4.8l 6-cyl - VIN [t] GMC Cab & Chassis R2500

ENGINE IDENTIFICATION

Note. For engine repair procedures not covered in this article, see ENGINE OVERHAUL PROCEDURES - GENERAL INFORMATION article in the GENERAL INFORMATION section.

Engine may be identified by eighth character of Vehicle Identification Number (VIN). VIN is stamped on metal tag attached to left side of dash, near windshield. Engine identification number is also stamped on machined pad, at right hand side of cylinder block, to rear of distributor bore.

ENGINE IDENTIFICATION CODES

Valve Arrangement

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

1. Remove distributor cap. Mark distributor housing at No. 1 and No. 6 rotor firing positions.
2. With distributor rotor in the positions designated in «VALVE CLEARANCE ADJUSTMENT»(/gmc/cab-chassis-r2500/1987-1987/remont/mechanical/#48l-6-cyl-vin-t) table, adjust valves. Loosen rocker arm adjusting nut until push rod lash (play) is felt. Tighten adjusting nut until all lash is removed. When lash has been removed, tighten adjusting nut one full turn.

VALVE CLEARANCE ADJUSTMENT

TROUBLE SHOOTING

Note. To trouble shoot engine mechanical components, see appropriate table in TROUBLE SHOOTING article in GENERAL INFORMATION.

ENGINE

See ENGINE REMOVAL - 6-CYL article in this section.

ENGINE OVERHAUL

Note. See ENGINE OVERHAUL PROCEDURES - GENERAL INFORMATION article in this section for detailed information on performing basic engine overhaul and checking procedures.

Removal

1. Disconnect negative battery cable. Remove air cleaner. Disconnect both throttle controls at bellcrank. Remove throttle return spring. Label and disconnect fuel line and vacuum hoses at carburetor.
2. Disconnect crankcase vent hose at rocker arm cover. Remove vapor hose at vapor canister. Disconnect exhaust pipe at flange. Remove manifold assembly attaching bolts and clamps. Remove intake and exhaust manifold assembly. For complete inspection and cleaning procedures see «ENGINE OVERHAUL PROCEDURES - GENERAL INFORMATION»(/gmc/cab-chassis-r2500/1987-1987/remont/engine/#engine-overhaul-procedures-general-information) article in this section.

Installation

Clean gasket surfaces. Place manifold in position with new gasket. Install and tighten clamps and bolts. Reverse removal procedure to complete installation.

1. Drain cooling system and disconnect upper radiator hose at engine. Remove intake and exhaust manifold assembly. Remove rocker arm cover. Remove rocker arm assemblies and push rods. Keep in sequence for reinstallation in original locations.
2. Disconnect and label all wires and vacuum hoses that may interfere with head removal. Disconnect air injection hose at check valve (if equipped). Remove cylinder head. For complete inspection and cleaning procedures refer to, in the GENERAL INFORMATION section, «ENGINE OVERHAUL PROCEDURES - GENERAL INFORMATION»(/gmc/cab-chassis-r2500/1987-1987/remont/engine/#engine-overhaul-procedures-general-information) article.

1. Ensure gasket surfaces are clean, and all head bolt threads and threads in block are clean. Coat head bolt threads with sealer.
2. To install cylinder head and remaining components, reverse removal procedure. Tighten head bolts in sequence. (Scheme 10) Lubricate rocker arm friction surfaces with Molykote. Adjust valves.

Cylinder Head Tightening Sequence Tighten front left-hand head bolt to 85 ft. lbs. (115 N.m), & all others to 95 ft. lbs. (129 N.m). Courtesy of General Motors. Scheme 10

ROCKER ARM STUDS

Use Reamer (J-5715) for .003" (.08 mm) oversize replacement studs. Use Reamer (J-6036) for .013" (.33 mm) oversize replacement studs.

Remove damaged stud with Stud Remover (J-5802-01). Install remover over stud. Tighten nut to remove stud from cylinder head.

Ream hole for oversize studs. Coat press-fit area of stud with hypoid axle grease. Drive rocker stud into place with Stud Driver (J-6880). When driver bottoms on head, stud is at correct height.

VALVE SPRING INSTALLED HEIGHT

Measure valve spring installed height from spring seat (or top of shim), to top of spring (or valve shield). Installed height should be 1.63-1.69" (41.4-43.0 mm). If height exceeds specification, install shims.

VALVE STEM OIL SEALS

"O" ring-type seals are used. Lightly coat seal with engine oil and install in lower groove of valve stem. Ensure seal is not twisted in groove.

VALVE GUIDES

Valves are available with .003" (.08 mm), .015" (.38 mm) and .030" (.76 mm) oversize stems. Use reamers in sequence when reaming valve guides.

