Unstable revs when slowing down in neutral MX5 NC

Technical Area Engine, Transmission, Exhaust etc
1

Weird problem,
i get unstable revs (going up and down) when i’m slowing down (like at a traffic light),
switching to neutral gear.
The weird thing it’s that when i full stop the car the revs become stable.
I’ve no other engine problem when using the car (like if i slow down without changing gear to neutral).
The problem is oftening happening when the car is cold.

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2

Have you had the battery disconnected recently ? If so, it takes the car a few miles to relearn the fuel trims and parameters before it settles down again.
If not…I have no clue

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3

no didn’t change or disconnected the battery

4

either TPS [ throttle position sensor ] OR air [vacuum hose] leakage in around the injection manifold would be the first candidates…w

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5

Have you checked the battery, I had this issue on an old original battery on my 2006 NC.
It finally died and with a new battery the idle is very smooth.

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6

Update,
first try, i’ve disconnected the battery and reconnected to reset the ecu.
The idle was very unstable, the car also stalled and engine got shut,
i’ve drove for some days (i’ve read the ecu will relearn the settings) but no luck.
After reading on another forum i’ve done the following:
Let the car engine warm up,
disconnected the battery,
reconnected.
Turned on ignition without starting the engine for 1 minute,
turned off for 1 minute.
Turned on the car without any electrical device on (radio, lights etc).
Idle for 15 minutes not touching gas or anything else (neutral gear).
Turned off, turned on and did Dsc steering wheel calibration.
All perfect! Idle is now really stable (900 rpm) even when engine is cold. :ok_hand:

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7

I’ve recently changed my battery, after resetting the ESC just letting the engine idle for around 15 mins does the trick. It should idle when settled at 800 rpm.:+1:

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8

yes, i wrote 900 rpm but looking is one tick under the 1 in rpm :slight_smile:
it’s 800 or 900? :smiley:

9

I think i found the main issue that first caused the issue.
It was low/faulty battery, i had the car stopped two days, and that morning the revs where unstable again.
I think when the battery is really low the esc resets itself. (the radio stations don’t suffer from the low voltage and are still memorized).
And this leads to the unstable revs.