My MX5 has spent the last two weeks sat on my parent’s driveway undriven as I had to borrow a more practical car for a while. Dad informed me the other day that the battery had gone - he hadn’t driven it, it had been sat in the cold and the battery was a bit suspect. Anyway, I replaced the battery today with a new one and after a bit of a panic when it didnt start (immobiliser…doh), it now works fine.
HOWEVER, it’s developed a bit of an odd idle problem. The car idles at it’s normal speed when started (about 850RPM when warm). But, if I rev the engine a bit, as I come off the throttle, the revs drop too low to about 500RPM or so, caushing a noticable judder, then it seems to catch and recover itself. It does this a few times, then returns to normal idle after a few seconds. It’s very annoying and has never happened before the battery was changed. It’snearly caught me out a few times sitting at traffic lights as well. Ive just taken a quick video (see link) after I took the car for a blast up and down the motorway, so the engine was warm.
What could be the cause of this? The car was fine before being left. The only thing that’s changed is the battery. The temperature guage is fine, the oil pressure is fine. The engine is smooth as ever, apart from this idle dropping problem. I have heard of the Idle Control Valve needing adjusting/cleaning, but it was fine before the battery was changed. Could the ECU be damaged after being without power for so long?Â
Any help much appreciated. The video below should show the symptoms, including the noticebale shake through the cabin you can see on the gear stick. Car is a 2006 NC 2.0 Sport (6speed).
 Hi, well it’s only dropping 100rpm before being caught. It has a knock sensor so it may well be an adaptive ecu. Having the battery off a few days will have reset factory settings. You can try one or two things, start up and leave it to idle for 15 mins see if it learns idle. Then take it for a drive round giving it a bit of work to do on hills and various speeds in different gears. Or just drive it normally and see if it learns and improves to the fuel and driving conditions over a couple of days. There are also load sensors for heated rear window, aircon, and probably power steering so see if not using these devices changes idling, particularly at this time of year with demanding electrical loads. I think just giving it a little time to learn the load settings will see a better idle return.Â
 I went out for a drive last night (partly to test it, but mainly because I wanted to go for a blast :D) and it actually cut out at the lights at the end of the road. I restarted with no issue and was fine for the rest of the drive.
This morning the idle was still lumpy. Even my girlfriend noticed it as she got in. I will update the thread if it improves but if it’s not settled by next week, I might have to get someone to take a look at it.
So this morning the car as no better. Still a rough idle, and it even cut out at the first set of lights as I set off to work.Â
On my lunch break I gave the local Mazda dealer a ring who had no idea but suggested I bring it into them for an “invstigation”. Knowing that the answer to any issue from them would probably be “New ECU needed, kthxbye” I politely declined and gave Freelance Mazda in Chatham a ring to get some advice. Helpful as ever, the guy there suggested that I give it a few days and if it’s no better, pop in and see them to run a diagnostic. He also suggested that he heard that it may have been a procedure to leave it idling for a certain amount of time. As the car sits right outside my office within view of my desk, I went outside, started it, and left it to idle for a good 50 minutes with all the electricals turned off. Shut down about 2.30PM and left it.Â
Got back in the car at 5.30PM to go home and hey presto…idle was steady, didnt drop at all and didn’t cut out. I drove all the way home and whenever I have to stop, the idle dropped back to a rock steady 800RPM just like it should do…no fuss whatsoever, back to how it was
So for future reference, if you have to change an NC’s battery it looks like letting it idle until warm is the best way for the ECU to sort itself out.
 Hi, that’s great, thanks for letting us know  It is a known one, see my first post, but I couldn’t find anything specific to MX5 Mk3 which is why it’s good to hear back from you. It probably isn’t finished yet and may store more settings as it learns them, depending on load, revs, emmissions, as you drive but that may not be as noticable of course as an engine that stalls.
How idle works I gather is to home in on cleanest emissions rather than pegging an absolute idle speed. Idle quality and speed will vary a little depending on the day, temp, humidity, fuel etc and it may hunt as it goes past lean then catches it and reverts the settings. A bit lumpy is normal, I can feel mine do it, and presumably they refine and store the settings on an ongoing basis.
