My 1990 nissan sentra has a noisy timing chain when running idle.

Here's a link to removing cover. Repair Guides Engine Mechanical Timing Cover And Seal AutoZone com Here's how to replace your chain. Repair Guides Engine Mechanical Timing Chain AutoZone com

Instructions

1. Remove Your Timing Chain
   • 1 Check your owner's manual for the recommended mileage for replacing your timing chain. Replacement for a Nissan Altima is 60,000 miles.
     
   • 2 Locate the timing chain. Replace it if the automatic chain adjuster (also known as the tensioner) is damaged or worn.
     
   • 3 Drain the cooling system and disengage the negative battery cable. Undo the spark plug wires and set the No. 1 piston at "Top Dead Center" (TDC) of the compression stroke.
     
   • 4 Disconnect the engine undercover, vacuum and fuel hoses, harness and connectors, drive belts, power steering reservoir, alternator and bracket, upper radiator hose, air duct, front exhaust tube, intake manifold collector supports, collector and exhaust manifold and the distributor.
     
   • 5 Prop up the oil pan and take out the front engine mounting, the valve cover, camshaft sprockets, cam bearing caps, camshafts, cylinder head bolts (reverse the installation order), cam sprocket cover, upper chain tensioner, chain guides and chain, idler sprocket bolt, cylinder head, intake manifold and cylinder head gasket and the steel oil pan.
     
   • 6 Place a steel cutter between the steel and aluminum oil pans. Slip the cutter around the oil pan's edge. Disengage the steel oil pan, baffle plate, oil strainer and the front tube.
     
   • 7 Secure the car on jack stands and place a seal cutter in between the oil pan and cylinder block. Slip the cutter around the oil pan's edge.
     
   Install Your Timing Chain
   • 8 Reconnect the crankshaft sprocket, oil pump drive spacer, idler sprocket and lower timing chain. Position the lower timing chain on the sprockets and line up the mating marks.
     
   • 9 Rub on an unbroken bead of liquid gasket and put in a new front cover and oil seal. Secure the bolt at 105 to 112 foot pounds (142 to 152 Nm).
     
   • 10 Eliminate any old material from the pan and cylinder block mounting surfaces. Rub on a 3.5 to 4.5mm bead of liquid gasket over the oil pan and cylinder block.
     
   • 11 Attach the upper timing chain to the idler sprockets and secure the cylinder head bolts.
     
   • 12 Refill the cooling system and add clean oil in the engine.
     
   • 13 Hook up the negative battery cable and any other disconnected parts or items. Start the engine and look for leaks.
     

Tips & Warnings

• Procedures for replacing timing chains in Nissan Altimas vary slightly depending on engine size.
  
• Make sure the new timing chain matches the chain you are removing.
  

• It's a timing chain not a belt.

• This diagram shows how to position the lower timing chain so that the mating marks line up.

• Lower timing chain tensioner and timing chain guide positioning.
  

Double check the mating mark alignment of the upper timing chain and idler sprocket.

• Make sure that the upper timing chain and camshaft sprocket mating marks align.
  

• There are the diagrams you requested.
• Hope this helps...

• 
  

• It's a timing chain not a belt.

• This diagram shows how to position the lower timing chain so that the mating marks line up.

• Lower timing chain tensioner and timing chain guide positioning.
  

Double check the mating mark alignment of the upper timing chain and idler sprocket.

• Make sure that the upper timing chain and camshaft sprocket mating marks align.
  

• There are the diagrams you requested.
• Hope this help.

Check your owner's manual for the recommended mileage for replacing your timing chain. Replacement for a Nissan Maxima is 60,000 miles.2 Locate the timing chain. Replace it if the automatic chain adjuster (also known as the tensioner) is damaged or worn.3 Release the pressure in the fuel system, drain the cooling system and disengage the negative battery cable. 4 Undo the upper radiator hose, engine drive belts, power steering pulley and pump with bracket, air duct, passenger front wheel and engine side and undercovers, front exhaust pipe, cylinder head front mounting bracket and head cover, rocker cover, distributor and distributor cap and spark plugs.5 Take off the intake manifold support and move the No. 1 piston at "Top Dead Center" (TDC) of the compression stroke.6 Disconnect the cylinder head front cover, water pump pulley, thermostat housing, upper and lower tensioner and slack side timing chain guide, idler sprocket bolt, camshaft sprocket bolts, sprockets, mounting caps and camshafts. 7 Disengage the cylinder head with the manifolds, idler sprocket shaft, upper timing chain, center crossmember, oil pan and strainer assembly, crankshaft pulley, engine front mount and bracket and the front timing cover. Remove the oil seal and unfasten the timing chain cover, idler and crankshaft sprockets and the oil pump drive spacer. 8 Unfasten the timing chain guide. install Your Timing Chain 1 Grease the oil seal lip with engine oil and place it in the front cover. Verify that the No. 1 piston is at "Top Dead Center" (TDC) of the compression stroke. 2 Reconnect the crankshaft sprocket (marks should point toward the engine's front), oil pump drive spacer and timing chain guide and the lower timing chain. Line up all mating marks.3 Hook up the crankshaft sprocket and pulley, lower timing chain, front cover assembly, engine mount and front mounting bracket, oil strainer and pan assembly and center crossmember.4 Reattach the upper timing chain, idler sprocket (to the back side), cylinder head assembly, idler sprocket bolt and exhaust and intake camshafts. Place the intake camshaft knock pin at 9 o'clock and the exhaust pin at noon. Connect the camshaft bearing caps and distributor brackets and secure all mounting bolts.5 Eliminate any old material from the pan and cylinder block mounting surfaces. Rub on a 3.5 to 4.5mm bead of liquid gasket over the oil pan and cylinder block.6 Attach the upper timing chain to the idler sprockets and secure the cylinder head bolts.7 Refill the cooling system. Hook up the negative battery cable and any other disconnected items. Start the engine and look for leaks.

Here you go This will help you

• Here are the diagrams you requested.

You want to wipe off the links of the lower timing chain next to the timing marks on the sprocket and put paint marks on the timing chain matching them with the timing marks on the idler sprocket and crankshaft sprocket.

Regards, Tony

the nissan sentra has a timing chain which should last the entire life of the car and should not require changing

Have you checked the oiler supply to the timing chain and the chain tensioner Just a thought. The guides usually only break from the lack of tension on the chains.