I’m looking for technical help or recommendations on: Hunting at Idle
Slipping accessory belt, big voltage spikes and a few too many replacement bulbs back in December, diagnosed to be the alternator at fault. I had this replaced. All was fine for a week or so, but since then the car has got progressively worse at idle, surging to 1400rpm after falling to below 300rpm, stalling occasionally. This problem exists cold, or hot. There are phases where it will idle for a few minutes sweet as a nut!
I’ve taken it to the garage who replaced the alternator. Read the code to P0102, changed HT leads, checked air intake for leaks and replaced the MAF sensor, all of which with no improvement.
I’ve since reset the ECU(unplugged battery, used brake lights to use residual and plugged back in as advised) and had it running - 15 minutes with no problems, but one touch of the throttle and as the revs dropped back towards idle, it kept going and stalled. Back to square one again.
Bit stuck on options, but have had a suggestion over the phone to check the ECU for water damage but any other thoughts or suggestions would be greatly appreciated!
Will check the ECU for water damage tomorrow and report my findings. Hope that all made sense!
How far has the car been driven since the alternator was replaced, or since you disconnected battery, because it takes a little while for the ECU to learn everything about the car again and it can take 15-20 miles to settle.
It’s worth a try, as all it will cost you is time and fuel?
I know that the nc suffers with this issue after having the battery off. Leaving it on tickover for about 15 minutes is enough to relearn the ecu, but you’ve tried this already.
I’m not sure if the nb has the same issue, if it does, then i don’t recall seeing it on here.
Maybe an unrelated question, but when did they change from OBD1 to OBD11 on MX5’s, because we used to have an early NB and that was OBD1?
I’m only asking because if it’s OBD1, it’s very unlikely that you would get a code showing in that way?
Hi all, thank you for the responses! bit of an update…
I reset the ECU last night as mentioned above. This morning it started fine, but showed the engine management light which went off shortly afterwards. I topped up with some super unleaded as advised by the garage and experienced only slight dips in revs all the way to work ~15 miles - great!
Started it after work to drive home and no problems at all, managing returning to idle after revving like it used to. @Micron, could be the re-learning process? @bainbridge I had some TB cleaner ready to get on with that this evening… that’s my next plan if this improvement is short lived.
Thanks again everyone. Will update the thread with any findings if it returns. Fingers crossed for now!