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I've owned my 2001 NB8B for around 3 months now. BP-4W, 120,042km, 6-speed manual.

Recently it's been stuttering after driving for seemingly irrelevant amounts of time. It's getting more frequent, too.

I'll be driving regularly and it'll happen without a warning. My temp gauge reads normal. The engine will start to stutter, as if it's misfiring or not receiving enough fuel. No amount of throttle I give it, it'll slowly decline in revs. At idle, the tacho constantly bounces between 1000 and ~400rpm. When I give it throttle, it'll pulse up every second or so. If I do it in gear, the car will jump forward in time with this pulse.

To temporarily fix it I pop the hood, turn the engine off and let it sit for 3 minutes or so. It'll be fine after that.

It doesn't happen every time I'm out driving. I have been driving for 2 hours straight and it hasn't occurred. Other times I've been driving for 15 minutes and it's happened.

Took it to my local Mazda dealership this afternoon. They took it for a drive and said it could be either the coils, the plugs or the leads, but they didn't seem too sure on it. It didn't happen when the guy drove it, either.

Any idea on what could be causing it?

Its very easy and cheap to install new plugs.... do that first.
They are often the cause of symptoms such as yours.

Check plugs, and leads for any signs of arcing or degradation.

If they're good than it could be your cam angle sensor, has happened to me and many others. When the sensor gets old and heatsoaked, the signal becomes degraded.

It sound more like a cam angle sensor fault. Plugs generally are pretty linear and won't be intermittent like that. Leads and coils, yeah maybe. Where are you? If you are near Northern beaches Sydney I'm happy to try and help you work it out. Maybe take some bits off mine and swap them to see if it makes a difference.

I'm in Perth, WA. Thanks for offering though!

Checking plugs and leads now.

No worries mate. One trick that can work, not sure if it has been tested on these cars though.
Is if cam or crank sensor has suspected heatsoak, keep a bottle of water in the car and if it starts to go funny, pull over and pour some water on it. Then restart car and if it goes away. You have a fair idea what it is. Obviously try it a few times to be sure. This trick works on a few cars.

ED_MX5 wrote:No worries mate. One trick that can work, not sure if it has been tested on these cars though.
Is if cam or crank sensor has suspected heatsoak, keep a bottle of water in the car and if it starts to go funny, pull over and pour some water on it. Then restart car and if it goes away. You have a fair idea what it is. Obviously try it a few times to be sure. This trick works on a few cars.

Definitely going to try this.

Plugs look like they could use a change. I'll put off buying them until I test with the spray bottle.

This is exactly what my NB was doing till I fixed it this week by changing the Cam angle sensor. As soon as the under bonnet temps got warmer it would stutter and die just as you described. Anything over about 80kph cruising and it wouldn't stall as the airflow would keep it cool enough.

Engine temp on gauge never went past halfway.

Is the season for CAS failure. Not just summer it's happened to a few forum members lately.


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Mine did similar things a while ago - on those 40+ degree days we had at the start of the year.

I'm not 100% sure what was causing it, but I replaced the cam angle sensor and it has never happened again. Not long after that I replaced the spark plugs anyway (they were definitely due), so who knows.

What did the old spark plugs look like?

you can get a pretty good idea of how the car is going by reading the plugs.
MX5s will normally have dry black plugs, as they run a bit rich normally.

Here is a radical idea and one which was a proven fix for a mate recently. Take the car to a mechanic / MX5 expert and see what codes if any are reported off the OBD. A mates car had the same sort of problem and a dealer got a code which said cam angle sensor and a replacement was ordered and fitted. However it can be other things and diagnosing them can be tricky. Also it does not have to be hot weather that causes the CAS to fail- its actually a fault in the unit itself.

PS I did mean cam position sensor ( not CAS)