFICATIONS»(/gmc/magnavan-g3500/1985-1993/remont/adjustments/#tune-up-v8) table. Reconnect Tan/Black stripe wire connector. Clear ACM trouble codes. See appropriate article in COMPUTERIZED ENGINE CONTROLS section.

IGNITION TIMING SPECIFICATIONS (DEGREES BTDC @ RPM)

1. Ensure no codes are displayed, idle air control system has been checked, and ignition timing is correct. See IDLE AIR CONTROL (IAC) SYSTEM CHECK chart in GENERAL MOTORS COMPUTER COMMAND CONTROL article in COMPUTERIZED ENGINE CONTROLS section.
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. See «CONTROLLED IDLE SPEED (RPM)»(/gmc/magnavan-g3500/1985-1993/remont/adjustments/#tune-up-v8) table. If within specification, idle speed is being correctly controlled by ECM.
4. If not within specification, go to "ROUGH, UNSTABLE OR INCORRECT IDLE; STALLING" TROUBLE SHOOTING in GENERAL MOTORS COMPUTER COMMAND CONTROL article in COMPUTERIZED ENGINE CONTROLS section.

CONTROLLED IDLE SPEED (RPM) (1)

Engine must be warm and choke fully open. Fast idle cam follower must be off fast idle cam. Turn idle speed screw to obtain specified RPM. See HOT (SLOW) IDLE SPEED RPM table.

Throttle Body Injected Engine

1. Check controlled idle speed and perform idle air control system check before adjusting hot idle. See IDLE AIR CONTROL (IAC) SYSTEM CHECK chart in GENERAL MOTORS COMPUTER COMMAND CONTROL article in COMPUTERIZED ENGINE CONTROLS section.
2. Remove air cleaner, adapter, and gasket. Ensure that throttle lever operates freely. With Idle Air Control (IAC) valve connected, ground terminal "B" to "A" at ALDL connector. (Scheme 4)
3. Turn ignition on, DO NOT start engine. 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.
4. Install tachometer, start engine, and check idle RPM. If incorrect, use an awl to pierce and remove minimum idle speed adjustment screw plug. With engine at normal operating temperature, allow engine RPM to stabilize.
5. Adjust idle RPM to specification with transmission in Drive (auto. trans.) or Neutral (man. trans.). See «HOT (SLOW) IDLE SPEED RPM»(/gmc/magnavan-g3500/1985-1993/remont/adjustments/#tune-up-v8) table. Turn ignition off. Remove tachometer. Reconnect IAC valve electrical connector. Install air cleaner, adapter, and gasket.

Scheme 4

HOT (SLOW) IDLE SPEED RPM (1)

THROTTLE POSITION SENSOR (TPS)

Note. "SCAN" tester may also be used to perform TPS adjustment. No throttle position sensor is used on 7.4L Carbureted engine.

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, check HOT (SLOW) IDLE SPEED RPM before replacing TPS. Remove voltmeter and jumper wires. Reconnect TPS connector.

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 is controlled by Electronic Control Module (ECM) on throttle body injected engines and is not adjustable.

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 5)
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 5

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 plugs (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. Check ignition timing and adjust if necessary.
3. Turn idle mixture screws in or out to set idle speed to 700 RPM. Readjust idle mixture screws to obtain highest idle speed. Check and adjust idle speed and fast idle speed. Turn off engine, remove tachometer, and install air cleaner. Remove block from drive wheels.

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

Note. Fast idle is controlled by Electronic Control Module (ECM) on throttle body injected engines and is not adjustable.

COLD (FAST) IDLE RPM

EMISSION CONTROL

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

FUEL SYSTEMS

CARBURETOR

TBI FUEL INJECTION

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

Relieving Fuel Pressure

Place transmission in Park (Neutral for manual transmissions) and apply parking brake. Loosen fuel filler cap to relieve tank pressure. The internal constant bleed feature on the TBI unit relieves fuel system pressure when engine is off.

Fuel Pressure Test

Relieve Fuel Pressure. Plug thermac vacuum port on TBI (if equipped). Disconect 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.

Note. The 7.4L (VIN W) carbureted engine uses a mechanical fuel pump.

FUEL PUMP SPECIFICATIONS

BATTERY

BATTERY SPECIFICATIONS

SERVICE INTERVALS

REPLACEMENT INTERVALS

BELT TENSION ADJUSTMENT

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

BELT ADJUSTMENT

CAPACITIES

ENGINE OIL CAPACITIES

COOLING SYSTEM CAPACITIES

AXLE, TRANSFER CASE & TRANSMISSION CAPACITIES

A/C SYSTEM REFRIGERANT CAPACITIES

See also:
• IGNITION TIMING SPECIFICATIONS