MX-5 OC Forum

I have a Mk1 1997 1.8 with an M45 supercharger fitted, along with air/water intercooler, crank overlay, Mk2 injectors and an emanage blue. Just lately my car seems to have developed an idle problem, where the idle bounces from 850 RPM upto 1500-2000 and then down again. The car seems to accelerate fine and is without issues, once its warmed up the idle settles down.

I have reset my base idle and checked for fault codes and nothing is showing. I have swapped the MAF over, changed the plugs and coil pack, engine coolant temperature sensor and the issue is still there. What else should I look at?

BTW - I have already reset the base idle.

I have a Mk1 1992 1.6 with an M45 supercharger (fitted by Mazdaspeed nearly 20 years ago!).  No intercooler.  Standard injectors.  Standard ECU, no piggy-back, not even MSD boost timing unit or Bipes ACU, so timing 8 degrees BTDC @ 950rpm.

However, it seems that I am getting the same problem as you - when cold, the idle bounces for a couple of minutes and then settles down.

I have also reset my base idle, checked for fault codes, cleaned and checked the PCV valve, replaced the plugs, HT leads and air filter.  Put the car on a rolling road last week and the fueling is fine and peak power is a healthy 178 bhp at the flywheel.

I will be replacing my oxygen sensor this week and was then, just like you, considering the MAF, coil pack, engine coolant temperature sensor, etc.

Please let me know if you find out the cause of the bouncing idle - I will do likewise!

I'll let you know if I solve the probelm. FYI, I replaced my O2 sensor about 2 months back and it made no difference.

Have you guys checked the idle air control valve?