Worth doing a diagnostic check to see if that shows any error codes but sounds like a change of throttle body may be a good idea.
Cheap to buy secondhand and reasonably easy to fit - within your capabilities I’m sure:-)
There is a separate Idle air valve on the MK1/Eunos 1.6 but all other cars have the functionality and components discussed within the throttle body.
Just re-read the original question. Do you mean it sits at about 1500 when first started from cold and gently drops back to around 900 as it gets warm? If so this is normal for all MX5s and indeed all cars. The ecu using temperature sensor etc automatically adjusts the fuel/air mixture as did the manual pull choke used on old cars to provide the car with a temporary richer fuel mixture until it has warmed up. As the car reaches optimum running temperature the fuel/air mixture is leaned out to its normal level and the revs drop to that expected at idle.
Mine is still going well and very easy to use. Last time I checked there was UK stock for around a tenner on ebay. When I bought mine x years ago it came from the orient.
There are far better ones available but for our purposes a cheap plug in unit that does the job effectively is my recommendation.
Don’t leave it in a damp car though - mine played up for a while when I did this.
I agree with rhino, sounds like its normal to me. All my 5s act the same way from cold to warm. If it stayed there or started ‘hunting’ for the revs then its needs looking at or if you perhaps can smell its running rich. But no harm in checking for codes.
An update for those who contributed to this thread (for which I thank you) following a few tests.
When you first start up it idles at ~1,400 rpm. If you leave it then the revs slowly drop as the temperature rises and after about 5 minutes, when the temperature gauge is just starting to creep onto the scale, the revs are at ~ 900. From what others have said this is perfectly normal.
I did also get an OBD reader but it didn’t find any error codes. I did check it on another car and it did find an error code, so the reader is working okay.
Thanks for everyone’s help. It looks like I was worrying unnecessarily but now I am reassured and I’ve learnt a few things along the way. I also have a code reader for the future so a worthwhile exercise I feel.
If it is taking a long time to raise the engine temperature, it could be that the thermostat is stuck open, and the rest of the control system is doing what it ought to. I would expect the engine to rise quickly in temp, in say one to two minutes (depending on the weather), and only then the will stat begin to open and hot flow begin to pass through the radiator. If warmish water is flowing immediately while the engine is still thinking it is ‘cold’ then this points to the stuck-open stat.
Feel the top hose for a minute or so after a proper engine-cold start, this should tell you what is going on. The water should be either cold or hot, and rarely just warm.
There are other hoses carrying hot water (heater, throttle body, etc), but the top hose to the rad is the key one to check first because the rad has by far the greatest cooling effect.