My 99 NB8A - Start up/Idle issue!

[MiataBoy]

All i can say is watch the video i posted and read the description I posted

https://www.youtube.com/watch?v=vJtJMQUO1Mw

[Trackphotos]

Seems quite similar to the misfire I had until a couple of days ago. Mine ended up being a dodgy ECU, but the most common cause is a failing coil pack. Known issue on NB8As. When it happens, disconnect one spark lead at a time from the coil packs to determine which cylinder/s aren't firing. Once you've narrowed it down, swap the coil packs over so the one that was failing is running the other 2 cylinders instead. Restart the car, and repeat the spark lead unplugging. If the fault moved onto the other cylinders, that coil pack is failing.

[Davidzferret]

That does sound like a cylinder or even two not happening, and ignition coils are a known issue with NB8A.

I've just got our NB8A running and picking up properly after a long period of chasing down faults of missing then coming good, poor torque below about 3000 RPM and stumbling / missing when throttle is opened from idle or trailing throttle.

Things I've tried:

- new no-name coil packs - these eliminate the bad missing and gave a temporary improvement but then went bad quite quickly - rough running rather than dead cylinders - and were sent back for a refund.
- new non-genuine (MX5 Mania) coil packs - improvement, but still stumbling whenever throttle opened quickly, and poor low end torque
- new platinum plugs and blue (MX5 Mania) leads - slightly better
- new fuel pump after old one failed - got the car going again but no improvement to running
- the fuel filter had been replaced in the last few years so I've left that alone.
- new crank and cam sensors - low end torque improved / now acceptable but still stumbling off idle
- Silicon grease to plug leads - helps a bit
- clean every connector I can find including those into the ECU with Servisol contact cleaner - no difference
- check Throttle Position Sensor for voltage and for smooth change of resistance, all TPS checks O.K.
- I was getting decidely cranky with the car by this point.
- finally saw a posting on a U.S. forum about the Mass Air Flow sensor. In my case this was the missing link in the puzzle.

Seems the MAF is supposed to keep clean by being heated by the ECU at power down, to burn off any deposits. Despite this it can get gunked up over time, and our car has 165000 km and about to turn 16 years old.

Gunk insulates the wire from the airflow and as the airflow is the main measure of engine load, the engine load data lags behind what the engine is doing. So if you open the throttle very slowly, there's no lag issue and all is well, but blip the throttle and the mixture is way off. This is what our car was doing and why I suspected the TPS.

Sensor uses very fine platinum wire (0.07mm!) so it's easy to damage. There are specially formulated MAF sensor cleaners - not as harsh as carby / throttle body cleaner - and a can of this worked wonders, car is back to normal and the process only took about 20'.

I wouldn't go pulling leads on a running engine, voltages very high / dangerous with electronic ignition. Plus AFAIK these coils fire one plug off each end of the high tension coil so I'd expect to lose both of the cylinders fired by that coil whichever lead you pulled. Maybe pull the low tension plug off one coil, and if the car won't start, that coil's the good one, if it starts on 2 cylinders, it's the bad one.

I can highly recommend MX5 Mania's non-genuine coils and leads and their service.

I hope you get it sorted quicker than the 14 months it's taken me and I hope the above is helpful.