NB8B Random Misfire

flamingsamurai · Postby **flamingsamurai** » Mon Apr 08, 2019 5:36 pm

Have been suffering from a misfire which seems to come on randomly - sometimes it begins from startup, sometimes at idle at a red light, sometimes while at speed. It occurs semi-regularly and sometimes the CEL comes on with a P0300 code, but it does not always happen.

Through random luck I've found that if I start the engine while the plug wire coming from the cylinder 4 coil (i.e. the lead to cylinder 1) is unplugged, the coil will arc to something and will temporarily cure the misfire next time the engine is started (nothing seems to happen when I unplug the lead from the cylinder 2 coil). Everything then seems perfectly normal and fine until the next time the misfire reappears.

My Dad has been taking care of the car while I've been living overseas and says he's replaced the plugs, the plug leads, the coils, and the MAF with no improvement.

Any ideas what could be the issue?

hks_kansei · Postby **hks_kansei** » Mon Apr 08, 2019 6:02 pm

Next time it does it take the plugs out and have a look as soon as you can.

I recall on my NB8A a while back that it developed a misfire (that would become a full time misfire roughly once a fornight)

That turned out to be a dodgy o2 sensor.
It didnt show up as a fault code since the o2 sensor was still sending out a signal that the ECU expected to see.
Bad part was it was constantly saying "lean" so the ECU kept dumping in fuel during closed loop which caused it to foul plugs.

Otherwise, look into replacing the cam and/or crank angle sensors.
Not a hard job (hardest part is setting the gap on the crank one), and the sensors are only $60 each or so.

Mr Morlock · Postby **Mr Morlock** » Mon Apr 08, 2019 9:02 pm

Coil packs may be the cause. My NB8B had this problem over quite a long period and finally I replaced the coil packs with OEM make and it ran very sweetly- I put up with the niggles for too long. No one can really tell you but coils do eventually start to deteriorate.

93_Clubman · Postby **93_Clubman** » Tue Apr 09, 2019 12:26 pm

Mr Morlock wrote:Coil packs may be the cause.

OP's already replaced them.

flamingsamurai wrote:replaced plugs, plug leads, coils, & MAF

flamingsamurai wrote:Any ideas what could be the issue?

https://corksport.com/blog/how-to-diagnose-a-misfire/

Craig · Postby **Craig** » Wed Apr 10, 2019 3:42 pm

Wasn't the crank angle sensor a previous suspect for this issue? :?:

manga_blue · Postby **manga_blue** » Wed Apr 10, 2019 6:31 pm

Do a compression test. If that's OK then put an OBD scanner on the ECU and check for sensible live data readings from all the control sensors, esp input air temp, MAF, engine coolant temp and O2 sensor.

manga_blue · Postby **manga_blue** » Thu Apr 11, 2019 10:35 am

Also occasionally the fuel injectors can gum up if the car has sat idle for a long time.

alexbwang · Postby **alexbwang** » Thu Apr 11, 2019 6:40 pm

Check the coil plug lead - you could try pulling on it with pliers to help increase the length of the conductor, this helps to maintain an optimum contact to the spark plug.

I once had a reputable MX5 garage install Ford OEM COPs which actually caused misfires. It was remedied by installing Mazda genuine OEM part. The leads were slightly different length (Mazda longer by a few mm).

Hope this helps.

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