mx5cartalk.com

Hi all, hopefully you guys can help me out here. I've got an NC with a 2.5 conversion. Donor motor was out of a 2009 Mazda 6 and had 65K on the clock. I've done around 9K since it's been in. I've got AEM CAI, Roadstersport maxpower header and full exhaust running a MotoEast tune. Have had zero problems until yesterday. Went to accelerate into traffic from my driveway, there was a bang and she just died and wouldn't restart.

Got her into the shop and the spark plug on no 1 has been destroyed. Where should I be looking? I doubt anything has got into the cylinder head but what else from the bottom end would cause it? Conrod failure? Piston pin? Is it even worth taking the head off to check; just drop the sump and look for bits?

I think I'm going to have to put the 2.0 back in to get my daily driver back because I'm just seeing $$ every time I think about it...

Any help, ideas, options appreciated.

First point of call would be to pull the head of.

But yes piston, piston pin, conrod, conrod bolt failure could all cause it.

Post a photo of spark plug

A friend had this happen to his supercharged Calais.
The top of the piston had broken off and hit the plug.
He installed a low km replacement engine.

I'd take the head off first and go from there. With any luck you might only have a couple of bent valves in the head and you can get that fixed while you work out what to do with whatever happened to the bottom end.

1. Drain the oil through some cloth/sieve - see what comes out.

2. Stick a little camera down the plug hole, see what you can see (get one from supercrap for 30 bucks or so, useful thing to have around).

3. Buy new (secondhand) engine. (Just guessing, but it doesn't look promising ... )

Thanks all. Fortunately I still have the original 2.0 wrapped up in my workshop. I'm going to break it out and do a swap to get back on the road. Once the 2.5 is on the stand I'll pull it down and see how expensive it is ;) In all honesty I was planning on stripping the 2.0 and building it up with forged internals and new cams (see if I can get a growl) +/- some sort of forced induction. I'll do that to the 2.5 now. I'll get photos of the damage as I pull things down and let you know what happened to it... Silver linings, I'll just keep telling myself that... It's not the kWs I'm going to miss but the torque

I'm sure Mike Mann from Moto East will be happy to supply all the bits you need for a killer 2.5

This sort of begs the question: is the 2.5 motor an interference fit motor? This failure sounds very familiar.

A couple of years ago I went to look at a Pug 207GTI, the one with the BMW/MINI N56 supercharged motor. Between arranging to look at it and actually going to look at it the owner called up and said the engine had made a funny noise, it doesn't sound good and he dropped the asking price by 75%. When I checked it out the cam cover was off and I could see that this head had twin cams with rocker operated valves. All the rockers flopped about with around 10mm clearance. When I put a borescope down the first bore I saw two valves staring back up at me from a debris field. At first I thought this has gotta be a faulty borescope. Then I realised that the stems of two valves had snapped, the valve heads had dropped down and speared themselves stemfirst through the top of the piston.

I did my research and found out all about the MINI death rattle (google it for the joy of knowing that you own an MX5, not a MINI). One day it rattles a bit on startup. The next day it starts and within the first few seconds the motor disintegrates. It was down to a faulty timing chain tensioner design which caused the chain to jump on the sprockets and then an unhappy marriage of the pistons and valves did the rest. Many motors died this way at about 7,000kms but then if they survived this they worked fine until the tensioner got more worn and they exploded after 70,000kms. That's exactly what had happened to this car at 72,000. Second hand engines were virtually unobtainable because so many were exploding and BMW are very shy about answering questions about them because of the class actions

Sooo,2.5 is out, 2.0 is going back in... here's my No1 piston, well, part of which fell out of the intake manifold:


And here's a peek inside:


That valve seat is FUBAR, can't see me resurrecting the head. Good news is there was nothing in the sump and the crank looked ok just have to see what sort of cylinder damage there is, ... It still may well be cheaper just to write this one off

I wonder what the cause was? Was it due to a Mazda manufacturing defect? I know finding the answer would require $, so hardly worth it.

The bearings on the crank might be squished a bit due to the piston bits jamming up in the head so if you reused the bottom end you'll need to check bearing clearances. I doubt it's done any damage to the rod but they can be deformed slightly if the chunks were large enough.

Pretty impressive. I wouoldn't use the crank without checking for bends and serious crack testing. Even then I'd always be worried about it. I assume #1 bore is fubar as well?

You don't do things in half huh?

Finally got the 2.0 back in the car and have her running, and had a little time to look at the 2.5. Single valve stem failure, head is definitely shot but what's left of the piston is still connected, sits properly at TDC and the bore is not too badly scored, I think a good polish would do the job but prolly a .5 overbore and some CP 12.5:1 piston heads All that damage caused by one 20c piece, so... anyone got a spare cylinder head Think I'll source a bare head, port and polish it, upgrade valves and springs and see how I go. Might as well get started on pulling down the bottom end...