Some of you may remember that I bought a hot Rebello motor for my early 260z and when I first started it there was a clunking sound.  Luckily that problem ended up being a clutch slave cylinder bolt that was too long and it was resolved pretty easily.  Since then I put about 200 miles on the new motor and its been reliable.  Yesterday I went to my garage to start the car for work and after running for about a minute there was a horrific clunking sound so I shut it down immediately and took the truck to work.  Its been cold (for the middle of California) in the low 30's (about 0 Celsius) but in the garage I estimate the temp to be in the mid 40's.  After work I took out all of the spark plugs and rotated the motor by hand to see if i could feel/hear any interference and the motor spun nicely.  With the spark plugs still removed I cranked the motor with the starter and still everything ran smooth.  After consulting with the elders I put new plugs in the car and attempted to start it again, this time on video.  As you can see the car starts and runs for about 30 seconds at about 1100 RPM before the noise kicks in.  What should I be looking for?

 

 

Edited December 30, 20159 yr by Kurbycar32

 That's weird. I would have expected it to relate to engine speed. It seems to be random. Just to rule one thing out, you don't have a "friend" in the garage who is a practical joker, right? I think you're going to have to trace it by the sound which sucks because you don't know if or how much damage is being done. Maybe a helper on each side to figure out where it's coming from ie; left-right, front-back, top-bottom. I often use a 4' section of garden hose as a stethoscope to reach difficult areas. 

 

Remove the valve cover and check that everything is assembled correctly. I assume oil pressure is constant also?

Yes the oil pressure is excellent immediately after the motor starts up and doesn't fluctuate 

It's something with your fan!  maybe the clutch?  It doesn't spin when you crank it.

I only heard two "clunks".  I assume that you turned the engine off then, it didn't die on its own?

Sounds like a spit-back through the carburetors, more than a clunk.  On the video anyway.

That is a really odd noise.  I've listened several times and I'm stumped so far.  It doesn't sound internal to the engine

I turned the engine off as soon as i heard the sound.  The sound is awfully metallic sounding

Doesn't the fan turn on y'alls car all the time? His isn't turning. Water pump or something is keeping it from turning in the video.

12 minutes ago, siteunseen said:

Doesn't the fan turn on y'alls car all the time? His isn't turning. Water pump or something is keeping it from turning in the video.

You're right, it sits still then starts moving, then there's the noise, and it stops.  Starts moving at 43.  Maybe pull the belt for a test.

The air filter is metallic.  You might pull those and see if the noise gets louder or goes away.  Noise will come back through the intake manifold faster than the air going in.

Edited December 30, 20159 yr by Zed Head

i'm betting the farm on it being related to the fan - is it hitting the shroud? i'm guessing the fan clutch is needing some fluid or otherwise worn, as perhaps it is slipping completely when cold, then starting to spin once it warms up a bit.

on another note, it seems like your motor is a bit hard starting - are you using the choke?

I agree with Zed. Watch the video, observing the fan. With a cold engine, our fans clutches should have enough drag to spin the fan. Your fans only moves one or two blades for 42 seconds, and then starts revving up to speed immediately before the clanking starts.

I just got out of the garage, everything is fixed and the car is running.  First the fan was an important clue but the problem was actually with the crank pulley.  It looks like the pulley separated into two pieces and the round part of the pulley was sort of spinning with the rest of the pulley most of the time.  When the broken pulley got out of alignment it would smack around against the other side of the pulley and the timing cover, creating the sound at random intervals.  I pulled the pulley off of my other Z and the problem is resolved.  The first photo is what it looked like on the car, you can barely tell its broken.  The second photo is what it looked like after it was pulled, two totally separate pieces

Edited December 31, 20159 yr by Kurbycar32