97' Code P0303 issues, but no obvious signs.

[Brap]

Hey MazdaRoadster.net

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.

[JamieH]

Check the coil packs. If those are fine, move on to the following:

https://www.miata.net/garage/tsb/006-98-1.pdf

Which as far as I know, throws a P0300. This is a specific issue for '97s. I experienced it with my '97 and the cure was swapping in a '96 ECU

[Brap]

I will check coils by swapping the 1&3 coils and seeing if the issue carries over.

When the code first popped up it was actually the P0300 "Random Misfire," the two consecutive times it was specifically P0303.

If it is the module issue, how would I service it? Do I have to take it to a proper shop or is it something I can do myself?

[JamieH]

If by module you mean the ECU, you can do it yourself. Just source a '96 ECU from one of the Miata breakers/used parts sellers.

I recommend Scratch N Sniff.

The ECU is located behind the passenger seat under the carpet on the wall. Disconnect the battery before removing the ECU. The connectors simply unplug, though may be stubborn.

[Brap]

It says Vin Number lower than JM1 NA35**V0 727842, My VIN is 3539, second set of numbers is 20024. Does my car fall into that category?

[JamieH]

If the last 6 digits are lower than 727842, yes.

[Brap]

So my car is in the window for the TSB. I'll call my local Mazda dealer/ Mazdaspeed distributor and see what they can do for me. Otherwise I'll pay for a 96' or the replacement ECU.

Someone on another forum said they swapped a fuel pressure regulator and it went away, I don't know if the fuel filter has been changed so I'll change that and see what happens.

Step 1 is Water Spray test to check for Vac leaks. Step 2 is checking the #3 injector to make sure everything is within spec. I've read somewhere someone had an issue with a tab on the Crank angle sensor being bent or broken causing an off reading. I'll check that last if the issue persists.

Anyone know the resistance specs for the injector? How much voltage should be going through the wires for it?

[Brap]

Update:

Injectors are all fine and within spec. As is injector wiring.

There are no Vacuum leaks.

I'll check Crank Angle Sensor but I don't think this would cause an isolated problem at the RPM range I am noticing it at, but if it's not too much of a hassle I'll check it anyway.

Fuel Filter is going in soon.