Others like Saab ecus push the settings to get more power/economy from higher octane E85 bio fuel giving 20% increase in bhp without the driver doing anything. It’s all interesting stuff and it would be good to hear from anyone who can expand the subject a bit for us!
I have a similar symptom with my '98 Mk2 1.8 iS…
After starting, the warmup rpm gradually settles as it’s supposed to; after a short drive (say 5 miles or so), when stopping, the idle also drops to about 650 on the rpm gauge for a few secs, then recovers to 850. If I drive it a bit more (or harder LOL) so the engine is well hot, symptom disappears…
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Could this be allied to the ECU having “lost” some settings and may benefit from the “extended idle” thingy? I’ve had the car a couple of months now & have driven 500-600 miles so it’s had opportunity to “learn” if that makes any difference.
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Oh BTW, battery is a bog standard (non-OEM) maintenance-free lead-acid type, cranks noticeably slower in cold temps so could also be in need of replacement.
Worth a shot. Leave the car idling for a while, maybe 30 minutes or so and see if it helps like it did with mine.
Couple of things to note though - mine was doing this even with the engine warm after a long drive. Also, yours is a MK2 so it has an adjustable idle speed (have a search on the forums/Google, adjusting your base idle on the NA and NB MX5 is quite well documented)
Hi, the battery q is interesting as I’ve been looking myself. The spec in my Mk3 2.0l handbook just says 36AH with no CCA rating but from memory the original one is 300 odd A.
Looking in Halfords for example it seems technology has moved on a bit and there is a choice of 3 batteries for all MX5s, check the Spec tab for each and you’ll see a big difference in AH capacity and CCA for the 063 size,
The replacement battery I bought was a Halfords one. I cant for the life of me remember the code, but it was ~ÂŁ75, it fits perfectly in the MK3 and it cranks with no problem. I will take a picture next time I pass the car and let you know.
Thanks, or just the Halfords code would do to identify which specification.
What I found surprising is the 3 qualities/g’tees/prices that were available in the same size battery. Looking more carefully I see they just quote “startup power” and don’t say if it’s actually; cold cranking, cranking, or hot cranking Amps. If it’s the same comparison temperature for all 3 there’s quite a difference in performance to be had. It’s also a very small battery for a 2.0 car and is the same size as the original on a Mk1 1.6 20 years ago so I’d probably go for the highest rated I could find.
Hi Bill, yes Halfords and others also have the Mk3 down as using an 063 same as the Mk1 and Mk2 which it doesn’t. It needs an 053 = JIS 46B24L(S) = DIN 545 23.
EDIT, just had a look and see your 075 is similar width and height but 30mm longer than 063 so is fine if it physically fits as you’ve found.
In the case of Mk3, the 063 is shorter than 053, but is wider and lower so it simply won’t fit at all
 Just fitted a new battery to ours. Original Panasonic was dated July 2005 so that’s done well. It was dropping to under 12V within a few days of charging so I guess it was time.
Fitted a Varta B32 45AH 330CCA from the local motor factor, fits perfectly, cranks well, and has a handle so I was able to clean out the leaves and rubbish from the battery box and leave off the tatty strap!
No issues restarting, the dsc flashes till the wheel is turned, then switched engine off then on again to lose the light. Radio needed retuning. Trip mileage was lost. Idle was a bit erratic but didn’t stall, worse with AC on. Went for a 20 mile drive and seems fine now. Scanned OBD before and after but nothing of note. So a good and easy job  Â
My 2000 1.8s does exactly the same! I recently had a new coil pack fitted & full service which I thought solved the problem but it’s still there (could be as it’s bloomin’ cold!). I’d be interested to see if the extended idling trick solves your problem.
 It is a known one, see my first post, but I couldn’t find anything specific to MX5 Mk3 which is why it’s good to hear back from you. It probably isn’t finished yet and may store more settings as it learns them, depending on load, revs, emmissions, as you drive but that may not be as noticable of course as an engine that stalls.
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