There are variations in the valve train design depending on if it's a single cam or double camshaft, years, and things like that. When you have push rods, GM and Ford used to run oil through them to lube the rocker arms. There was a hole in the rocker arm that lines up with the hollow push rod's hole. Oil would squirt from those holes so you had to fashion a shield out of cardboard to make the oil run back into the head. If you found one with no oil squirting out, the most common cause was a plugged push rod, and next was a lifter with a plugged oil feed hole. Those usually didn't make noise until excessive wear took place between the push rod and rocker arm.
Roller lifters can oil the rocker arms the same way or the oil can get there through passages from the camshaft bearings. That's how Chrysler did it years ago. Actually, there's so many variations, you're better off looking in the service manual for your specific engine. There will be a diagram that shows the oil passages.
If you have a low-oil pressure problem, all of the lifters will make noise. When there's just one that's noisy, you often have to disassemble things to look for wear patterns. There are a few engines that use "mushroom" lifters. You have to remove the cylinder head to get those out. For the regular lifters you can slide them out, then stroke them in a container of oil to see if they'll pump up. If they do, you know they're okay. If one doesn't, that may confirm the noise you heard it making, but some just don't pump up unless they're running in the engine with pressurized oil feeding them.
Thursday, July 2nd, 2015 AT 10:30 PM