Snapped Valve spring - how the chuff?
 

On Tuesday evening whilst driving home on the motorway, my knocklite suddenly lit up like a christmas tree, I was down on power and there was a god awful racket.

Shut the car off got towed home and started the diagnosis. Worked out I had zero compression on clyinder no 3. Figured I'd blown a ring or piston or something and started to strip the car down to pull the head.

Only when I realised the head bolts are Torx heads did I realise why I had 0psi compression.

https://www.miataturbo.net/attachmen...ine=1316120434

Below this was a snapped valve spring.

1) How the feck has this happened.

2) I'm going to have to continue pulling the head to assess the damage, but I'm guessing as this happened at 4000rpm and the engine had been started since in an effort to diagnose, plus the noise, that the valve and piston are going to be fucked.

3) Seriously how has this happened? I rarely hit the limiter, and my tune is good and safe.

your knock lite hooked up to anything?

Man that sucks. I wouldn't think a broken valve spring would be caused by anything other than plain ole shitty luck. I'm no expert but I wouldn't think a bad tune or an over-rev would be any reason for the spring to break. Hopefully it kept the valve up enough to stay away from the piston, but I wouldn't think the noise you described good sign. :ohnoes:

Yeah, a knock sensor :idea:.

It lit up because of the horrendous noise the snapped spring caused.

It wasn't det that caused this failure I can tell you that 100%

Your sig says you have a 2001 motor. Correct?

NB motors have the flimsiest valvesprings. If you are on a budget, I would suggest that you upgrade to the 94-97 valvesprings (I think they used the same springs during that span). The earlier BPs have a heavier spring because of the hydralic lifters, which are heavier than the solid lifters in the NB. Mazda was also trying to get a tad more power by using a spring that compressed easier.

Weak springs are, well... weak.

Now would also be a good time to get shim under lifters, which will make the valvetrain lighter, and less prone to valve float. Will make a tad more power too.

Hope you did not hurt the piston too bad. That shit will polish right out.......Maybe.

you mean cam? or lifter?

Am I the only one that suspects a jammed lifter? Or did you verify that it's moving freely somehow?

I've not thought of that curly.

I now have the head off and can confirm that there is zero damage to the piston and valve which is good (and incidentally zero det has ever occurred on my engine, which pleases me greatly!).

I should have a valve spring by weds/Thursday so I'll start dismantling/removing the camshaft then and will check that the lifter isn't stuck.

If it is stuck how the heck could it have happened? Debris in the head somehow?

Debris or lack of oiling. Or a failed lifter, not sure how it would fail in a way that would jam. Or if it even could.

For starters, does the valve move up and down, or is it stuck in position?

This would indicate that the valvespring is intact, and the lifter is holding tension on the spring and keeping the valve open.

The valve spring is 100% broken, I can see that, I'll try to grab a pic as proof :-P

The valve also moves freely up and down with my finger, smooth as silk.

Pretty sure they're 12 point 12mm.

correct. unless ARP or something.

That'll be why a 12point 12mm socket got them off then.

DOH...

In my defence, for high torque stuff I always reach for the 6 sided sockets, the last thing I wanted to do was round a head bolt. FAIL on my part there!

its okay. now you know, and youll impress your friends next time you need to pull a head and happen to bust out the 12mm 12-sided knowledge.