Tune-Up - v6 GMC Cab & Chassis S15

ENGINE IDENTIFICATION

Engines can be identified by the eighth character of the Vehicle Identification Number (VIN). The VIN number is stamped on a plate attached to the top left side of dash.

ENGINE CODES

TUNE-UP NOTES

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

Due to late changes and corrections, always refer to Emission Control Label in engine compartment before attempting tune-up. In the event of a conflict between specifications given in this manual and label specifications, label specifications prevail.

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 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 system. Remove air cleaner. Open throttle and choke fully. Remove all spark plugs. 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. Twist spark plug boot 1/2 turn before removing. Using an ohmmeter, check resistance while gently twisting wire. If resistance is not within specification, or fluctuates from infinity to any value, replace cable.

HIGH TENSION WIRE RESISTANCE (OHMS)

2.8L

1. E-I-I-E-I-E (Left bank, front-to-rear).
2. E-I-E-I-I-E (Right bank, front-to-rear).

4.3L

1. E-I-E-I-I-E (Left bank, front-to-rear).
2. E-I-I-E-I-E (Right bank, front-to-rear).

VALVE CLEARANCE

All engines use hydraulic lifters.

VALVE CLEARANCE ADJUSTMENT

DISTRIBUTOR

Federal 4.3L models are equipped with Delco High Energy Ignition (HEI) distributors. All 2.8L models are equipped with Delco HEI-Electronic Spark Timing (HEI-EST) distributors. California models equipped with 4.3L carbureted engines are equipped with Delco HEI-EST distributors with Electronic Spark Control (ESC).

2.8L Firing Order & Timing Marks. Scheme 6

4.3L Firing Order & Timing Marks. Scheme 7

Federal & High Altitude 4.3L Models Only

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.
2. Check and adjust ignition timing with engine at normal operating temperature, distributor vacuum advance hose disconnected and plugged, and transmission in "D" (auto. trans.) or Neutral (man. trans.).

All California & 2.8L Federal Models

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. Disconnect 1-wire connector at distributor. DO NOT disconnect 4-wire connector at distributor.
2. Check and adjust ignition timing with engine at normal operating temperature and transmission in Park or Neutral. Reconnect 1-wire connector at distributor. Clear trouble code by removing ECM power fuse for 10 seconds.

IGNITION TIMING SPECIFICATIONS (DEGREES BTDC @ RPM)

HOT (SLOW) IDLE RPM (2.8L & 4.3L TBI)

Idle speed is not adjustable on TBI models. Idle air speed screw is preset and sealed at the factory. Idle speeds are automatically controlled. DO NOT attempt adjustment.

IDLE SPEED (RPM)

Note. Depending on engine size and vehicle application, either a vacuum operated throttle kicker assembly or an electric idle speed solenoid (ISS) is used.

Throttle Kicker

1. With engine at operating temperature and choke wide open, adjust idle speed screw to obtain specified curb idle speed. Apply 20 in. Hg vacuum to throttle kicker to fully extend plunger.
2. Manually open throttle slightly and then release against plunger. With plunger held inward, turn plunger in or out to obtain specified curb idle RPM. See Emission Control Label.

Idle Speed Solenoid (ISS)

1. With engine at operating temperature, choke wide open and ignition timing set, adjust curb idle speed to specification. If equipped with A/C, disconnect A/C compressor lead at compressor. With automatic transmission in Drive or manual transmission in Neutral, turn A/C on to energize solenoid.
2. Open throttle slightly to allow solenoid plunger to fully extend. Turn solenoid screw to set curb idle RPM to specification. See Emission Control Label.

IDLE MIXTURE

Note. Idle mixture adjustment is not a normal tune-up procedure. Idle mixture should be adjusted only if vehicle fails emissions testing or carburetor has been disassembled.

