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G'day,

Seems my NC has developed a knocking noise. I'm fearing the worst.

Its only noticeable under light throttle above 1500rpm. Once loaded up it goes quiet, and similarly on deceleration its not there. But when its ticking, it seems pretty loud!! Took it to the dealer who suggested its likely a bearing 'somewhere in the engine'...not really helpful but I guess they don't have xray vision. They serviced it, popped in an additive and suggested it should quieten down in a couple of hundred kms. They said it will eventually need a rebuild, but to see how it goes.

I'm a little more pessimistic. Was going to run up a few K's on it over the weekend, and see how the additive worked, but really don't want to drive it as is.

Was thinking most likely big-end bearings and was wondering if they can be changed from underneath (last time I did that was a Holden red motor!). On the other hand, if the crank is already cactus it would just be a wasted effort. Car has 140,000km on it.

Thoughts?

Ghoti wrote:Took it to the dealer who suggested its likely a bearing 'somewhere in the engine'..

That doesn't give me any confidence in that dealer.

Worn main bearings normally growl/rumble.
Worn big/little ends normally clatter under load. https://www.youtube.com/watch?v=jT57SsRNLX0 I think you would have to drop the subframe to do it in car.
Other things on the MZR that can be noisy are the cam VVT gear, timing chain https://www.youtube.com/watch?v=B4mN3hbxtzY and serpentine belt tensioner/idler.

What year is your car?
Manual or auto?
Any noises at idle or only under light throttle above 1500rpm?
Does it go away at any particular higher revs?

I don't know that motor but in general it's rare for a modern motor to develop a big end bearing knock at only 140kkms, although you definitely do have symptoms that would point that way. It can happen because of either an engine over-rev (eg hitting 2nd instead of 4th on a fast upshift from 3rd) or oil failure (either starvation or age and quality).

I'd only assume the worst if you know in your heart that one of these has happened. Otherwise keep looking as JBT suggests.

A second opinion wouldn't hurt. Mechanics do have stethoscopes, you know, that can pinpoint a sound much better than 'somewhere in the engine' if they take the time. Big ends are just about the hardest thing to pick up on a stethoscope but just about anything else can be pinpointed.

The early NC engines have cast internals and are prone to spinning bearings if hammered and/or suffer extended oil change intervals. I know of two cases. The second iteration with forged internals is far stronger.

Absolutely certain that you need second/ third opinions. If it were my car I can ask the local guy who specialises in engine building to listen / diagnose if possible and you need to tap that sort of resource. For your own info ask the question what is estimate of cost for a rebuild of the engine- engine out etc. As the other guys said this is premature failure if indeed its a bearing. Any chance it ran low on oil in the past- i.e. known history?

Not that i am an NC expert but the 1st gen motors are notorious for conrod related issues , it is a flawed design.

Dann

Sounds like a good justification for a 2.5 swap ...

Either that or a base for a built motor or an upgrade to a NC2. Unless it's something stupid we all missed like a stick hitting the AC pulley.

Given the way most of these 'help me diagnose this noise over the internet' go it's maybe an empty stubby in the glove box.

Big end knock in an NB. This drove OK (no real noise) under 60-70kph in 6th, above that she started rattling and banging.

Got me home from Eastern Creek to Newcastle, was an interesting trip up the expressway at 60k's at nighttime........

https://www.youtube.com/watch?v=9-iFFmTyXvA

manga_blue wrote:Given the way most of these 'help me diagnose this noise over the internet' go it's maybe an empty stubby in the glove box.

So true

I will be interested to learn what is the cause. Please let us know in due course.

Thanks for all your replies!

JBT wrote:

manga_blue wrote:Given the way most of these 'help me diagnose this noise over the internet' go it's maybe an empty stubby in the glove box.

So true

I'll second that. Doen't hurt to try though.

So, for history...

