I have knocking under my vavle cover what could be the problem

a diagram of what ?
if you read your post you will see what I mean

Lots of early Nissans had a fuel feed pipe problem The plastick pipe split causing fuel starving engine missing

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uunbolt the vavle cover ,pull vavle cover off replace gasket put back in reverse order. just look to see what migth have to be took off to get cover off.

it located on the side of block or cylinder head it detects "spark knock" . the pcm will adjust the timing if spark knock is detected . other words some gas has least octane so if engine is put in strain it will adjust the timing for more power and keep engine from detonation(burnin vavle or piston)

First off you need to disconnect the battery. then you will need to take the plug wires off be sure to number them per cylinder. NOTE on the passenger side you have cylinder starting from the front of the engine 1-2-3 3 being at the back of the engine. on drivers side you have 4-5-6 6 being at the back of the engine. firing order is clockwise- 1-4-2-5-3-6.. then you will have vacuum lines that have to come off so you would need to mark those as well as to where they go, you can use masking tape and a fine point black marker. You must take out the altenator for better clearance on the passenger side and you may have to loosen the distributor to have clearance because of the ignition module if it is in the way. most hoses and fuel lines can be lifted up with a bungie cord and connected to the hood for getting those out of the way. the biggest pain is the wiring harness for the engine that runs through the pentium you can wiggle it enough to bungie it up out of the way. you may also find that you have to remove the heater hoses also since that will give you more clearance on the passenger side. the bolts are a 10 mm they also should have a spreader on them that you must look for under the bolt very important to put these back on. carefully lift and remove the vavle covers. clean all gasket and material off of the engine and on the vavle covers you want them clean enough to eat off of.. check vavle covers where the bolts attach to see if the vavle cover is straight if not then use a board under the vavle cover lip to tap the holes back down from the underneath side to get them straight. once all this is done then you need to get i suggest the cork felpro gaskets and some black sealant for valve covers. once the vavle covers are clean and ready for install then coat the valve cover where the gasket goes with sealant and let it setup- very important. then coat the gasket on one side and put it on the valve cover the valve covers have cutouts to where the tabs on the gaskests go then install on the valve cover. then coat the gasket again and let it setup for awhile very important. then reinstall the valve covers using a criscross pattern staring from the center out then tighten all bolts the same . very important do not over tighten the bolts just snug them up otherwise it will leak-reason the valve covers will warp by the holes.. and let the vehicle sit for about 12 hrs to make sure the silicone is set up.. it is a good weekend project did my fathers took about 2 days sat-sun but never leaked since..

if i sounds like lifter noise try adding lucas oil treatment

They are Located UNDER Intake Manifold !! #1 bank in Front #2 Back.

1. Remove the intake manifold.
2. Gently pry up the rubber covers.
3. Disconnect the knock sensor electrical connectors.
   1. Remove the knock sensors.
   INSTALLATION PROCEDURE
   
   NOTE: Refer to Fastener Notice in Service PrecautionsInstall the knock sensors. Tighten Tighten the sensor to 20 N.m (15 lb ft)
   
   1. Connect the knock sensor electrical connectors.
   2. Push down on the rubber covers.
   3. Install the intake manifold.
   
   

Hi, lifter valves cannot be "reset". I never heard of lifter valves being reset. When they are worn, you need to replace them, engine overhaul.

Most likely it could be a vavle or it could be something with the exhast. Hope for the exhast. Replacing vavles are costly.