Jeep Cherokee (XJ): Description and operation

Engine

DESCRIPTION

The 2.5 liter (150 CID) four-cylinder engine is an In-line, lightweight, overhead valve engine.

This engine is designed for unleaded fuel. The engine cylinder head has dual quench-type combustion chambers that create turbulence and fast burning of the air/ fuel mixture. This results in good fuel economy.

The cylinders are numbered 1 through 4 from front to rear. The firing order is 1-3-4-2 (Fig. 1).

The crankshaft rotation is clockwise, when viewed from the front of the engine. The crankshaft rotates within five main bearings and the camshaft rotates within four bearings.

The engine Build Date Code is located on a machined surface on the right side of the cylinder block between the No.3 and No.4 cylinders (Fig. 2).

Fig. 1 Engine Firing Order
Fig. 1 Engine Firing Order

The digits of the code identify:

  •  1st Digit-The year (8 = 1998).
  •  2nd & 3rd Digits-The month (01 - 12).
  •  4th & 5th Digits-The engine type/fuel system/ compression ratio (HX = A 2.5 liter (150 CID) 9.1:1 compression ratio engine with a multi-point fuel injection system).
  •  6th & 7th Digits-The day of engine build (01 - 31).

FOR EXAMPLE: Code * 801HX23 * identifies a 2.5 liter (150 CID) engine with a multi-point fuel injection system, 9.1:1 compression ratio and built on January 23, 1998.

Fig. 2 Build Date Code Location
Fig. 2 Build Date Code Location

1 - NO. 4 CYLINDER
2 - NO. 3 CYLINDER
3 - DAY
4 - MONTH
5 - YEAR
6 - MACHINED SURFACE

Lubrication system

DESCRIPTION

A gear-type positive displacement pump is mounted at the underside of the block opposite the No. 4 main bearing.

OPERATION

The pump draws oil through the screen and inlet tube from the sump at the rear of the oil pan. The oil is driven between the drive and idler gears and pump body, then forced through the outlet to the block. An oil gallery in the block channels the oil to the inlet side of the full flow oil filter. After passing through the filter element, the oil passes from the center outlet of the filter through an oil gallery that channels the oil up to the main gallery which extends the entire length of the block.

Galleries extend downward from the main oil gallery to the upper shell of each main bearing. The crankshaft is drilled internally to pass oil from the main bearing journals (except number 4 main bearing journal) to the connecting rod journals. Each connecting rod bearing cap has a small squirt hole, oil passes through the squirt hole and is thrown off as the rod rotates. This oil throwoff lubricates the camshaft lobes, distributor drive gear, cylinder walls, and piston pins.

The hydraulic valve tappets receive oil directly from the main oil gallery. Oil is provided to the camshaft bearing through galleries. The front camshaft bearing journal passes oil through the camshaft sprocket to the timing chain. Oil drains back to the oil pan under the number one main bearing cap.

The oil supply for the rocker arms and bridged pivot assemblies is provided by the hydraulic valve tappets which pass oil through hollow push rods to a hole in the corresponding rocker arm. Oil from the rocker arm lubricates the valve train components, then passes down through the push rod guide holes in the cylinder head past the valve tappet area, and returns to the oil pan.

Oil Lubrication System-2.5L Engine
Oil Lubrication System-2.5L Engine

1 - CAM/CRANK MAIN GALLERY (7)
2 - TAPPET GALLERY
3 - TAPPET GALLERY
4 - CAMSHAFT BEARING
5 - NUMBER 1 CAMSHAFT BEARING JOURNAL
6 - CAMSHAFT SPROCKET
7 - TAPPET GALLERY

Cylinder block

DESCRIPTION

The cylinder block is a cast iron inline four cylinder design. The cylinder block is drilled forming galleries for both oil and coolant.

Fig. 3 Cylinder Block-2.5L
Fig. 3 Cylinder Block-2.5L

1 - CYLINDER BLOCK

Cylinder head

DESCRIPTION

The cylinder head is made of cast iron containing eight valves made of chrome plated heat resistant steel, valve stem seals, springs, retainers and keepers.

The cylinder head, valve seats and guides can be resurfaced for service purposes.

The cylinder head uses dual quench-type design combustion chambers which cause turbulence in the cylinders allowing faster burning of the air/fuel mixture, resulting in better fuel economy.

