NB Miata Engine Noise

[Brian Jackson]

Are there any Miata experts who might be willing to lend me an expert ear?

The engine in my '99 Miata has started making progressively more ticking noise. Now it starts earlier, at about 3k rpm and comes-n-goes with throttle position. 

PIstom slap? That's my guess but I've never (knowingly) dealt with piston slap so I cant identify it. 

Lifter? Probably not since throttle affects it. 

Rod knock? Doesn't sound like it. 

Cracked piston? Hope not. 

Wrist pin? Who knows? 

I'm going to do a leak down test and potentially check the oil pan soon. But I figured I'd have a Miata expert make his best guess if possible. 

[John]

I suggest searching for answers on the miata.net forum. You can also post your question there too.

 The MX-5 Miata Forum 

Here's a thread you might find interesting.

Does my NB have rod knock? (sound clip) - MX-5 Miata Forum

[Brian Jackson]

Yep, that's rod knock. 

That's not the sound I have not at such low rpm. 

Honestly, if the Corvair made this sound I'd think nothing of it. In fact, it made basically the same sound for the 6 years that I raced it haha! 

I did do quite a bit of research in the interim. NA lifter tick dominates search results for NB cars haha. I went ahead and tried a Liquid Moly treatment and switched to 5W-30 anyway. Predictably, very little difference if any. I'll run the car some more tomorrow and do one more oil change tomorrow night. 

[John]

Did you try this test for play in the rod bearings? If a rod bearing has excessive play, you may be able to hear or feel it.

"I forgot to mention that I also did the rod bearing test where you turn each piston slightly past TDC and push down on it. I got a small but noticeable click from cylinder 1 but no clicks on any others. I'm assuming cylinder 1's rod bearings are toast."

[Brian Jackson]

I haven't done the loose rod bearing test yet but I think I'm about to. My sound is VERY similar to this one: 

Mine appears mostly during acceleration though... But maybe that's because I only drive one way - underpowered and always at full throttle haha. 

Such an easy test on a DOHC I4. I'll do this next. 

[smeyers]

Not had this experience with my three Miatas, thank goodness, but if the rod bearings are that loose wouldn’t the oil pressure be significantly low?

How many miles? Any history of lots of racing or autocrosses? Heavy usage?

[Brian Jackson]

Scott, this car is new to me. The PO said he rebuilt the engine himself due to piston slap. The car has 99k miles. So the PO may not have been the best rebuilder. It's possible he didn't rebuild anything and did a swap on a blown motor. Who knows. 

As for the oil pressure, the gauge indicates that it's fine. No indication of what the pressure actually is of course. The engine makes racket for about 1 second on startup after sitting overnight. My guess for now is that oil pressure quickly quiets the slightly-loose rod bearing. 

I pulled plug wires and drove on 3 cylinders to see which produced the noise.

Cylinder #1. 

The TDC test didn't reveal anything though. Maybe the bearing isn't loose enough. One more autocross should help! 🤣 

I'm still interested in an NB enthusiast driving the car and hear what they think. But then it's getting real close to getting quotes to rebuild the engine. Know anybody good? 

[John]

Sadly your 1999 Miata doesn’t have a real oil pressure gauge. Starting in 1994, Mazda changed from a real oil pressure gauge to a fake gauge (idiot light). The fake gauge only has two positions which are driven by an on-off oil pressure switch. Ever notice how the oil pressure gauge doesn’t vary no matter what the oil temperature or engine rpm is?

[Brian Jackson]

16 hours ago, John said:

16 hours ago, John said:

Sadly your 1999 Miata doesn’t have a real oil pressure gauge.  

 

I suspected as much. 

Anyway, whatever the problem was, the engine decided to make loud banging sounds this morning the moment I started it to back it out of the garage. Definitely sounds like a bad rod bearing now. No oil on the floor so I don't think I poked a hole in the block or pan. Mystery solved! Contacting rebuilders this week. 

Edited February 24, 2024 by Brian Jackson

[John]

I you find a good rebuilder, let me know. Rebuilding has gotten so expensive, a good used engine might be a better option. I wish Mazda sold new short or long blocks.

[Brian Jackson]

Shoot! I was so put off by all the loose bolts on this engine, the damaged crank, and the probable need for a rebuild... I never considered that some of those loose bolts might be causing the noise I was so concerned about. 

Removing #1 plug wire made the noise go away. 

#1 exhaust manifold bolts were loose. 

I inspected the exh manifold gasket tonight and tada! Evidence of an exhaust leak through the gasket at #1. (The dirty-looking end of the gasket in the pic) 

Hopefully that was the source of the sound (probably). I wish I had put 2 & 2 together before pulling the engine. I could have listened for the leak at the manifold with a stethoscope (yeah right, garden hose!) and confirmed the source of the noise 100%. 

Anyway, I'll double check all the crank bolt torques, mic and plastigage everything, check the new-used crank, have it ground if necessary, and reassemble with new bearings and gaskets. 

I suppose I can inspect the head, pistons, and potential leftover cylinder damage too. I'm still concerned whether loose-nut-larry missed/skippedanything during his rebuild haha. 

[Brian Jackson]

Which one do you think made more noise on throttle? The exhaust leak or this rod bearing? Both on cylinder #1.