1. Drain cooling system and remove radiator. Remove drive belt(s), fan and pulley. Remove crankshaft pulley and vibration damper. Remove oil pan-to-front cover attaching bolts and front cover-to-block attaching bolts.
2. Pull cover slightly forward to permit cutting of oil pan front seal. Cut oil pan seal flush with block at both sides of cover. Remove cover with attached portion of oil pan seal.

Scheme 11

1. Clean all gasket mating surfaces. Cut tabs from new oil pan front seal. (Scheme 11) Install seal to front cover, pressing locating tips into holes in cover.
2. Coat front cover gasket with sealer and position on cover. Apply 1/8" bead of RTV sealer to joint formed by oil pan and cylinder block. Install Oil Seal Aligner (J-23042-A) in front cover seal and install cover to cylinder block.
3. Install and partially tighten oil pan-to-cover bolts. Install front cover-to-cylinder block bolts. Remove Aligner and tighten bolts. To install remaining components, reverse removal procedure.

Removal (Front Cover Installed)

Remove vibration damper. Pry seal out with screwdriver. Use care not to damage cover seal surface.

Install new seal with open end toward inside of cover. Drive seal into place using Seal Driver (J-23042-A).

Removal (Front Cover Removed)

Pry seal out with screwdriver. Use care not to damage cover seal surface.

Install new seal with open end of seal toward inside of cover. Support cover at seal recess area. Using seal installer, drive seal into position.

1. Remove engine from vehicle. Remove valve cover and loosen all rocker arm nuts. Rotate rocker arms to side and remove push rods. Mark parts for reassembly reference.
2. Remove valve lifter side cover and lifters. Remove front engine cover and fuel pump. Align crankshaft and camshaft timing gear marks. Remove camshaft thrust plate bolts through access holes in camshaft gear. Remove camshaft. For complete camshaft checking procedure, see «ENGINE OVERHAUL PROCEDURES - GENERAL INFORMATION»(/gmc/cab-chassis-r2500/1987-1987/remont/engine/#engine-overhaul-procedures-general-information) article in this section.

Coat camshaft lobes with Lubriplate. To install camshaft, reverse removal procedure. Ensure timing marks on gears are aligned. Use care to avoid damage to camshaft lobes or bearings. Install new valve lifters. (Scheme 12)

Timing Gear Mark Alignment Remove camshaft thrust plate bolts through access holes in camshaft gear. Courtesy of General Motors. Scheme 12

Inspection

With engine front cover removed, check timing gear backlash. Backlash should be .004-.006" (.10-.15 mm) for new gears, and .004-.008" (.10-.20 mm) for used gears. Check crankshaft and camshaft gears for runout. Maximum camshaft gear runout is .004" (.10 mm). Maximum crankshaft gear runout is .003" (.08 mm). Refer to ENGINE OVERHAUL PROCEDURES - GENERAL INFORMATION article for timing gear backlash checking procedure.

Remove camshaft from engine. Position camshaft and gear on an arbor press, and properly support hub of gear. Ensure thrust plate is positioned so Woodruff key in shaft will not be damaged when camshaft is pressed out of gear. Press camshaft from gear. (Scheme 13) Remove crankshaft gear from crankshaft, using Gear Puller (J-8105).

Removing Camshaft from Gear Properly support hub of camshaft gear to avoid gear damage. Courtesy of General Motors. Scheme 13

1. With crankshaft properly supported, use Hollow Driver (J-5590) to drive gear onto crankshaft.
2. To install camshaft gear, install camshaft in press with camshaft supported at back of front journal. Place gear spacer ring and thrust plate over camshaft and install Woodruff key in keyway. Press gear onto camshaft until it bottoms on gear spacer ring.

CAMSHAFT END PLAY

Check camshaft end play. End play should be.003-.008" (.08-.20 mm). See ENGINE OVERHAUL PROCEDURES - GENERAL INFORMATION article to check camshaft end play. (Scheme 14)

Scheme 14

Remove camshaft, oil pan and oil pump. Drive camshaft rear plug from block. Drive camshaft bearings from engine. Remove front and rear bearings after center bearings have been removed.

Install front and rear bearings first. Oil holes in cam bearings must align with oil holes in block. Install new camshaft rear plug.

OIL PAN

See ENGINE OIL PAN REMOVAL - 6-CYL article in this section.

1. Remove oil pan, oil pump and cylinder head. Remove ridge at top of cylinder bore.
2. Mark connecting rods and caps. Remove connecting rod cap nuts and cap. Cover rod bolts with rubber hose and push piston and rod assembly out top of cylinder block.

