4.8l 6-cyl - VIN [t] GMC Cab & Chassis S15

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 CLEARANCE ADJUSTMENT

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 ADJUSTMENT table, adjust valves as follows: 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 1 full turn to complete procedure.

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 article in the ENGINE section.

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.

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. If equipped, disconnect air injection hose at check valve. Remove cylinder head.

1. Ensure gasket surfaces of cylinder head and block are clean, and all head bolt threads and threads in block are clean. Coat threads of head bolts with sealer.
2. To install cylinder head and remaining components, reverse removal procedure. Tighten head bolts in sequence. Tighten front left-hand head bolt to 85 ft. lbs. (115 N.m), and all others to 95 ft. lbs. (129 N.m). (Scheme 22) Lubricate rocker arm parts with Molykote. Adjust valves.

Cylinder Head Tightening Sequence. Scheme 22

VALVE ARRANGEMENT

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

ROCKER ARM STUDS

Rocker arm studs that are loose in head or have damaged threads, can be replaced with oversize studs. Use Reamer (J-5715) for .003" (.08 mm) oversize replacement studs, and Reamer (J-6036) for .013" (.33 mm) oversize replacement studs.

Remove damaged stud using Stud Remover (J-5802-01). Install remover over stud. Tighten nut to extract 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.

1. Remove valve cover. Remove spark plug, rocker arm assembly and push rod of cylinder to be serviced. Install air hose and adapter in spark plug hole, and apply air pressure. Do not remove air pressure until all components have been reinstalled.
2. Use valve spring compressor to compress valve spring, and remove retainer locks. Release spring compressor and remove spring retainer or rotator, shield, spring, damper (if equipped), and oil seal. (Scheme 23)

Exploded View of Valve Spring Assembly. Scheme 23

Install damper, spring, shield and retainer. Use spring compressor to compress valve spring, and install new oil seal and retainer locks. Remove spring compressor. Remove air hose and adapter. Install remaining components in reverse order of removal.

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 specifications, install shims to meet specifications.

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 GUIDE SERVICING

Valve guides are integral with cylinder head. If guide is worn, it must be reamed for installation of valves with oversize stems. Valves are available with .003" (.08 mm), .015" (.38 mm) and .030" (.76 mm) oversize stems. Use reamers in sequence when reaming valve guides.

HYDRAULIC VALVE LIFTERS

Disassemble and thoroughly clean lifters. Inspect all components for wear and damage. If any components are worn or damaged, complete lifter assembly must be replaced. If push rod seat or lifter body wear is noted, inspect mating engine components for wear. Replace lifters as complete assemblies; do not interchange parts between lifters. (Scheme 24)

Cutaway View of Valve Lifter Assembly. Scheme 24

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, then 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 25

1. Clean all gasket mating surfaces. Cut tabs from new oil pan front seal. Use sharp instrument to ensure clean cut. (Scheme 25) Install seal to front cover, pressing locating tips into holes in cover. (Scheme 25): Oil Pan Front Seal Modification
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 of cover with screwdriver, using 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 of cover with screwdriver, using 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 withdraw push rods in sequence, for reinstallation in original locations.
2. Remove valve lifter side cover and remove 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. Carefully remove camshaft.

Coat camshaft lobes with Lubriplate. Reverse removal procedure to install camshaft. Ensure timing marks on gears are aligned. Take care to avoid damage to camshaft lobes or bearings. Install new valve lifters. Remove camshaft thrust plate bolts through access holes in camshaft gear. (Scheme 26)

Timing Gear Mark Alignment. Scheme 26

Inspection

With engine front cover removed, check backlash between timing gear teeth. 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 with dial indicator. Maximum camshaft gear runout is .004" (.10 mm). Maximum crankshaft gear runout is .003" (.08 mm).

Remove camshaft from engine. Position camshaft and gear on 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. Properly support hub of camshaft gear, to avoid gear damage. (Scheme 27) Remove crankshaft gear from crankshaft, using Gear Puller (J-8105).

Removing Camshaft from Gear. Scheme 27

1. With crankshaft properly supported, use Hollow Driver (J-5590) to drive gear onto crankshaft.
2. To install camshaft gear, position 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 with feeler gauge. End play should be.003-.008" (.08-.20 mm). (Scheme 28)

Checking Camshaft. Scheme 28

CAM LOBE LIFT

1. With valve cover removed, remove rocker arm assemblies. Mount dial indicator on rocker arm stud. Position dial indicator and ball socket adapter on push rod. If readings are not within specifications, replace camshaft & lifters. (Scheme 29)
2. Slowly rotate engine in direction of rotation until lifter is on base circle of camshaft. Zero dial indicator. Rotate engine until push rod is fully raised. Record lobe lift reading and compare with specifications. If not within limits, replace camshaft and lifters.

Checking Camshaft Lobe Lift. Scheme 29

With engine removed from vehicle, 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, to act as guide for pilot of installer, and to center remaining bearings being pulled into place. Oil holes in cam bearings must align with oil holes in block. Install new camshaft rear plug.

