Engine Knock
 

I have a 1986 naturally aspirated 300zx, just picked up for a great price because it has a tick. We noticed that the sound was coming from the passengers side of the engine. We were sure it was a lifter and possibly broken rocker. Until we pulled the valve cover, and none of the rocker arms were bad and the lifters seemed okay. The sound isn't always there, it ticks when the engine is idling then stops, then starts again etc, then when revving it tick, then stops, then starts etc. It's almost more of a tick tick tick tick noise that's loud then quiet and can stop. Any ideas what could be wrong, we're pretty sure it's not a rod because it doesn't sound like a rod. Other people said it could be a bent valve but everything I've read said that a bent valve would equal no compression and a non-running engine, the timing belt may have slipped and the cams aren't in sync, I guess what I'm asking is, are there any tests to see if the lifters are bad, or a valve is bent, or anyone have any ideas, I'm at a loss. I've worked on plenty of engines, never worked on anything with independent cams for each cylinder bank or with hydraulic lifters....:confused:

Is it a knock or a tick? Is it a metallic sounding knock or tick? Have you checked your oil?

I checked the oil, but there is no owners manual so I don't know what type of oil it takes, I'm guessing something like 10W30. The oil smells a bit old, and the levels are a little low. I can't get an oil pressure reading because the battery and oil pressure assembly isn't working properly. I was going to do a seal test on the valves to see if those are the problem. It sounds like a mechanical tick, pulled the valve cover ran it at idle with the cover pulled, didn't see any of the valves stuck like they were bent. The rockers were moving freely and the lifters seemed to be okay. It looked as though on cylinder 3 the innermost valve (I'm guessing the intake valve) seemed to kind of hangup and the knock seemed to be in sync with that lifters movement. I just am at a loss to where to go from here. I could change the lifters, pull the head check all the valves and dick around to just find out that the oil was low the whole time, so I'm just looking for any leads as to what could be wrong and how to test different possibilities. Thanks

Buy yourself a Haynes or Chiltons Manual, get an oil change, take it to a Oil shop and get the works,.... chassis lube, see if your radiator needs flushing etc, so you have a base line for maintenance. Get some Royal Purple if you want the best.

Yeah I went to Advance Auto Parts, they said there wasn't a Haynes for the 300zx, but they're full of crap, just found them on Amazon. I'll order one. I heard somewhere that if you add some oil and the tick stops, its a lifter that will eventually go bad, can anyone confirm this?

Z31 lifters can be rebuilt, its fairly easy once you get to them, here is a link to a site that covers the subject in good detail http://www.xenonz31.com/Rebuilding%20Lifters.html