Tune-Up - Diesel GMC Cab & Chassis R2500

ENGINE IDENTIFICATION

Engines can be identified by the eighth character of Vehicle Identification Number (VIN) which is stamped on a tag at the top left corner of instrument panel. The 6.21, engine is also identified by code letters, located on a label at front and rear of right valve cover and also stamped into block on left front corner.

ENGINE CODES

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.

Adjustment of injectors or internal adjustment of injection pump must be done in a properly equipped injector shop with clean environment.

Prior to checking compression, be sure battery is fully charged to avoid battery run down. When turning engine over during test, 6 strokes per cylinder should be used to obtain reading.

ENGINE COMPRESSION

Note. Do not add oil to cylinders during compression check as extensive engine damage will result.

Remove air cleaner. Disconnect wire from fuel injection pump solenoid terminal. Disconnect glow plug wiring, and remove all glow plugs. Use Compression Gauge (J-26999-10) to test individual cylinders.

COMPRESSION SPECIFICATIONS

FUEL INJECTION

All 6.2L engines are equipped with General Motors diesel fuel injection.

FUEL PUMP

All models use a mechanical fuel pump mounted on front right side of engine to deliver fuel to fuel injection pump on engine. Prior to testing, disconnect Pink wire for fuel injection pump electric shut-off solenoid.

FUEL PUMP SPECIFICATIONS

VALVE ARRANGEMENT

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

VALVE CLEARANCE

The 6.2L engine uses hydraulic lifters with roller followers. Valve lash is not adjustable.

GLOW PLUGS

Glow plugs are small 6-volt heaters operated by an electronic relay. They cycle on and off, powered by 12 volts to give rapid heating. Glow plug light on dash should glow as plugs cycle on and off.

If test light is connected to glow plugs and ground, it should flash on and off. Relay can be heard clicking on and off after ignition has been on for approximately 25 seconds.

6.2L Firing Order. Scheme 14

INJECTION PUMP TIMING

1. Check alignment of injection pump timing marks on top of engine front cover and injection pump flange (half circles on California models and scribe marks on Federal models).
2. If timing marks are not aligned, loosen 3 retaining nuts, and align mark on injection pump with mark on front cover. Tighten nuts to 30 ft. lbs. (41 N.m) and adjust throttle linkage.

Scheme 15

1. Set parking brake and block drive wheels. Warm engine to normal operating temperature and install Tachometer (J-26925). Adjust low idle speed screw on pump to obtain curb idle speed. (Scheme 15)
2. Remove connector from fast idle solenoid. Run an insulated jumper wire from battery positive terminal to solenoid terminal to energize solenoid. Open throttle to ensure plunger is fully extended.
3. Adjust fast idle speed by turning plunger hex head. Turn engine off. Remove jumper wire and test equipment. Install fast idle solenoid connector.

IDLE SPEED

VACUUM PUMP

The vacuum pump is designed to aid the engine in maintaining a proper vacuum level for the power brake system and other vacuum operated accessories. The vacuum pump is a diaphragm type pump which requires no periodic maintenance.

Vacuum pumps on "C", "K" & "P" series engines are gear-driven. Gear-driven pump is driven by a cam inside drive assembly to which it mounts. Drive housing assembly has a drive gear on lower end, which meshes with cam gear in the engine. Drive gear causes cam in drive housing to rotate.

Vacuum pumps on "G" series engines are belt-driven. Belt-driven pump is mounted at right front of engine with a pulley attached which is driven by the generator belt. Vacuum pump is replaced as an assembly with the exception of the pulley.

Scheme 16

1. Connect vacuum gauge to vacuum pump inlet. If equipped, disconnect outlet hose from outlet tube on pump and plug end of hose. Do not plug vacuum pump outlet tube. With engine idling, vacuum should reach 21 in. Hg at sea level in 30 seconds. (Scheme 16)
2. If pump is okay, leak is in system other than at pump. If belt driven, check belt tension and pulley fit to shaft. If reading is low or fluctuates, check gauge and connections for leaks. Check idle RPM. If okay, go to step 3). If vacuum is still low, replace pump.
3. Reconnect vacuum hose with a "T" and vacuum gauge located near pump inlet. With engine idling, there can be a maximum of 3 in. Hg vacuum less than reading measured in step 1). If vacuum is okay, problems are not within vacuum system. If vacuum is low, check all attaching hoses and accessories for leaks.

INJECTION NOZZLES

Note. Perform all tests separately. If injector nozzle passes all tests, nozzle holder assembly can be reused. If injector nozzle fails any one test, replace complete nozzle holder assembly.

