Mavvrikk

Recently I was driving home and noticed my CEL flashing which we all know only means one thing; A misfire. It had been about 30K since I had done a tune up so that’s initially what I assumed my problem was. But when I was driving home I noticed my CEL flashing again. I have 0 loss of power, no weird idle and it revs just fine. I’ve attached a video of me driving it on the interstate while it was ‘misfiring’ showing it drives fine. It only seems to do it on the highway. I’ve cleaned the MAF but it was replaced not that long ago.

Meat Head

How old are the coils. They fail over time. Revision C Mazda or BHR Globally Compliant or similar for replacement.

Static Flownut

The ECU is picking up a misfire, but where does it pull the data from? Is it from the front O2 sensor or maybe the knock sensor. It is possible that a sensor is giving a bad reading to the ECU causing the flashing, but I'm not sure how likely that would be.

Loki

iTrader: (1)

My understanding is that misfires are detected through ignition coil voltage or small changes in crank rotation speed. If the plug doesn't fire/doesn't ignite fuel, there will be more resistance through the spark plug, which is detectable, and there will be a drop in crank rotation speed compared to what the ECU expects had the rotor fired on that rev. O2 sensors are way too slow to react to pick up misfires, you need those direct measurements.

I don't know which of the two methods the 8 ECU uses, but suspect its crank position based.

Mavvrikk

they’re a day old. I replaced them and drove home but this morning it started again. Everything is NGK or intermotor because I understand they’re what Mazda used?

Mavvrikk

how do I test that?

Loki

iTrader: (1)

You don't, the car won't run without coils and the crank position sensor (ESS), so you can't really isolate each one. Misfires aren't always noticeable, especially if you're cruising or idling and engine load is low, and the misfires aren't super frequent. VW direct injection engines are famous for misfiring at idle once the intake valves get sufficiently carboned up, but you don't feel them, you just get the CEL.

Let's say the coils are new, I would try the following:

​​​​​​- reset the ESS profile and let the car relearn by idling. Also known as the "20 time brake pedal stomp".
- go over your coil and spark plug connection and make sure they're snug
- clean the ESS if it's dirty- is the intake stock? poor aftermarket intakes can cause misfires and other issues by not controlling airflow over the MAF properly
- miles on the car? if your injectors were very dirty you might get misfires. Try injector cleaner, though if you're using good quality gas there shouldn't be that much dirt to begin with​​​​​​- swap rotor 1 coils to rotor 2 and see if the misfire travels with the coils. One might be a dud.

Mavvrikk

Everything is stock on the car. The misfire is frequent driving down the interstate. I’ll swap the coils and see if it moves.

the car has 129K, it was maintained by a rotary mechanic before me and I take as good of care of it as I know how. The fuel is always premixed 93 octane. I’ll clean the ESS and injectors and too

Shaozhou Zhang

From the FSM:
The PCM monitors eccentric shaft position sensor input signal interval time. The PCM calculates the change of the interval time for each rotor. If the change of interval time exceeds the preprogrammed criteria, the PCM detects a misfire in the corresponding rotor. While the engine is running, the PCM counts the number of misfires that occurred at 200 eccentric shaft revolutions and 1,000 eccentric shaft revolutions and calculates misfire ratio for each eccentric shaft revolution. If the ratio exceeds the preprogrammed criteria, the PCM determines that a misfire, which can damage the catalytic converter or affect emission performance, has occurred.