The car is a 2005 NC manual, so would have been an early series. Purchased the car almost 2 years ago as a fun weekender. Got it perhaps a little cheap, but it has a lot of dings in the doors that look like carport poles In that time would have done less than 15,000k. Had a reasonable service history, initially dealer then general garage servicing. Was recently serviced when I bought it, so I got it serviced again after about 9 months and yes, in terms of elapsed time, it was overdue for another service (7,000k since last service, but almost 12 months since last service).

Loaned it to my son for a couple of months, and he was going to service it at work (apprentice diesel mechanic). Suffice to say I got it back making the noise and immediately checked the oil thinking it was low. It wasn't, but it looked pretty dirty to me. Called my son who sworeit had been serviced and been making the noise all along. I know at least one of those statements not to be true! Called the local mechanic that serviced it for me and he suggested Mazda would be better able to diagnose the fault (OK...so know not to go back there).

JBT wrote:The early NC engines have cast internals and are prone to spinning bearings if hammered and/or suffer extended oil change intervals. I know of two cases. The second iteration with forged internals is far stronger.

This is my fear. Extended oil change in terms of time or miles? While not dríven often, when dríven was usually upwards of 100km round trip so definitely was at operating temperature. I must admit I wonder if its gotten low, started making a noise, and then been topped up??? Optimist in me is hoping for something less drastic.

Took it for a short drive tonight as am reluctant to run the engine while there is a remote chance of repair. Observations are:
> No noise when cold. Low revs, high revs, under load or coasting.
> Noise starts once temperature gauge is on 3rd notch, so off cold but not far off.
> Starts between 1500 and 2000 RPM, under light throttle. Sounds like a spinning wheel off a game show!
> Seems to disappear about 4500 RPM, or is covered by exhaust noise.
> Doesn't occur if engine is loaded up (i.e. low revs/high gear, heavy acceleration, up hill)
> Doesn't occur when revved with gear in neutral
> Doesn't occur on deceleration
> Other than the noise, no apparent performance impact as far as I could determine on public roads.

Any suggestions on a good place to take it for a second opinion in the Clayton/Oakleigh Vic area?

JBT wrote:

Ghoti wrote:Took it to the dealer who suggested its likely a bearing 'somewhere in the engine'..

That doesn't give me any confidence in that dealer.

Worn main bearings normally growl/rumble.
Worn big/little ends normally clatter under load. https://www.youtube.com/watch?v=jT57SsRNLX0 I think you would have to drop the subframe to do it in car.
Other things on the MZR that can be noisy are the cam VVT gear, timing chain https://www.youtube.com/watch?v=B4mN3hbxtzY and serpentine belt tensioner/idler.

What year is your car?
Manual or auto?
Any noises at idle or only under light throttle above 1500rpm?
Does it go away at any particular higher revs?

Just checked those You tube links. The VVT noise is certainly an issue which the car has done since before I had it and was passed off as 'normal'. More fool me and another strike against the mech who serviced for me. Doesn't seem to be doing that after the Mazda service though? Dunno if the did something without reporting it (unlikely) or if the additive may have just quietened it.
As for the bearing clatter clip...its the noise alright, but mine doesn't do it if you just rev it while stationary.

Sorry, cannot help you with service/advice in VIC.

As for the bearing clatter clip...its the noise alright, but mine doesn't do it if you just rev it while stationary.

That's not good

What oil? I have always used 5 W40 synthetic Penrite or Shell. I generally change oil/filter every 6 months - oil is cheap. Our car is 9 years old with 127,000km of mostly highway km but a fair bit of motorsport use.

Is your car manual transmission? The NC manual tranny is very agricultural below about 2000 (ish) rpm when hot and makes a horrible rattling/growling noise in all gears 1st to 4th. The noise (I hate it) goes away in mine above 2000 rpm. You sure it's not transmission related?

Ghoti wrote:Any suggestions on a good place to take it for a second opinion in the Clayton/Oakleigh Vic area?

There was, but since hks_kansei discovered how they 'fixed' his NB8A we don't recommend them for mechanical repairs. But if you're prepared to drive to Somerville, Mammi Motors are good with MX5s: 10 Industrial Drive Somerville, 03 5977 7973: http://www.mammimotors.repcoservice.net ... contact-us