OIL PAN

See OIL PAN REMOVAL article.

1. Remove oil pan, oil pump and cylinder head. Position piston at bottom of stroke, and cover with cloth to collect metal cuttings. Remove ridge at top of cylinder bore with ridge reamer.
2. If necessary, mark connecting rod for cylinder identification. Remove connecting rod cap nuts and cap, and cover rod bolts with rubber hose. Push piston and rod assembly out top of cylinder block.

1. Before installing piston and rod assembly, ensure ring gaps are properly spaced. (Scheme 30) Lightly coat pistons, rings and cylinder walls with engine oil. Install rod bearings in rod and cap, and lubricate with engine oil.
2. Compress piston rings with ring compressor. Do not allow position of rings to change. Cover connecting rod bolts with rubber hose. Install piston and rod assembly, with notch on top of piston facing front of engine. Install and tighten connecting rod cap.

FITTING PISTONS

1. Using telescopic gauge and micrometer, measure cylinder bore diameter 2 1/2" (64 mm) from top of bore. Measure piston diameter across piston skirt, at center line of piston pin. Difference between the two measurements is piston-to-cylinder bore clearance.
2. Using cylinder bore gauge, measure cylinder bore taper by working gauge up and down in bore. Measure cylinder bore out-of-round. Take measurements at different points in bore, by rotating gauge horizontally, around entire circumference of bore. Out-of-round must not exceed .002" (.05 mm).
3. Taper must not exceed .005" (.13 mm). If taper or out-of-round are not within limits, hone or bore cylinders for installation of new pistons.

FITTING RINGS

1. Position ring into cylinder bore about 1/4" above ring travel. Ring must be square in bore. Measure ring end gap with feeler gauge.
2. Before installing compression rings on pistons, check side clearance. Insert outer edge of ring in its respective groove, and slide ring around entire circumference of groove. Ring should slide freely in groove.
3. Check side clearance of compression rings, with feeler gauge inserted between ring and ring groove. Install rings with gaps properly positioned. Note that anti-rotation tang of oil ring spacer is inserted into oil hole (or slot) of piston. (Scheme 30)

Desired Ring Gap Locations. Scheme 30

Using arbor press and piston pin remover/installer, press piston pin from piston and connecting rod.

Check clearance of piston pin in piston, replace piston and pin assembly if not within limits. Lubricate piston pin holes in piston and connecting rod. Position connecting rod on piston and press in piston pin, using pin remover/installer and arbor press. Check piston for freedom of movement on piston pin.

CONNECTING ROD BEARINGS

Note. 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. During repairs, always replace both inserts as a set.

1. Ensure rod caps are marked for cylinder identification. Remove rod cap and bearings. Inspect bearings for wear or damage and replace as necessary. Check crankshaft rod bearing journal for out-of-round or taper. 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. If clearance is still excessive, crankshaft must be reconditioned.
3. Clean crankshaft journal and bearing surface in rod. Insert bearings in rod and cap, then lubricate bearing surfaces with oil. Pull piston and rod assembly down onto crankshaft. Install and tighten rod cap.

MAIN BEARINGS

Note. 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. During repairs, always replace both inserts as a set.

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 or identify main bearing cap before removing. Support crankshaft weight, using jack placed under counterweight next to bearing being checked. Loosen drive belts from crankshaft pulley, prior to checking No. 1 bearing.
2. Remove main bearing cap and place a piece of Plastigage across full width of bearing, about 1/4" off center, and away from oil holes. Install cap and tighten to specifications. Do not allow crankshaft to turn.
3. Remove cap and measure width of Plastigage with scale furnished. 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.
4. 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, cotter pin may be bent, as necessary, to do the job.
5. 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 (as used in removal), 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, then check crankshaft end play. Check end play by prying crankshaft forward, and inserting feeler gauge between crankshaft counterweight and forward face of rear main bearing cap.

THRUST BEARING ALIGNMENT

Ensure all other main bearing caps 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 31

1. Fabricate oil seal installer, if one is not supplied with replacement oil seal. (Scheme 31) 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 31): Rear Main Oil Seal Installer
2. Roll seal around crankshaft, using installer as "shoehorn" 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. For further information on cooling systems, see ENGINE COOLING SYSTEM article in the ENGINE section.

CRANKCASE CAPACITY

Crankcase capacity is 5 qts. (4.75L). Add 1 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. 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, timing Gears are lubricated by oil splash. (Scheme 32)

Engine Oiling System. Scheme 32

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 later reassembly of gears in same meshing position. Remove idler gear and drive gear with shaft, from pump body. Remove pressure regulator valve retaining pin, then remove spring and valve.

Oil Pump Assembly. Scheme 33

Note. If any part of oil pump requires replacement, entire pump assembly 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, and tap tube into place, using plastic hammer. 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 OVERHAUL PROCEDURES - GENERAL INFORMATION