Idals rough
This one could be caused by so many issues - I'll try to cover a few of the common ones but more information would probably be very helpful.
Check to see if the car idles better with the clutch and the transmission in gear, or with the clutch out and the transmission in neutral (and all combinations) etc. If one of these combination corrects the problem, you may have a faulty electrical switch which informs the computer to adjust the idle revs slightly for one of these conditions.
You may also wish to experiment with increasing your idle revs slightly if it is dipping too low - there is a screw on the top of the throttle body for this purpose. Try adjusting it and see if it helps.
There is also a small dashpot towards the front of the throttle body which prevents over-dip when dropping to idle from power. The spring and piston on this can seize sometimes, prevent the correct power dip. Check it runs freely and anti-seize it if necessary.
NOW - the more mechanical problems.
POOR AIR SUPPLY
Is your filter fresh? If not, replace it. Poor air intake will cause all kinds of idle issues. Clean and check your air intake and fittings - leaks or dirt will create problems. The idle air bypass in the throttle body is often a problem - check this and clean the TB. Be careful not to destroy the seal on the throttle plate.
The idle air control valve (IAC valve) sometimes fails to operate correctly - this valve allows the air through the bypass to stop the engine starving. The test procedure for this is a little more complicated - I will post it as a secondary solution if required (ie. none of the easier options work).
The MAF (mass air flow) sensor and the air temperature sensor in the air intake box may also be playing up - this will be indicated by the engine diagnostic codes. It would pay to check your engine error codes in case they can save you some time. If you're driving an american model WITH a check engine light on the dash, put a paper clip between terminal 10 and GND in your diagnostic box (right side of the engine bay near the air intake - has a big DIAGNOSTIC written on it). Then hop into your car, turn the key to ON (but don't start the engine). The light should go on steady - then off for 4 seconds. After this any diagnostic codes will be displayed. Long flashes are for the first digit in a two digit code - short flashes are the second digit. There will be a 4 second pause between each code AND the codes will repeat. If you get codes, record them and post them and I'll see what I can suggest.
Once air is ruled out, you can turn your attention to fuel. If the car is otherwise running OK, I think the fuel pump and injectors are probably fine (no guarantees though!). The one part which you can probably check on the fuel system easily is the fuel filter. If this hasn't been replaced in the last 60000 miles, it should be.
Finally, (and most likely), your issue could be caused by poor ignition. Change your spark plugs and spark plug wires if they are due (every 30000 miles). The original wires in the NA series MX5 tended to deteriate somewhat and sometimes didn't even make it to 30000 - replace them with good quality replacements.
Check your diagnostics! The ignition system relies on a lot of inputs - one dodgy value from a sensor can really throw things out (especially at idle). Common culprits include - MAF and air temperature sensor, coolant temperature sensor, Cam Angle Sensor - and wiring faults to these sensors. You may also wish to check your coil pack for correct resistance and operation.
Hope these points help - there is no ONE issue that anyone can point a finger at - and idle problems on MX5's are relatively common. If you get a diagnostic code - you may find it quite easy to fix. Otherwise it's a long slow job of checking everything, tuning it and maintaining it if required. If your MX5 hasn't been serviced in a while, I'd probably start with all the things that really need servicing.
Cheers,
Sherwin
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