1. Before installing piston and rod assembly, ensure ring gaps are properly spaced. (Scheme 15) Lightly coat pistons, rings and cylinder walls with engine oil. Install rod bearings and lubricate with engine oil. Compress piston rings with ring compressor.
2. Install piston and rod assembly, with notch on top of piston facing front of engine. Install and tighten connecting rod cap. See «ENGINE OVERHAUL PROCEDURES - GENERAL INFORMATION»(/gmc/cab-chassis-r2500/1987-1987/remont/engine/#engine-overhaul-procedures-general-information) for complete piston and rod installation procedure.

FITTING PISTONS

Ensure piston-to-cylinder bore clearance is within specification. Out-of-round should not exceed .002" (.05 mm). Taper must not exceed .005" (.13 mm). For bore measuring procedure, see ENGINE OVERHAUL PROCEDURES - GENERAL INFORMATION article in this section.

FITTING RINGS

1. Check ring end gap and ensure it is within specification. Check side clearance of compression rings. Ensure rings slide freely in groove.
2. Install rings with gaps properly positioned. Note anti-rotation tang of oil ring spacer is inserted into oil hole (or slot) of piston. Hole is located within arc of oil ring spacer gap "A". (Scheme 15)

Ring Gap Locations Insert tang of oil ring spacer in oil hole (or slot) of piston. Courtesy of General Motors. Scheme 15

CRANKSHAFT & ROD BEARINGS

Main and connecting rod bearings are precision insert type. Shims are not used for adjustment. During production, selective fitting of bearing inserts may be necessary to achieve close tolerances. Therefore, half of a standard bearing insert and half of a .001" (.03 mm) undersize bearing insert may be found on the same journal. Always replace both inserts as a set.

CONNECTING ROD BEARINGS

1. Ensure rod caps are marked for cylinder identification. Remove rod cap and bearings. Maximum crankshaft out-of-round or taper must not exceed .001" (.03 mm).
2. Check rod bearing clearance using Plastigage method. If clearance exceeds specifications, standard, .001" (.03 mm), or .002" (.05 mm) undersize bearings may be used in combination to produce correct clearance.
3. If clearance is still excessive, manufacturer recommends replacement of crankshaft. Manufacturer does not recommend refinishing of crankshaft. For complete rod bearing clearance measurement procedure, see «ENGINE OVERHAUL PROCEDURES - GENERAL INFORMATION»(/gmc/cab-chassis-r2500/1987-1987/remont/engine/#engine-overhaul-procedures-general-information) article.

MAIN BEARINGS

Some production crankshafts are ground to .009" (.23 mm) undersize at the assembly plant. They are identified as follows

1. Crankshaft counterweight of undersize journal, will be stamped on one side with the number "9", along with large spot of Light Green paint.
2. Main bearing cap will be painted Light Green on each side.

1. Mark main bearing caps before removal. Check main bearing clearance using Plastigage method. Standard, .001" (.03 mm) or .002" (.05 mm) undersize bearing halves may be used in combination to obtain correct clearance. Always replace both upper and lower bearing halves. For complete main bearing clearance measurement procedure, refer to «ENGINE OVERHAUL PROCEDURES - GENERAL INFORMATION»(/gmc/cab-chassis-r2500/1987-1987/remont/engine/#engine-overhaul-procedures-general-information) article in this section.
2. With exception of rear main bearing, main bearings are removed from cylinder block using Bearing Remover/Installer (J-8080). Insert remover/installer in crankshaft oil hole and rotate crankshaft clockwise. If bearing remover/installer is not available, a cotter pin may be used.
3. To remove rear main bearing from block, partially drive out bearing with drift. Use pair of pliers, with jaws taped, to hold bearing thrust surface to oil slinger. Rotate crankshaft to remove bearing.

1. Lubricate journal and bearings. Insert plain end of new bearing between crankshaft and notched side of block. Insert bearing remover/installer into crankshaft oil hole and rotate bearing into place. To install rear main bearing, use pliers to aid in installation.
2. Install lower bearing half into cap. Install and tighten main bearing caps with arrows pointing toward front of engine. Align thrust bearing and check crankshaft end play.

THRUST BEARING ALIGNMENT

Ensure all main bearing caps, except rear, have been properly tightened. Tighten rear main bearing cap to 10-12 ft. lbs. (14-16 N.m). Tap crankshaft rearward, then forward, using lead hammer. Tighten rear main bearing cap. Retighten all main bearing caps.

Remove rear main bearing cap and pry out old seal. Remove upper half of seal by tapping end with brass punch until seal protrudes enough to be removed with pliers.