The valve guides are integral to the cylinder head, They are not replaceable. However, they are serviceable.

Crankshaft

DESCRIPTION

The crankshaft is constructed of nodular cast iron

Fig. 4 Cylinder Head
Fig. 4 Cylinder Head

1 - CYLINDER HEAD BOLTS
2 - CYLINDER HEAD GASKET
3 - CYLINDER HEAD

Fig. 5 Crankshaft-Typical
Fig. 5 Crankshaft-Typical

1 - MAIN BEARING JOURNALS
2 - COUNTER BALANCE WEIGHTS

Piston and connecting rod

DESCRIPTION

The pistons are made of a high strength aluminum alloy, the piston skirts are coated with a solid lubricant (Molykote) to reduce friction and provide scuff resistance. The connecting rods are made of cast iron.

Fig. 6 Piston and Connecting Rod Assembly
Fig. 6 Piston and Connecting Rod Assembly

Camshaft

DESCRIPTION

The camshaft is made of cast iron with eight machined lobes and four bearing journals.

Fig. 7 Camshaft-Typical
Fig. 7 Camshaft-Typical

1 - CAMSHAFT
2 - LOBES
3 - BEARING JOURNAL

OPERATION

When the camshaft rotates, the lobes actuate the tappets and push rods forcing upward on the rocker arms which applies downward force on the valves.

Rocker ARM

DESCRIPTION

The rocker arms are made of stamped steel and have a operational ratio of 1.6:1.

Fig. 8 Rocker Arms-Typical
Fig. 8 Rocker Arms-Typical

1 - CAPSCREWS
2 - BRIDGE
3 - PIVOT ASSEMBLY
4 - PUSH RODS
5 - ROCKER ARMS

OPERATION

When the push rods are forced upward by the camshaft lobes the push rod presses upward on the rocker arms, the rocker arms pivot, forcing downward pressure on the valves forcing the valves to move downward and off from their seats.

Valves

DESCRIPTION

The valves are made of heat resistant steel and have chrome plated stems to prevent scuffing. All valves use three bead lock keepers to retain the valve spring and promote valve rotation (Fig. 9).

Valve spring

DESCRIPTION

The valve springs are made of high strength chrome silicon steel. The springs are common for both intake and exhaust valves.

Fig. 9 Valve and Keeper Configuration 2.5L Engine
Fig. 9 Valve and Keeper Configuration 2.5L Engine

1 - VALVE LOCKS (3-BEAD)
2 - RETAINER)
3 - VALVE STEM OIL SEAL)
4 - INTAKE VALVE)
5 - EXHAUST VALVE)
6 - VALVE SPRING

Cylinder head cover

DESCRIPTION

The cylinder head cover is made of die cast aluminum and incorporates the Crankcase Ventilation (CCV) Hoses and the oil fill opening.

Hydraulic tappet

DESCRIPTION

Valve lash is controlled by hydraulic tappets located inside the cylinder block, in tappet bores above the camshaft.

Valve guide

DESCRIPTION

The valve guides are integral to the cylinder head, They are not replaceable. However, they are serviceable.

Fig. 10 Cylinder Head Cover
Fig. 10 Cylinder Head Cover

1 - AIR INLET FITTING
2 - CYLINDER HEAD COVER
3 - FIXED ORIFICE FITTING

Oil pan

DESCRIPTION

The oil pan is made of stamped steel. The oil pan gasket is a one piece steel backbone silicone coated gasket.

Fig. 11 Oil Pan
Fig. 11 Oil Pan

1 - OIL PAN
2 - OIL PAN DRAIN PLUG

Valve stem seal

DESCRIPTION

The valve stem seals are made of rubber and incorporate a garter spring to maintain consistent lubrication control (Fig. 9).

Intake manifold

DESCRIPTION

The intake manifold is made of cast aluminum and uses seven bolts to mount to the cylinder head. This mounting style improves sealing and reduces the chance of leaks.

Fig. 12 Intake and Exhaust Manifo
Fig. 12 Intake and Exhaust Manifo

Exhaust manifold

DESCRIPTION

The exhaust manifold is log style and is made of high silicon molybdenum cast iron. The exhaust manifold shares a common gasket with the intake manifold.

The exhaust manifold also incorporates a ball flange outlet for improved sealing and strain free connections (Fig. 12).

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