97' Code P0303 Issues but no obvious symptoms.
 

Hey Miata Forumz,

My 97' keeps throwing up the CEL and when I check the code its P0303 (#3 Cylinder Misfire). It only pops up when I'm doing freeway on-ramps pulls, between 6700 and 7000RPM. It's done it 3 times now, once even after changing spark plugs, all the same situation. Previous owner changed spark plug wires to NGK blues. I changed the plugs literally 3 hours before posting this. I used NGK BKR5EGP's for plugs, gapped to .41 and torqued to 13lb-ft. I put in 4 quarts of Mobil 1 10w30 and a Mobil 1 M1-110 Filter.

This happened after I changed my oil. Before the oil change I could go pulls all I want with no CEL.

There is absolutely no stutters, rough idle, pinging, knocking, power loss, jitters or anything to indicate a misfire other than the CEL. She even picked up a bit of pep after the oil change and a bit more after the plug change.

What could be the issue? I'm stumped.

So I can tell you that this will not affect the operation of the car.

Verify good plugs, wires, coil packs.

I also read somewhere that there was a bug on the PCM that would sometimes throw the CEL at 6800. Sounds like you might fall into this. I would research this option a little more.

You also might want to verify there is no looseness to the harmonic balancer. The keyways are known to waller out.

Hope some of this helps

Thank you.

My car does fall into the TSB window for the PCM logic issue so I'll call my local Mazda/ Mazdaspeed dealer and see what I can get.

I read on another forum someone swapped a fuel pressure regulator and it fixed it, I have a fuel filter to swap in and see what happens.

Thanks for the advice on the HB. Step 1 will be to spray some water on the intake and check for vacuum leaks. Step 2 is to check the #3 injector to make sure it's in spec. Step 3 is to check the Crank angle sensor for broken or bent tabs. I'll check the Harmonic Balancer after that.

Update:

Injectors and ignition are all 100% fine.

There are no vacuum leaks.

If the CAS is not too much of a hassle to get i'll check that.

So far it's pointing to the ECU and not any engine components.