Mx5 NC1 1.8 Manual Idle Issue.

[mituc]

Since it's an NC there's a change that the rubber/silicone o-rings between the intake manifold and head are crusty and not sealing any more. So you may want to take a look at that, because the symptoms and the inconsistency of how they appear (usually for vibration/heat related reasons, which is hard to replicate) kind of fit.

[@NG_MX5 Niall]

Little update... I think I might have solved it.

I tried some nice simple resolutions first before I dismantled items.

Did the old, disconnect the battery and clean up the terminals, leave disconnected for 20 mins, then reconnect.

No dice.

Then left it idling for 20 mins without touching anything.

Still no dice.

So then tried holding the car at 3k rpm for 3 minutes and then the ECU "reset itself".

This is where after 3 minutes the engine sends itself back to idle without moving your foot on the accelerator.

After this, the problem seems to have vanished completely.

So for now, I think this may have been all that was needed. But I'll touch base should any more problems crop up again.

At which point I might try dismantling and cleaning out the throttle body. As I've never done this and have no record of it ever being done, so will probably help. But the throttle body / intake gasket seems to be out of stock in all the usual places at the moment anyway.

[mituc]

So fuel trims are fuel trims, these are deviations between what the ECU expects and what it gets.
Idling for 20 minutes doesn't reset anything. Keeping the engine at 3k rpm doesn't reset anything. I have no idea where this procedure came from. Doing all this just warms things up and if you have one of those intake manifold o-ring issues then heat will simply fix it. But the problem will come back when the car is cold, 892.64% guaranteed.