MX5 NC - Idle speed dropping too far after a battery change.

Technical Area Engine, Transmission, Exhaust etc
21

No it didn’t change anything, though the problem seems much less frequent now I’ve put some miles on the car with the “occasional” spirited driving! Have also been adding Millers additive to clean/de-gunk any lines or injectors that may be a bit varnished.Big Smile

22

Hmm,

I’m having the same problem reported originally in this thread. The idling drops and at times is close to stalling the car. Since my NC is a 2005 model and I doubt the battery was ever changed (2nd owner), I’m suspecting the battery is somehow to blame. I just picked up the car from a bumper repair where it presumably spent three days switched off and the problem started happening again (it hadn’t occured in a while). I’m suspecting the battery is not retaining charge and hence the ECU is losing it’s setting. Does that make sense? Are there any other tell-tale signs of a battery needing replacement (it doesn’t pose much of a problem starting usually).

 

Joe

23

 Hi Joe, could be, things that happen if the battery has gone flat or removed are;

the ecu fault codes are lost, the radio station tuning is lost, and the DSC may light on the dash and need lock-to-lock movement to reset. With the ecu seems to go the loss of idle setting and a base idle is restored that takes a while to adapt/learn again.

Others have noticed the handbrake light showing under load with the lights on with a weak battery.

If your battery is original it may be a Panasonic wetplate (with filler plugs). On the top is a long black/yellow sticker all in Japanese except, across it stamped in a box, is a date code as day month year.

24

Dying batteries are now starting to cause lots of problems!

 

25 

									    Re: Strange problem - handbrake light - advice please!
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Thanks for that!  I had the battery effiecency checked yesterday, and the battery is dying..... not critical at the moment, reading 12.1 volts without any demand on the battery, but I am not happy! ..........the battery has done 96,000 miles and has been in the car from new.

Comments on the USA forum mention batteries just dying without warning!

Run the car without any headlight, heater, stereo, or other ‘demand’
on the battery and the ‘caution’ light disappears…

I have just ordered a new battery… http://www.advancedbatterysupplies.co.uk/carbattery053.html

HiQ wanted £86.00 …dont think so!

26

Thanks All. I think the battery had already been replaced as it wasn’t a panasonic. That said I decided to replace it anyway since it was some unknown brand and it was leaking acid and there was corrosion all round it’s terminals. Besides, it wasn’t maintenance free and had been left dry once already… Car now seem to be starting better so I think it’s just as well that I changed it. The idling is once again hunting but this is to be expected. I guess it will settle down with some use.

 

Thanks again for the pointers.

Joe

p.s. i replaced the battery with a sealed Varta B32.

27

My NC has this very problem - the battery died over xmas/new year when the car wasn’t being used.  I replaced it with the Halfords 053 and now it has the droopy idle …

I’ll run it for half an hour at tickover tomorrow, hopefully it will sort it.

A snatchy throttle on my Triumph Bonneville was cured by this method - idle for 20 mins, let the adaptive ECU adapt.

28
Hi, just a long shot that may be worth a try. I have this prob on my BMW motorbike (Canbus wired like the Mk3.5 MX5) after the battery has been disconnected for any length of time - i.e erratic tick over when it's started up again. The problem there is that with everything being electronic, the throttle position sensors have to be re-set after the power has been removed.  It's done after reconnecting the battery by turning on the ignition but not starting the motor, then opening the throttle fully 3 x times. It ticks over fine afterwards.
Sorry about the italics - they just appeared mid typing and I can't get rid of them    Smile
29

I worked as a techy at a modern car dealers for many years and with the adavent of multiplexing or can bus whatever is your desire to call it came very clever systems that adapt to driver style and conditions it was common for the cars to do a fast motorway jaunt and then at the end of slip road it used to stall (citroen owners ) this was because the adaptive system was to slow to pick up and reset .

After a few miles everything was ok and back to normal it is possible that the battery going flat on the mazda or any modern car may allow the Ecu/computer to revert to base settings and then it would need a procedure to set these back into the Ecu hence leave it idling for 15 mins or so.

We used to update the ECU with new software at service time or when recalled to cure many problems and afterwards the car had to learn full throttle and minimum throttle via a laptop,  then it was taken on a road test and driven at different speeds to learn all parameters. If this was not done the customer immediately came back with stalling and idle issues.

If it was an auto it applied even more set up . I don’t doubt for one minute that the MK3 has to have this done after battery or software upgrade but they do seem to like to eat batteries.Big Smile

 

30

Hi there,

Very useful picture and info.  I think I have a pending battery failure on my 2007 2.0 Sport - open circuit voltage around 12.4 volts (12.7 is the norm) and noticeably sluggish cranking even in the summer.Sad

Please can you tell me how to unclip the water hose that runs along the side of the battery cover?  I don’t want to dislodge anything - is it just a case of pull?

