Engine starts ok but revs to more than 1500 then drops to stalling point when cold

Technical Area Engine, Transmission, Exhaust etc
1
  1. My model of MX-5 is: NBFL
  2. I’m based near: Grimsby
  3. I’m looking for technical help or recommendations on: engine runs fast on start up when cold then revs drop to the point of stalling. Problem clears when engine is warm. Any ideas? Main dealer has checked it over but no faults found but then the system is not very sophisticated!
2

I would start looking at the air idle control valve first, this is probably more likely culprit.

next possibly a coolant temperature sensor fault but that would probably show up more on a diagnostic test.

Another place to check would be all vacuum hoses to see if anything is dodgy.

After that it might tps or very dirty throttle bodies.

Rich

3

Having been reading about these problems on and off for 8 years if this was happening to my NBFL I’d start by checking the plugs, and HT leads, strangely puny items on my ‘02 when I got her, and coils which can get tempremental with age and temperature. So when cold after the revs drop I’d verify I was getting a spark. Once sure the sparks are flying when cold I’d look at cleaning the throttle valve and Idle Air Control (IAC) valve and check the IAC valve resistance is approx 12 ohms. Then look for vacuum leaks.

4

I’d start by setting the BASE IDLE.

Then air leaks

Then plugs and ht leads/plugs/coil