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I need some help to diagnose an idle problem with my NA8 please.
It came to me with a known dud engine, with severe oil blow-by in cylinder #3.
With the help of some forumites (thanks again Emily and Adam!) I have installed a replacement NA8 engine
The replacement engine was reported to have been running with no problems, although I didn't get to see it running to confirm that.

When first run after the install, the engine idled and ran fine when dríven for about 30 mins, then it started a surging idle cycling between about 1500 and 2500 revs around each second.
- When started from cold, after about 5 mins it starts this surging idle behaviour
- When started from hot, it starts the surging idle immediately,
- If dríven, the surging continues, although it isn't as obvious as at idle
- If the IAC valve is disconnected it goes to a steady idle
- I have measured the resistance of the IAC valve at 13ish ohms
- After an ECU reset by disconnecting the battery overnight, on initial ignition on and again after an engine run, no diagnostic error code is displayed on a led (+ 1.8k resistor) between B+ and FEN

With the severe oil blow-by of the original engine, it is likely that a lot of oil has gone down the exhaust.
Could this have affected the O2 sensor and be causing this behaviour?

Any other suggestions on the cause of this problem, or other tests I can do to assist with diagnosis?

Thanks
David

Further information.

The replacement engine was complete, and was installed with the attached coil packs, plug leads, engine sensors, intake manifold and throttle body.

probably wouldn't hurt checking out the O2 sensor in the exhaust.

If its blocked, it could be causing the car to run super rich, which could affect your idle

Thanks Regie, I will remove and inspect the O2 sensor.

FWIW, here is a video showing the surging idle.

might need a quick clean of the ICV also

nice voice over there mate

Yes, cleaning the throttle body and idle control valve look like the next move.

Today I removed the O2 sensor from my NB8A, which has no known problems, and tried it in the NA8.
No change :-(

Glad you liked the voice over. I have a face for radio, and a voice for print media :-)

That surging isn't uncommon & mostly at least seems to be due to a sticking IACV.

Removed the throttle body today, but I could not separate the IACV valve assembly even after trying an impact driver on the 4 screws.
So I just used some carby/throttle-body cleaner on the entire assembly.
Unfortunately, no change, with the surging idle still there.

As an experiment I disconnected the TPS while the engine was running, and the idle immediately went to about 3,000 rpms, but stable with no surging. I wondered if the idle-switch in the TPS could be faulty, so I joined together the ground and idle-switch pins of the TPS cable plug, simulating the throttle at idle, and again the car demonstrated a surging idle.

I am out of ideas. Any suggestions please?

david_syd_au wrote:Removed the throttle body today, but I could not separate the IACV valve assembly even after trying an impact driver on the 4 screws.

I am out of ideas. Any suggestions please?

I had a look on Miata.net, & the surging is commonly due to sticking or faulty IACV. If you can't remove the IACV assembly screws, & here's a recent video by a forumite on doing so:
How to remove IAC valve:
viewtopic.php?f=29&t=61437

Then if possible may be carefully hacksaw the screw heads off, or replace entire TB including IACV.

Also check the PCV valve on top of the cam cover. It should let the manifold suck when there's low vacuum (e.g. running with some throttle) but close off when there's high vacuum (e.g. at idle). If it's opening and closing at idle instead of just staying closed then you'll get idle surging. Take it out and shake it to make sure it moves inside, then run the car with the PCV still off to see if you can hear what it's doing and test if blocking the inlet with your thumb makes any difference. I replace them after about 100 ks with $10 generics from Repco - they're effectively consumables.

Given the history of the past engine then it's possible you have a partially blocked cat convertor as well. I'm not sure that it would affect idle but it will affect power.

Well, the problem has been solved, so I though it best to write a post to close this topic.

FWIW, I did try another known-good throttle body, with no improvement.

In the end I gave up and handed the car over to MX-5 Mania, and they soon diagnosed the problem as coming from the ECU. Fitting a replacement 2nd-hand ECU has solved the problem.

In hindsight, it should have been obvious that the ECU could be suspect, as when I got it, this car was waterlogged on the passenger side from blocked drains. It is quite possible that the ECU suffered water damage.

Now I can get this little beastie registered and back on the road :-)

Thanks to everyone for your advice and suggestions.