locating a engine noise valve train or rod knock?


The Grumpy Grease Monkey mechanical engineer.
Staff member
Hello all, I recently picked up an 85 C4 L98 4+3, hasn't run in over a year, price was right. We got it into the garage put a new battery in it and cranked it over and this is what the outcome was. Its a large video so I posted on youtube, I hope that is ok.
Any ideas on what the issue could be would be a big help.
Thank You

or search youtube for 85 C4 noise

step one

pull the valve covers one at a time and rest your palm and apply pressure on each of the 16 rockers in turn,
as it rocks /reciprocates with the engine running ,
if it changes the sound its obviously valve train related,
as your application of pressure on the rocker mutes the slack and resulting sound,
and yes you'll notice the difference,and probably feel the difference,
in the rocker, movement. its usually obvious,
if not its most likely lower rotating assembly or clutch related, like a cracked flex plate or loose flywheel.
Id remove the dipstick at some point as its a potential source of the noise if its bent and contacting the rotating assembly,
and Id pull the oil pan to make sure the rods are not hitting the windage tray ,
or the oil pump pick-up, before pulling the engine,
if you think its rotating assembly,
pull each spark plug wire in turn and see if that changes the sound,
and a compression test would also help here.
once and if you find its related to one or a couple rockers,
pull and replace those rocker arms, and adjust the valves,
if it does not improve, your likely to need a new cam & lifters




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