Well, that was fun...

[rascal]

Yes, I barely drive anything anywhere these days, and the car cover has not been off the MX5 since before Christmas. This can’t be great for battery health.

Get your self a trickle charger and that will solve the battery health problem.

I have a tiny 3kg bike battery in my Mx5 which only gets used once a month and it’s 8 years old and still going strong. It goes on the trickle charger as soon as I roll the car off the trailer and into the shed, until I put it back on the trailer for the next meet.

[Rocky]

As a footnote to this saga, My Auto-Electrician removed the Alternator on Monday and pulled it apart.
Showed me photos of the guts and said that there was less wear on everything than expected for 84000 klms .
I took it for a run as usual on Tuesday and as expected everything was normal.
I still don't really understand why a dying battery would cause the problem, and neither does my mechanic but hopefully it won't happen again.

[Rocky]

Well, it happened again!
Took the MX5 for a run today - up the Mt Morgan Range. This is a very tight, twisty road - probably 20 corners.
Ran just great going up but as per last time, a couple of hundred meters over the crest on the down run and it starts losing revs/power, pig-rooting/missing - whatever. I eventually pulled over and tried to keep the engine from dying and managed to start off again and get a couple of hundred metres further up the road to a turnoff where I figured there was plenty of room for a tow-truck. As soon as I stopped revving it, the motor died. I gave it 15 minutes to cool, started it first go and drove it the 20 klms home without any problems whatever. Same as last time.
It is booked in to the local Dealer next week for a comprehensive (read 120K) service (it has actually only done 87,000 ks) but I want everything checked and replaced. $$$$
So after last time it got a new battery and an overhaul on the Alternator. Feels like fuel to me. Strange that it only happens when it has been hurled through a series of tight corners, and not then while it is actually happening, but as soon as the 'load' comes off.

[rascal]

The car dying, and then restarting and running fine once cool reeks of CAS issue.
I'd try swapping in a 'known' one (not just a new one) and see if issue repeats...

[greenMachine]

^^^^
Wot he said .

I'd go one step further, and replace both the crank and cam sensors. It is usually the cam sensor in the NB8A, not so sure about the 8B, and I have a box with several of each which was/is part of my NB8A/SE racecar track kit (SE has no VVT so essentially an NB8A sensor-wise).

[Rocky]

Thanks Guys.
Wot's a "CAS"?
How does a Crank / Can Sensor fit into the picture?

[rascal]

CAS = Cam Angle Sensor (though technically could also mean Crank Angle Sensor. I meant Cam..)

In an NB8B its found on top of the rear of the valve cover. (on the front of the valve cover on an NB8A, though its exactly the same sensor,)

[Rocky]

Why/how would it produce the symptoms it did?

[rascal]

Why/how would it produce the symptoms it did?

When the sensors get old and flaky they play up when they get hot.
Letting them cool down allows them to start working again.

[Rocky]

So when they get hot (how do they get hot?) what do they do to cause the strange effects?
Like- do they interrupt fuel flow or something to do with the electrics?

[rascal]

So when they get hot (how do they get hot?) what do they do to cause the strange effects?
Like- do they interrupt fuel flow or something to do with the electrics?

They get hot because they are sitting on top of an internal combustion engine.
Thousands of hours sitting there getting hot, and then cold when the engine is off eventually breaks them down.

In the case of an NB8A, there is little nubs on the inlet cam gear at a specific point. In an NB8B there is nubs on the cam itself that trigger the sensor.
When these nubs pass by the sensor it generates a signal that the ECU reads to know at what angle the cam is at. Hence the name cam angle sensor.
This signal tells the ECU where the cam is in the cycle, so it knows the correct time to fire the injectors/coils.

If no signal is sent to the ECU then no fire. (eg no spark and/or no fuel)
If intermittent signal then intermittent firing.
Neither which is good for a smooth running engine....

[Red_Bullet]

^^Great explanation that.

[greenMachine]

I didn't realise that while the VVT engine changed the CAS location, it was the same sensor . Live a little, learn a little!!

My CAS selection includes one brand new, two 'known working when removed'. Having these on hand has resulted in me never having to use one. Highly recommended!! I have had to use one of my spare crank sensors though.

Yes, good explanation Rascal

[93_Clubman]

Rocky, these are the two items in question:
https://mx5mania.com.au/products/camsha ... 1092&_ss=r
&
https://mx5mania.com.au/collections/ele ... -1998-2004

[Rocky]

Thanks Rascal (great explanation) and Clubman (good references).
I am always traumatised by the idea of taking a problem to a Dealer as I have never had a successful solution from one and they never listen to what you try to tell them about the problem. They invariably have their own preconceptions about the nature of the problem, when if they listened to what you told them they might actually get some vital information about causation.
The key bit of info here is that the car runs at about 3 - 4000 rpm consistently for about 10 mins max as it gets tossed left and right going up the range and that the problem doesn't occur whilst this is happening but does occur within about 30 seconds of it ceasing when the car is at low revs in 6th gear going down the other side of the range. After stalling, it only takes about 5 or 10 mins before it will restart and run normally (not enough time to cool appreciably, I would have thought) - and the engine didn't seem particularly hot when I pulled over and lifted the bonnet.
Anyhow, your explanations are very helpful in understanding a likely cause and it gives me a 'prompt' for the Dealer mechanics, if nothing else.