I've read through everything I can find on this and I haven't seen this particular behavior addressed.

Car starting to feel weak, like the coils are going, and got the misfire CEL. It's a little early at about 20K, but they are generic coils so not unexpected. Plugs look pretty bad and the coils have white spots. I put in new iridium plugs and new generic coils. Starts up great. Feels much better starting out. I drive down the block and back home, maybe 1/2 a mile. I come back because I forgot to clear the CEL before I left. Clear the CEL and take off again. After about 2 miles the CEL flashes. I slow down and it goes off, but engine is no longer smooth. I turn around and make it home, but it feels terrible, the CEL flashed once or twice more on the way, and it died immediately at idle.

What happened?

I tested the old coils with an ohm meter, and they were very consistent. All roughly:
A-B 1.3K
B-C 3.2M
C-A 0

Then I tested the new coils and they were also very consistent with each other, but totally different than my old coils
A-B 936
B-C 4.1M
C-A This was the weird one that I don't know what it means. They each started out with a reading, but then steadily went to zero after about 5 seconds. Measure again and they would be zero. Short the pins and measure again and they will have a reading that slowly goes to zero again.

I know different coils can have different ohm readings so that the differences in the A-B and B-C readings may or may not be meaningful, but what do those C-A readings mean?

Based on the behavior of the car, should I just put in some better coils, or might there be something else wrong that I need to check? Tomorrow I will put the new coils back in and run a spark test. I have an in-line spark plug tester, but not the HEI spark tester. Any issues with that?

Finally, which aftermarket coils at least work, even if they may not last as long? Yes, I know of the BHR solution, but unfortunately that isn't going to happen right now and I need this car to run.

Thanks

 

All of that is meaningless; testing both coils and wires under load is the only effective method:

https://www.rx8club.com/series-i-do-...-wires-222641/


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I'd check your cat for clogging. Coils aren't the only reason for misfires.
Check your OBD fuel trims as well. Replacing coils required taking apart the intake, possible you've left gaps.

 

Definitely no gaps in the intake.

I do have some concerns about the cat. It was replaced under warranty about 50K ago, has gone through an engine failing and a round of coils failing where each time my other driver probably pushed a bunch of gas through it trying to get home. It had a CEL on the oxygen sensor a year ago, but replaced the sensor and it went away. My wife did get another CEL on the oxygen sensor shortly before it started misfiring, but it hasn't come back.

What is the best test for the cat? Visual through the sensor hole? I am in California. Did anybody ever come out with a reasonable aftermarket cat solution that is California legal?

 

I understand that an ohm test is limited and I will test them on-car today. I basically have 2 questions:

1) I was not expecting the C-A reading to vary from a reading to OL in 5 seconds. Does that make sense and what is physically happening? That behavior is not listed on the Mazda test or any other reference I can find, and it is not how my old coils act, but the Mazda test very specifically says that a reading of 0 is a failure. Is it?

2) Will this be reasonable for the spark test, since I have it but I don't have the HEI?

 

Definitely check out the cat.

You could have created a vacuum leak.

You could have mixed up the wires.

You could have not snapped a wire on snugly.

You could have a bad wire.

You could have received a defective coil.

Check all of that before you panic.

 

An in-line timing light should work to check the coils.

Delphi coils are supposedly fine so you can try those.

And white spots under the coils are pretty meaningless for the most part. It's better to not have it, but having it doesn't mean your coil is dead.

 

Aargh! Sat for a week and now I have to catch a stupid rat before I run it. I actually saw the stupid thing running down low near the rear of the engine. Air box is already removed, so I set a spring rat trap there, then set a couple of glue traps over the battery and PCM. This morning, the spring trap was sprung, but no rat. One glue trap was on the far side of the engine, full of fur and with a tail stuck to it, but no rat. What #&@^! am I dealing with???

Sorry, had to share...

 

Coils all fire through wires. Spark plug wires are not swapped, and I labeled the connectors as I took them off the coils. Checked vacuum connections. It's getting down to the cat, so I'm going to go ahead and start panicking a little.

I've never diagnosed a cat, so I'm off to dig through the archives to make sure that is the problem before I go down that pricey road. Last I looked I didn't like my options in California.

 

I cracked open the exhaust at the exhaust manifold and stuck in a boroscope. Looks pretty toasty, but not necessarily clogged. This cat passed California smog 3 months ago!

I'll try the original coils that may be weak to see if the new coils are just bad. Something started off fine with the new coils and then went very south over the course of 2 or 3 miles. I don't think the cat is doing cat stuff at all, but I'm skeptical that it suddenly became fully clogged at the exact moment I swapped out the coils. It looks more likely that the cat is already acting at least a bit like a straight pipe.

 

Sweet Jesus. This cat is destroyed. If the first element looks like this, it's a good bet that either its fragments are resting against the second element, which makes a clog, or the second element is also destroyed.

Either way you can't drive like this. Just because it has a hole through it doesn't mean it flows as well as it did stock.
Get it off and at least clear the exhaust.

Cats are usually killed by bad coils, so... Things must have been bad for a while...

 

New engine was put in 40K ago. Coils, plugs and wires were all replaced, but the cat was not checked. In hindsight I think the cat probably at least mostly went out with the first engine. Like I said, it passed smog 3 months ago. How?

 

To my knowledge, California smog test only checks for an active CEL for newer cars. They only sniff test older cars.

The cat may just be barely above that threshold to throw a CEL light.

Loki is absolutely correct, gut this one ASAP and get a new one if you need to pass emission tests. You are literally playing with fire if you don't act - there are people who lost their 8s to cat fires.

40k miles on ignition components are quite long BTW. Spark plugs need replacement every 30k miles or so, and usually OEM coils and wires should be replaced as well.

 

I'm back home after a few weeks away and finally ready to get back on this.

The new engine has 40K but I've already replaced the coils and plugs once, so there were only 20K on them. I'm still concerned why it started and ran perfectly, then degraded very quickly to where it won't come close to starting. It seems like too much of a coincidence that in that moment the cat completely clogged. I'm concerned what else may have happened. Spark, gas and air all still seem fine.

I'm going out to gut the cat now, then I'll go from there.