Also, does the battery fit well?  I’ve seen another post where the terminal size was wrong and had to be spaced (alarmingly, the person used a screw or similar which I would think could easily form a high resistance hot spot).

Comments welcome - thanks.Smile

Mark B.

31

 Hi Mark,

Yes that’s about the right timescale, mine went at a similar age with low voltage as you find.

I went for a Varta B32, £50 from local factor, g’teed 4 years, case and terminals fit perfectly. I’m not sure why the other member you mention had trouble, maybe the terminal wasn’t pushed down onto the tapered post fully, but it is the correct battery and works great. It’s sealed type btw, no removable filler plugs, so keep charge voltage below 14.4V to avoid gassing and losing water if charging. I took the opportunity to buy a smart charger as I can see most batteries will need this soon. Alternative is 053 from Halfords or elsewhere.

Don’t worry about the battery cover or that hose, just pull the hose out of the clip, it won’t break, and unfasten the clips to remove the cover top, the Varta fits perfectly Thumbs up 

32

Thanks Rich, that’s a great help (quick reply too!).  I’ve got a service and MoT to sort out first (next friday) but rest assured that before the Summer is out (and before I get stranded somewhere), I’ll be going for a Varta B32.

Cheers, Mark

 

33

Hi,

Just an update on this. The Varta B32 I installed is just fine. It turned out though that the idling issue had nothing to do with the battery…

Initially I had left the car idling for 30mins as suggested in this thread and that had seemed to solve things…but only for a few weeks. After a few weeks the issue started again. Whenever I’d drop the car out of a gear and into ‘free’ the revs would drop to low and would keep hunting from about 500rpm to 2000rpm.

I took it to my mechanic and he he suspected it was some dirty sensor. He dismantled the air intake and cleaned one of the sensors there (not the air flow sensor in the air intake assembly; there’s another sensor further inwards - closer to the engine). After that, no issues. He told me it’s quite a common problem with cars after a certain age. Apparently, one gets a carbon deposit from combustion on this sensor. Over the years, this (unwanted) builds up and can upset the sensor’s reading - hence the revs issue.

 

Hope that helps.

Joe

34

Odd, browsing and found this. Replaced my battery on my mk1 1.6 when I bought it as the car had the original in it from 1994 from Japan Shock

Anyway, since doing so, it randomly drops down in revs but picks it’s self back up after a second or 2. This is if I come from high revs down. Not every time, but most times.

Would leaving it idle for a while help?

35

Some extra feedback:

Was working in the boot (fuel filler pipe NRV replacement) today, & of course disconnected the battery…

30 mins later, went for a drive & refuel: the “low idle after battery replacement/disconnection” symptom returned…

Left idling for approx 10 mins & problem seems to have gone; so it DOES seem the ECU can lose the “active” idle setting/ idle droop after decelerating, & it CAN be recovered by extended idle by the looks of things!

Wouldn’t have known this of not for this forum, thanks guys!

 

Regards, Bill

1 Like
36

I have just experienced the OP’s symptoms after I had disconnected the battery to rectify a loose earthing cable from the battery negative to the chassis (black with yellow stripe) (2.0L 2006 NC) . I had to remove the battery & battery box to gain access to the bolt which clamps the cable onto the chassis. The loose cable has not caused any running or charging problems with the car/battery because there is a braided cable from the battery to the engine block, but it seamed as tho’ it had never been tightened down in the first instance as the bolt was not loose and could not be turned by hand.

I did think that the issue may well be the “Keep Alive Memory” of the ECU and that it needs to re-learn various driving characteristics. This was two days ago and it seams to be getting better, but I might heed the advice of leaving the car to idle for a while if it doesn’t get any better soon.

Thanks for the advice :slight_smile:

37

If the car won’t idle, just hold the revs at 3k for three minutes should reset itself.

38

 

UPDATE!

after an few days driving to & from work wondering if the engine’s going to cut our (which it did a couple of times) the idle has now reverted to its correct 800 or so rpm. It got better as the week progressed but it finally settled down after an hour & quarters “stuck in traffic” run of 17 miles to the Yorkshire Ridings Social Night. 

Happy MX5 and owner again Cool

 

39

I have used this feature of the ECU loosing a few settings just before long trips to get better MPG as it has to relearn…:slight_smile: (my best being 47.5 MPG)

The RPM does take a while to settle down back to normal after about 20-30 minutes of driving.

40

If the battery has been disconnected you need to re set the ecu.

Start engine.  Use a peice of wood to hold the throttle down so that it holds 2500rpm.  Then leave it running at 2500rpm.  Don’t try doing it with your foot as small movements in your leg muscles will spoil the process.  After about three minutes the engine revs will suddenly drop down to normal idle event hough the throttle pedal is held in place.  At this point the ECU should be reset.  

You will also need to turn the steering wheel fully to the left and fully to the right after a new battery as the car needs to re learn the full left and right lock of the steering.

Until you do this the traction control light remains on.

 

 

 

 

 

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