Scheme 16

1. Fabricate oil seal installer, if one is not supplied with replacement oil seal. (Scheme 16) Coat seal lips with engine oil. Keep oil off of seal ends. Position installer between crankshaft and seal groove in cylinder block. Position seal between tip of installer and crankshaft. (Scheme 16): Rear Main Oil Seal Installer Installer may be supplied with replacement oil seal. Courtesy of General Motors.
2. Roll seal around crankshaft, using installer to protect seal from sharp corner of seal groove in cylinder block. Installer must remain in position until seal is positioned with both ends flush with block.
3. Install lower seal into bearing cap. Feed seal into cap using light pressure with thumb and finger. Apply sealer to cap-to- block mating surface of cylinder block, being careful to keep sealer off the seal split line. Install and tighten bearing cap.

Disconnect negative battery cable. Drain cooling system. Remove drive belts, fan and pulley from water pump. Remove lower radiator hose and heater hose from water pump. If necessary, remove alternator adjusting bracket from water pump. Remove water pump.

Clean all gasket mating surfaces. Install components in reverse order of removal procedure, using new gasket.

Note. Refer to, in this section, DRIVE BELT ROUTINGX7 article and/or X7LB T0013835ENGINE COOLING FAN article for any further information on cooling systems.

CRANKCASE CAPACITY

Crankcase capacity is 5 qts. (4.75L). Add one additional qt. (.95L) when replacing oil filter.

OIL FILTER

Replace oil filter at every other oil change, or more often under severe conditions.

NORMAL OIL PRESSURE

Normal oil pressure with engine at operating temperature should be 30-45 psi (2.1-3.2 kg/cm 2 ) at 1500 RPM.

OIL PRESSURE REGULATOR VALVE

Oil pressure regulator valve is located in pump body. Valve is nonadjustable.

ENGINE OILING SYSTEM

Gear-type pump provides full pressure lubrication. Oil drawn through pick-up screen is pressurized through pump and routed to oil filter. A by-pass valve allows oil flow to main gallery in the event backpressure is encountered at filter.

Main gallery supplies oil to camshaft bearings, lifters and main bearings. Connecting rod bearings are supplied oil from crankshaft main bearings, through cross-drilled passages.

Oil passing through hollow push rods lubricates valve train. Oil drains back to crankcase through drain holes. Piston, piston pin and timing gears are lubricated by oil splash. (Scheme 17)

Scheme 17

Remove oil pan. Remove oil inlet tube attaching bolt and oil pump attaching bolts. Remove oil pump and tube assembly.

Disassembly

Remove oil inlet tube assembly. Remove pump cover. Mark idler gear and drive gear at a meshing point for reassembly reference. Remove idler gear and drive gear with shaft from pump body. Remove pressure regulator valve retaining pin, spring and valve.

Oil Pump Assembly Be sure to mark gears at meshing point. Courtesy of General Motors. Scheme 18

Note. If any part of oil pump requires replacement, entire pump must be replaced.

1. Clean all parts. Inspect pump body and cover for cracks and excessive wear. Inspect pump gears for damage or excessive wear. Check drive gear shaft for looseness in pump body.
2. Inspect oil inlet tube assembly for damage. Check pressure regulator valve for fit in bore.

Reassembly

Install oil inlet tube assembly. Apply sealer to end of tube. Using plastic hammer, tap tube into place. Install idler gear into pump body with smooth side of gear toward cover opening. Reassemble remaining components in reverse order of disassembly.

Prime oil pump with engine oil prior to installation. Install oil pump and pick-up tube in reverse order of removal.

TORQUE SPECIFICATIONS

TORQUE SPECIFICATIONS

GENERAL ENGINE SPECIFICATIONS

GENERAL SPECIFICATIONS

VALVE SPECIFICATIONS

VALVE SPECIFICATIONS

PISTON/PIN/RING SPECIFICATIONS

PISTONS, PINS & RINGS

BEARING SPECIFICATIONS

CRANKSHAFT MAIN & CONNECTING ROD BEARINGS SPECIFICATIONS

VALVE SPRING SPECIFICATIONS

VALVE SPRINGS SPECIFICATIONS

CAMSHAFT SPECIFICATIONS

CAMSHAFT SPECIFICATIONS

See also:
• ENGINE REMOVAL - 6-CYL
• ENGINE OVERHAUL PROCEDURES - GENERAL INFORMATION
• ENGINE OIL PAN REMOVAL - 6-CYL
• DRIVE BELT ROUTINGX7 article and/or X7LB T0013835ENGINE COOLING FAN
• VALVE CLEARANCE ADJUSTMENT