Carbureted Models

1. Remove carburetor from engine and drain fuel. Invert carburetor. Make 2 parallel cuts, one on each side of locator points, with a hacksaw. Cuts should not extend more than 1/8" beyond locator points. (Scheme 8)
2. Place a flat punch near end of saw marks. Hold punch at a 45° angle. Drive punch into throttle body until casting breaks away, exposing steel plug. Hold a center punch in a vertical position and drive it into plug. Now hold punch at a 45° angle, and drive plug out of casting.

Removing Idle Mixture Plugs. Scheme 8

Propane Enrichment Procedure 4.3L Federal Models

1. Warm engine to normal operating temperature and turn A/C off. Set parking brake and block drive wheels. Disconnect and plug vacuum advance hose. Connect tachometer to engine. Adjust timing and reconnect vacuum advance hose. Set idle speed.
2. Disconnect crankcase ventilation tube from air cleaner. Using Adapter (J-26911), insert hose with rubber stopper (from propane valve) into air cleaner snorkel. Ensure propane cartridge is in vertical position.
3. With engine idling in "D" (auto. trans.) or Neutral (man. trans.), open propane supply control valve slowly. Engine speed will increase. Add propane until engine speed begins to drop from over-richness. Note maximum enriched idle RPM.
4. If enriched idle speed is within specifications, idle mixture is correct. Proceed to step 8). If enriched idle speed is not correct, remove carburetor and remove idle mixture plugs.
5. Install carburetor on engine. Lightly seat idle mixture screws and then back out equally, just enough so that engine will run. Place transmission in "D" (auto. trans.) or Neutral (man. trans.).
6. Back out each screw 1/8 turn at a time until maximum idle speed is obtained. Set idle speed to enriched idle specification. Turn mixture screws in 1/8 turn at a time until idle speed reaches value given on Emission Control Label.
7. Recheck enriched idle speed with propane. If not within specification, repeat adjustment beginning at step 5). After adjustment is completed, seal idle mixture screw openings using RTV.
8. Check idle speed and fast idle speed, adjust if necessary. Turn off engine, remove propane adapter and connect crankcase ventilation tube. Unplug and reconnect vacuum hoses and install air cleaner.

PROPANE MIXTURE ADJUSTMENT (4.3L)

E4ME & E4MED MIXTURE ADJUSTMENT

Note. Idle mixture control plunger travel should be checked before mixture adjustment or disassembly.

Mixture Control Solenoid Plunger Travel

1. Hold plunger down against solenoid stop. Use Driver (J-28696-10 or BT-7928) to adjust lean stop screw, until solenoid plunger just contacts Gauging Rod (J-33815-1) when placed over throttle side metering jet rod guide.
2. Adjustment is correct when solenoid plunger is contacting solenoid stop and gauging rod at the same time. (Scheme 9)

Mixture Control Plunger Adjustment. Scheme 9

Note. Mixture must not be adjusted unless carburetor has been disassembled for cleaning, parts replacement or a systems performance check indicates that carburetor is cause of malfunction.

Idle Air Bleed Valve Adjustment

1. Install air bleed valve Gauging Rod (J-33815-2 or BT-8253-B) in throttle side "D" shaped vent hole in air horn casting. (Scheme 10) Upper end of gauge should be positioned over open cavity next to idle air bleed valve.
2. While holding gauging rod down lightly, so that solenoid plunger is against solenoid stop, adjust idle air bleed valve so that gauging rod will pivot over and just contact top of valve. No further adjustment is required.
3. Disconnect and plug canister purge hose at canister. Connect dwell meter. Place a.10-.75" (3-19 mm) spacer between accelerator pump lever and TPS plunger. Start engine and allow it to reach operating temperature. Check idle dwell. If not within 25-35°, idle plugs will have to be removed and mixture screws adjusted.

