Engine Knock 1991 300zx NA

[table_flipper]

Hello everyone. I just recently bought a 1991 300zx NA. The reason that this car was sold was that the engine was knocking quite loudly. Here is a video of it that I recorded: http://www.youtube.com/watch?v=SzPhBDPZ_4w
does anyone know what is wrong with it? I'm told by the person that sold it to me that the compression is fine and that is it an upper engine problem, potentially something wrong with the lifters or valves. I tried looking at my engine number because I read that if it is below a certain number then those 300zxs tend to have valve problems. I hoping for a response that tells me it is just that the timing is off, but that seems unlikely.

[J30tChumpCar]

Lifters tick, you sound like a a knock, heres another z with fatal noise, compare
http://www.youtube.com/watch?v=hlkOkB6t6Fg
Disconnecting cyls like in this vid can help decide, with a knock the noise usually subsides when the bad cyl is located and disabled, try that.

Compression being fine is no indicator in many cases, the damage is above/below the pistons.
Lots of things get torn up when engines knock, metal shavings get into everything, the oil squirter's below the piston get damaged, you will need a new crankshaft and several rods, in my opinion, rather then spend 4x the money rebuilding, go find a used engine, perhaps one from a less abused J30

[twinturbozxtacy]

sounds like rod knock to me....

[table_flipper]

I guess I will try running the engine and disconnecting the coil packs to see where the problem cylinder is. Thanks for the advice

[twinturbozxtacy]

that wont help you at all. it will still knock either way, your motor is shot and you need to rebuild it.

[table_flipper]

So should I just open up the plenum and take a look to see what is wrong then go from there?

[twinturbozxtacy]

knock happens when a bearing in one of the connecting rods has spun. therefore leaving the space it took up for the rod end to bounce around on the crank journal. it is not something you could see without taking the heads and or oil pan off. but hey it cant hurt, go for it, after all i could very well be wrong i am not a genius(far from it lol!)

[t.mcginley.jr]

knock happens when a bearing in one of the connecting rods has spun. therefore leaving the space it took up for the rod end to bounce around on the crank journal. it is not something you could see without taking the heads and or oil pan off. but hey it cant hurt, go for it, after all i could very well be wrong i am not a genius(far from it lol!)

No, you're right that's exactly why rod knock happens. Even if there weren't metal shaving throughout the whole engine (although there probably is), you would need a new crank and possibly rod(s) at the very least. It will probably be cheaper to pick up a used engine especially since its only an NA.

[ThisIsSparTTa]

Change your oil, if there's a ton of metal shavings, don't even bother refilling the oil. You've spun a bearing, have rod knock, have to rebuild. Don't run your engine any more than you need to in this state, or you'll risk not being able to reuse your crankshaft (or getting your core charge).

Btw, rod knock will follow the RPMs. Here was mine with 1 spun bearing:
https://www.youtube.com/watch?v=mfw2CW7vqIg
https://www.youtube.com/watch?v=kT8gmlimxJg

[nissanfreak12]

Where is the noise coming from? Is it more pronounced under the car or above the engine? If under, its bearing/rod knock. If on top its lifter.

At idle is it a fast or slow knock? Lifter noise will be a fast tapping, rod knock will be slower, just like spartta said.

[table_flipper]

Where is the noise coming from? Is it more pronounced under the car or above the engine? If under, its bearing/rod knock. If on top its lifter.

At idle is it a fast or slow knock? Lifter noise will be a fast tapping, rod knock will be slower, just like spartta said.

I think it is just the lifters since it sounds like it is coming from the top of the engine and the noise sounds like more of a fast tapping.

[J30tChumpCar]

I never understood the urge to rebuild a NA, there are cheaper options (used) that work out just fine(sometimes better, machine shops are not all that sometimes)


This is NOT great advice, especially if you decide to rebuild or need to reuse the oil pan/pickup tube (for a used J30 engine I mentioned earlier) but free revving the engine will diagnose the problem with 100% certainty, either the lifter tick just continues to tick or the loose rod pounds so hard you just know and or it lets go destroying all possibility of rebuild.
Have a fire extinguisher and oil dry ready.