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My introduction post veered a little off course when going over the issues I'm having.

I'm basically getting an intermittent issue with my MX where it idles rough and struggles to accelerate. I'm unsure why it's rough, but it's missing and smells rich.

I was hoping that somebody was around Gold Coast with an NB8B who could meet Thursday night and lend me a coils and/or cam angle sensor for a quick bolt-on check before I try sourcing them myself. Any input would be good too. I don't know any owners yet, so throwing it out there.

I'm on Chevron Island so the closer to Surfers Paradise the better, as it's like driving a tractor and I hear you can damage your cat when running this rich. That said, I'm not about to be fussy.

If this falls through I'll be organising a mechanic after the weekend anyway, just thought that it wouldn't hurt to ask.

Cheers!

Is it only doing this when hot or all the time?

I don't have a reliable pattern.
It seems that it often occurs more when it gets warm, however it's also come good after driving for 10 minutes before.

That said, it's usually either good or bad when first starting the car and stays that way until ignition is off (then next start is random) It's apparent immediately when starting the car and usually dips to 500rpm before popping back up to ~1000rpm when bad (When good it's about 900-1000rpm immediately)

I tried pouring cold water on the cam angle sensor when it was rough, but that didn't seem to do anything. Maybe it didn't cool enough.

I've also made up the diagnostic light (2000 ohm resistor and red LED) which resulted in a quick half second flash when switch to ignition, and nothing more.

The only thing I've replaced so far are the plugs, which were black.
I've been running it on 98 (v-power last time) not that I thought that should matter...

If you look at the Mazda workshop manual is goes thru a trouble solving listing- whether you have the equipment to check or knowledge is another thing. Join the club of not being the full bottle on all things for diagnostics on modern engines. If its running rich I don't think it will have anything to do with cam angle sensors but take that as something not to be relied on. Replacing plugs will make no difference i.e. if they are black then you are looking for root cause - the black plug is the result. Mechanics can check the diagnostics on the car. Its good if you can do swaps of components but that is not always possible esp if you do not have mates with this model. It may also be related to the coil(s) but if it cannot be replicated in a workshop its tricky.

So its an intermittent issue or happens quit regularly?

Its NB8B's they do odd things sometimes that are difficult to diagnose.

Iv had all sort of little things happen to me. I was driving up to mx5 plus one day in traffic on the m1. We were stopping and starting so much the car must have got sick of it. I went to take off it almost stalled, running rough, idle going up and down and wouldnt rev over 2.5k rpm then I couldnt get it over 4k rpm. Then the next second it sorted itself out and drove normally lol. Still not sure what caused that but hasnt happened since.

Iv replaced both the cam angle sensor and the crank angle sensor and it ran much more smoothly after that. Looking to replace the green EGI relay, 2 days in a row this week I started it in the morning to have the revs to go up to about 2000 rpm then drop to about 400 rpm the first day it caught itself and revved back up to 2000 rpm then settled on 1500 rpm for cold start but second day it done it it stalled itself.

Im near Helensvale so I could probably meet you Thursday arvo/night if you wanted to try the cam angle sensor since mine is fairly new. Yours could be the EGI relay on its way out too if its running rich. Mine smells very rich on a cold start, do you get poor fuel economy?

When it plays up does it idle rough and drive rough or does it clear up with some rpm on board?

Mr Morlock wrote:If you look at the Mazda workshop manual is goes thru a trouble solving listing- whether you have the equipment to check or knowledge is another thing. Join the club of not being the full bottle on all things for diagnostics on modern engines. If its running rich I don't think it will have anything to do with cam angle sensors but take that as something not to be relied on. Replacing plugs will make no difference i.e. if they are black then you are looking for root cause - the black plug is the result. Mechanics can check the diagnostics on the car. Its good if you can do swaps of components but that is not always possible esp if you do not have mates with this model. It may also be related to the coil(s) but if it cannot be replicated in a workshop its tricky.

Thanks! I didn't expect plugs to change anything, just mentioning it. It's the only thing I changed since I bought it (A little over a week ago)

I know nothing about modern engine diagnostics, but open to learning. I was hoping that I'd get some flashes out of the LED as per various internet how-to's but wasn't particularly optimistic about it.

I was hoping that it would actually start happening consistently as I know that as soon as I book it into a mechanic it will behave. I've had this with any intermittent issue previously.

The cam angle sensor is only on my list because it seems people have mixed issues with it.. so worth just trying.
I had a hunch about o2 sensors, but figured the engine diagnostic would have picked that one up.
(Only thought o2 because of mix. I also thought air-flow sensor, but feel I've ruled that out.)

I was hoping to just make sure it's not the coils as it's something I can try easily and replace myself. I don't know anybody with an MX-5 in the area (let alone the correct model), hence this post to see if anybody would be keen on helping me out with a diagnostic swap. Doesn't hurt to ask, right?

I started reading up on bench testing coils, but it would be easier to plug another in. I'll try to find the workshop manual and look over the steps you mentioned.

speed freak wrote:So its an intermittent issue or happens quit regularly?

It happens quite often but not consistently. It was fine when cold this morning, and when I drive it home at lunch, then rough when I hopped back in. It feels like it's happening a bit less than half of the time.

speed freak wrote:Im near Helensvale so I could probably meet you Thursday arvo/night if you wanted to try the cam angle sensor since mine is fairly new.

I could make it to Helensvale, I'll touch base with you on the day and make sure it's still fine. I really appreciate it.

speed freak wrote:Yours could be the EGI relay on its way out too if its running rich. Mine smells very rich on a cold start, do you get poor fuel economy?

Certainly has some poor fuel economy. I think I got ~300km to my last tank with it mainly being drove when good. I swear I was watching the needle drop when driving rough from Burleigh to Broadbeach on Saturday morning.

speed freak wrote:When it plays up does it idle rough and drive rough or does it clear up with some rpm on board?

When it plays up, it idles rough and revs take a LOT of effort. It's hard to bring up to 60 and doesn't want to play, particularly bad at the lower end though (around 2k rpm) it will rev up, but havent taken it past ~3500rpm as it really doesn't feel like it wants to.
It has only made popping sounds once and usually just feels like it's struggling.

Well Im more then happy to help/take a look. Im no mechanic but know a few things and we can try a few different items off my nb8b and see if it works.

Sounds like we should be able to get it to play up hopefully, if not when cold it sounds like it usually runs rough when warmed up.

It very well could be the cam angle sensor but Iv been thinking it might not be too. Normally the tell tail sign of the cam angle sensor on its way out is it might cut out/stall when its hot/at running temp. Or it will be difficult to start or not start at all and only crank over after its already been dríven and the engine is still warm. If it does this like you said thats when you can pour some cool water over the cam angle sensor to cool it down then it will normally start up.

Must be running really rich. Around town you should normally get about 500 km to a tank (about 38-40 L so not entirely empty) It could be the o2 sensor, which would cause it to run rich and I would suspect it to be sluggish but maybe not as sluggish as you described.

So you have replaced the spark plugs? It could be the leads or the coils. Iv replaced my spark plug leads too so we can try those aswell if nothing else works.

NB8B's seem to be funny, some do get these intermittent issues which can be hard to diagnose because I know mine everytime it goes to the mechanics and I tell them about it, it runs perfect for them. At one stage they had it for an entire week and drove it everyday and it ran perfect

Im sure its only something small though its just finding the culprit.