Positioning Idle Air Bleed Valve (Hold Gauge Down Lightly). Scheme 10

Adjusting Idle Mixture Screws

1. Turn each mixture screw clockwise until lightly seated. Back out mixture screws 3 turns. Warm engine to operating temperature. While idling in "D" (auto. trans.) or Neutral (man. trans.) adjust both mixture screws equally in 1/8 turn increments until dwell reading varies within 25-35° range, as close to 30° as possible.
2. If reading is too low, turn mixture screws counterclockwise. If reading is too high, turn screws clockwise. Allow time for readings to stabilize after each adjustment. When adjustments are completed, seal each screw with RTV sealer.

COLD (FAST) IDLE RPM

Note. Procedures listed are for carbureted engines with adjustable fast idle.

Warm up vehicle. Remove and plug EGR and purge control vacuum lines. Place fast idle screw on highest step of fast idle cam. Start engine. On ECM controlled engines, wait until engine enters closed loop operation. Adjust fast idle screw to obtain fast idle RPM.

FAST IDLE RPM

2.8L TBI

Connect digital voltmeter between TPS connector terminals "B" and "C" using jumper wires. With ignition on and engine stopped, TPS voltage should be .42-.56 volts. Rotate TPS to obtain correct voltage reading. Turn ignition off and remove jumper wires. Tighten TPS mounting screws. Recheck voltage reading to ensure sensor did not move.

Note. DO NOT remove TPS adjustment screw plug unless TPS is not adjusted correctly or if it is necessary to replace air horn assembly, float bowl, TPS sensor or TPS adjustment screw.

4.3L E4ME Calif. Only

1. Using a 5/64" drill, drill a 1/16" to 1/8" deep hole in aluminum plug covering TPS adjustment screw. Use care not to damage screw head. (Scheme 11)
2. Start a 1/2" long No. 8, self-tapping screw in drilled hole, turning screw in only far enough to ensure good thread engagement. Using a screwdriver between screw head and air horn, pry out plug.
3. Using TPS Adjuster (J-28696), remove TPS adjustment screw. Connect a digital voltmeter from TPS connector center terminal "B" to bottom terminal "C". With ignition on and engine stopped, install TPS adjustment screw to obtain specified voltage with A/C off and throttle at idle.
4. After adjustment, install new plug in air horn. If new plug is not available, use Delco Threadlock Adhesive X-10 on screw threads then adjust voltage, as in step 3). Clear trouble code memory after adjustment.

THROTTLE POSITION SENSOR SPECIFICATIONS

4.3L Calif. E4ME TPS Adjusting Screw. Scheme 11

AUTOMATIC CHOKE

All engines have nonadjustable chokes.

Federal 4.3L models are equipped with Delco High Energy Ignition (HEI) distributors. All 2.8L models are equipped with Delco HEI-Electronic Spark Timing (HEI-EST) distributors. California models equipped with 4.3L carbureted engines are equipped with Delco HEI-EST distributors with Electronic Spark Control (ESC).

DISTRIBUTOR PICK-UP COIL RESISTANCE

IGNITION COIL

IGNITION COIL RESISTANCE

EMISSION CONTROL

For information on emission control systems, see EMISSION section.

FUEL PUMP

FUEL PUMP SPECIFICATIONS

FUEL SYSTEMS

Some 4.3L models use a 4-Bbl. carburetor. All other models are equipped with Throttle Body Injection (TBI), using General Motors Model 220 throttle body.

CARBURETOR

BATTERY

BATTERY SPECIFICATIONS @ 0°F (-18°C)

STARTER

All engines are equipped with Delco overrunning clutch starters.

STARTER SPECIFICATIONS

ALTERNATOR

All engines are equipped with Delco alternators with integral voltage regulator.

ALTERNATOR SPECIFICATIONS @ 80°F (27°C)

ALTERNATOR REGULATOR

All engines are equipped with Delco nonadjustable voltage regulators, integral with alternator.

BELT ADJUSTMENT

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

BELT ADJUSTMENT

SERVICE INTERVALS

REPLACEMENT INTERVALS

CAPACITIES

FLUID CAPACITIES

SYSTEM REFRIGERANT CAPACITIES