Opening Pressure Test

1. Using a nozzle tester and Test Oil (ISO 4113) at room temperature, check opening pressure. Connect test line to nozzle holder assembly and tighten fittings.
2. Close shutoff valve at pressure gauge and operate tester handle sharply several times. Check for proper nozzle position. Spray should be injected into a clean container.
3. Open shutoff valve at pressure gauge 1/4 turn. Operate tester handle slowly to determine injection opening pressure. Observe gauge reading just before oil is sprayed from tip. Minimum opening pressure is 1500 psi (105 kg/cm 2 ).

Leakage Test

Open shutoff valve at pressure gauge an additional 1/2-1 1/2 turns. Dry injector nozzle tip with compressed air. Apply 1400 psi (98 kg/cm 2 ) pressure to nozzle. Observe injector tip. Tip should remain dry. A drop is allowed to form but not drop off within a period of 10 seconds for injector to be performing properly.

Note. Some injectors may chatter louder than others. As long as there is chatter, injector is acceptable.

Chatter Test

Close shutoff lever at pressure gauge. Slowly depress test stand lever and note whether chatter noise can be heard. Injector should chatter. If there is no chatter, move lever faster until injector chatters. The injector may make "hissing" or "squealing" sound, instead of chatter, this is also acceptable. Replace injectors that do not chatter.

Spray Pattern Test

Due to design of injector, spray pattern test may not be accurate. Do not replace injector for spray pattern test failure.

Throttle Position Switch (TPS)

1. Loosely assemble throttle position switch to fuel injection pump with throttle lever in closed position. Disconnect throttle position switch connector. Attach an ohmmeter between ignition Pink wire and EGR Yellow wire of TPS switch.
2. Insert proper "switch-closed" Gauge Block (J-33043), between gauge boss on injection pump and wide open stop screw on throttle shaft. Rotate and hold throttle lever against gauge block.
3. Rotate throttle lever and hold wide open throttle stop screw against gauge block. Rotate TPS until there is continuity between terminals. Hold switch body at this position and tighten mounting bolts to 53 INCH lbs. (6 N.m). NOTE: Switch point must be set only while rotating switch body in clockwise direction.
4. Release throttle lever and allow it to return to idle position. Remove "switch-closed" gauge block and insert "switch-open" gauge block. Rotate throttle lever against "switch-open" gauge block. There should be no continuity between terminals.
5. If no continuity exists, switch is set properly. However, if there is continuity, then switch must be reset by returning to step 1) and repeating entire procedure. (Scheme 17)

Scheme 17

TPS GAUGE BLOCK SPECIFICATIONS

Heavy Duty Emissions (VIN J)

1. Loosen vacuum regulator valve so that it is free to rotate on fuel injection pump. Connect vacuum pump and apply 18-21 in. Hg vacuum to bottom port.
2. Attach vacuum gauge to top vacuum port. Insert Gauge Block (J-33043), between gauge boss on injection pump and wide open throttle valve stop screw on throttle lever (switch on position).
3. Rotate throttle shaft and hold it against gauge block. Slowly rotate vacuum regulator valve clockwise (facing valve) until vacuum gauge reads 11-12 in. Hg vacuum. Hold valve body in this position, and tighten mounting screws to 54 INCH lbs. (6 N.m).
4. Valve must be set while rotating valve body in a clockwise direction only. Check by releasing throttle shaft, allowing it to return to idle stop position. Rotate throttle shaft back against gauge bar and determine if vacuum gauge reading is 11-12 in. Hg vacuum. If vacuum reading is outside specifications, reset valve.
5. Confirm detent switch point by reading resistance between ignition terminal Pink/Black wire and detent terminal Orange wire. Insert Switch Gauge (J-36142) between wide open stop pin and wide open stop screw on throttle lever.
6. Hold throttle at WOT. Meter should show no continuity. Release throttle lever. Remove gauge. Slowly rotate throttle to WOT. Meter should show continuity just before throttle reaches WOT. If specifications are incorrect, repeat entire procedure.

EMISSION CONTROL

See EMISSIONS section.

BATTERIES

All models use Delco "Freedom" type batteries. All V8 diesel vehicles use two 12-volt, negative ground, sealed-top batteries. One battery is located on each side of engine compartment and they are wired in parallel.

BATTERY SPECIFICATIONS

STARTER

STARTER SPECIFICATIONS

ALTERNATORS

On 6.2L, alternator supplies current to both batteries. There are no switches or relays in charging circuit.

ALTERNATOR SPECIFICATIONS

ALTERNATOR REGULATOR

All models are equipped with Delco nonadjustable regulators, integral with alternator.

REGULATOR SPECIFICATIONS

BELT ADJUSTMENTS

BELT ADJUSTMENT (TENSION IN LBS. (KG) USING STRAND TENSION GAUGE)

SERVICE INTERVALS

REPLACEMENT INTERVALS

CAPACITIES

FLUID CAPACITIES

SYSTEM REFRIGERANT CAPACITIES