Miata engine noise diagnostics

Check the crank nose. I had one make that kind of noise.

The ticking is definitely lifters.  I hear a lower thud sound too.  I'm not sure what that is.  I know the ticking is lifters though.  If you let a Miata engine sit a while and let all the oil drain out of the lifters to the crankcase this will happen.  That's why you really need to start and drive your hobby cars weekly.  Once you get the lifters sorted out maybe the thud will go away or become more clear. 

Apexcarver said:

Check the crank nose. I had one make that kind of noise.

Good point, I had a failing crank noise make the most incredible rattle. Sounded different than this IIRC. But if the crank bolt was messed with during the recent work (108k - could have been a timing belt service) it's definitely worth checking. It's pretty unlikely it spontaneously failed at the same time.

I'd say it needs an ls or an lfx, either will fix that right up. 

We did the timing belt ~8k miles ago, so no recent activity there.  I didn't detect anything amiss there while rotating the engine to check for valve lash.  How do I check for the crank nose issue?  Just check pulley and bolt for tightness or slop?

8k miles ago is pretty recent. Make sure that crank bolt is tight. The best way to identify the crank nose problem is to pull the pulley off and inspect, but a visibly wobbling pulley is definitely a danger sign.

Carbon (Forum Supporter) said:

I'd say it needs an ls or an lfx, either will fix that right up. 

Then it will make a different kind of funny noise.

My daughter wants more power, but I don't think she's ready for that yet!

Wow, I'm sad that it's been 4 months.  Got the engine out a couple months ago, and finally got a chance to tear into it.  Bottom end bearings range from good to nearly new looking.  Can't detect any wrist pin play when rocking the crank back and forth while touching the bottom of the piston.  So, I pulled the head.  

There is something sitting on / stuck to the top of the #4 piston.  It's flush with the top of the head gasket, and it's in an area where the combustion chamber is flat with the bottom of the head.   I can imagine the piston making contact with the head through this bit, but I'm having a hard time picturing the bit not becoming detached and the symptoms changing.  Anyone have any thoughts on that?

 I'm trying to add a picture, but I can't get it to work.  I'll see if I can do that from my phone.  Edit: that worked  

So, that piece on the piston is folded non-ferrous sheet metal.  There are mating marks on the head.  I'm now assuming this caused the noise.  I've ordered gaskets, and in the meantime I'm going to retorque the main and rod bolts and clean things up so I can just reassemble it when the parts come in.

Any ideas as to what that foreign body might be?

In reply to pkingham (Forum Supporter) :

Maybe casting flash from the intake manifold? Highly unlikely, but I have no idea where else there would be thin metal to suck into the engine.

Examine the air filter carefully looking for a void that could admit debris. I would also pull that piston out and examine the top ring groove to make sure it has not pinched the ring.

In reply to TurnerX19 :

Good idea on the ring groove. Just checked it, and it's fine. 

Here's the wayward piece. It actually stayed stuck to the cylinder wall when I lowered the piston. 

Oh that is weird!

It almost looks like it was a piece of something tubular that got folded.  Did the injectors have little metal caps on the ends?

It would appear that wayward piece was the cause of the noise. I put everything back together with only a new head gasket, and after a few miles to get the lifters to quiet down, it seems perfectly normal.