What's that engine sound? Everyone's favorite game. (Video included)

[Rill Cosby]

Where to start? First off, my username didn't age well. Secondly, it's been years since I've bothered you guys but I need the help this time. I usually try to do as much as I can on my own before I bother you nice people and usually there is enough here already to fix most problems. I have read a lot on engine noise but I cannot pinpoint this one. 

A couple of years back my 1974 260z went through a rod bearing. The actual nut on the rod cap backed out while driving and you can guess what happened next. The motor then went through a rebuild about 7 months ago containing a new crankshaft, some new rods and pistons. Everything went pretty smoothly. Roughly about 6,000 miles on the rebuild now. 

Then problems started to happen. First, valve train noise to which I found to be rocker arm jumped off a lash pad. After that was fixed there has been a little noise there since. I know these heads can be noisy but it sounded different. I've checked valve clearance 3 times since and all lash pads seem to be intact, also the springs. I checked and cleaned the oil bar as well.

Where my stupidity comes in, I went with a 20 weight oil (my defense, it's also very hot here) and only after about 2 short distance drives I realized that was a mistake and I switched back to 10w-30. I only mention this because this is when that sound seemed to get worse and started to sound not so much from the head. I thought the thicker oil may have had trouble getting into rod bearings so last night I had a look at those. I'm not an expert in this at all but from what I could tell all bearings looked perfectly fine. Everything was properly torqued before and after. 

Compression is about 170 through all pistons. Plugs seem fine, maybe a little rich. Checked timing multiple times. The car still runs strong, maybe not as strong as before the rebuild which bothers me a bit, but she cranks up and runs fairly smoothly every time. The knocking sound only occurs around 600-800 RPMs. Seems like it's coming from more from the front of the engine. Pulling plug wires while running doesn't seem to change much.

At this point I'm not thinking logically anymore and about to start throwing money. Possibly going through all new ignition parts again. There seems to be some fresh black carbon buildup on piston #1 which maybe isn't getting a good spark for some reason? I don't know. 

Sorry for the long post and I'm sure there are things that are missed. Hopefully the video will do a better job of describing what words can't. Thank you!

[240260280]

Idle then pull each plug wire one at a time and see if the clunk changes.

A mechanic's stethoscope will help to isolate.

[Rill Cosby]

39 minutes ago, 240260280 said:

Idle then pull each plug wire one at a time and see if the clunk changes.

A mechanic's stethoscope will help to isolate.

I'll give it another go later tonight. I mentioned in my post that I have tried that. I read somewhere that even pulling two side by side might narrow down something (bearingwise?) but I don't recall off the top of my head but I've tried that too. I've tried the flathead screwdriver to the ear method but like I said I'll give it another go tonight and report back with something more accurate.

Edited May 11, 2019 by Rill Cosby

[240260280]

Could be water pump or timing chain area.  It sounded louder when filming at the front and at the back.  It seems somewhat random occasionally so it may not be crank related.

[AZ-240z]

Have you checked for an exhaust manifold leak at the block?

[Rill Cosby]

5 hours ago, 240260280 said:

Could be water pump or timing chain area.  It sounded louder when filming at the front and at the back.  It seems somewhat random occasionally so it may not be crank related.

I think you may have hit the nail on the head with the water pump!

One, it was such a deep sound I would have never expected it. Two, I'm not so confident with engine rebuilding I was quick to assume that something had gone wrong internally. I filmed the video panning across front to back for that very reason. I definitely heard more from the front and I'm glad it came across in the video. I really had a feeling it was piston #1.

Something simple, I never would have thought of the water pump sadly. I pulled it off, ran it for a quick second and the sound seemed to have disappeared. 

Thank you for the help, bud. I've read so much of your information on this site. If I remember correctly you went under a different alias in the past.

4 hours ago, AZ-240z said:

Have you checked for an exhaust manifold leak at the block?

I think it may have been solved already. The exhaust manifold was one of the first things I looked at, I should have mentioned in my original post. I know when that leaks some wonky sounds can occur. 

Thanks for chiming in!

Edited May 12, 2019 by Rill Cosby