We all can learn from MY mistakes. 2007 cx9 misfire
No, nobody is perfect. True. But in this industry, lessons are either learned thru training, or experience. Although all training is good, not all experiences are. Learn from my mistakes, as I did.
Vehicle in question is a 2007 Mazda CX9 3.5L.
This beat down of a lesson, already took place a while ago. It is posted on another site, but not as detailed as it is here. This is how it happened.
This vehicle came in from another shop(they had changed the spark plugs, unbeknownst to me) with a misfire(jolty misfire) on cylinder 2. Code verified. I did the relative compression test, and it passed. I suggested to the customer that we should replace the spark plugs--EDIT(The idea was to get an approval to remove the plenum, replace the plugs due to mileage, 122K, and shift the #2 coil and injector to the front bank, reassemble and retest). Client declines and states that the previous shop had already done so.
I proceeded to scoped the coil current circuit triggered off of number 2 secondary, using a paddle probe, tucked underneath the plenum. The coil fires as it should, but it also fires again in between, Where it should not fire. none of the other cylinders do this, they fire as they should when its their turn.
I scoped cam and crank signals and found what I believed to be a noisy ugly signal from the ckp(sorry I did not save the waveform). I then removed the CKP, which I found metal shavings on and replaced. Misfire persists.
Mode 6 data was absent and no misfire counter was present, but the same scope test revealed the same behavior from cyl 2, firing when it shouldn't. and not intermittently either, like clock work every time it fires at what i believe is close to its intake valve opening.
So I called it, told the manager the PCM is firing that coil when it shouldn't, Or so I thought. We waited a week to get a rebuilt one. The programmer(I honestly don't do reflashes or programming) did his thing, and lo and behold nothing has changed. Same exact symptoms. The programmer has my lifetime in experience and he also mentioned it felt like closed gap misfire, no testing, just sat in it and felt it, and I took it to heart, which is why I then pulled the intake and replaced the plugs(even though it was changed).
I pulled that spark plug, and it had a hairline crack across its insulation. So technically Garland(The programmer was right, since the spark energy made its way to ground with ease due to atmospheric pressure). Put it all back together, and all was right with the world.
At this point I felt like a punching bag. Why did I assume that just because the other shop changed the plugs everything was fine? Things like this really have you doubt yourself. How could this have gotten thru? What is that second firing? Is that energy a result of the misfire?
If customers only knew the amount of pressure we put ourselves thru when we work on their cars.
So I hope you can learn from my mistakes. This was my worst mistake ever, and although some of you may have had figured it out with ease, not all of us are equal.
May you learn this lesson the easy way.
Why was it firing a second time? I have never even heard of that, let alone seen it.
You could have just as easily installed a cracked plug yourself, so don't beat yourself up too bad. You probably couldn't even see the crack until after it misfired a whole bunch of times and changed color.
If we second guess everything all day long we will never accomplish anything. Gotta trust that new parts are good, then just start from scratch if we don't fix it.
I think the one thing I would have done differently (not that you asked) would have been to scope the coil driver at the PCM to see that errant spark command, or not.
Multistrike ignition ,the second strike is the true firing event , the slope of the firing event is off. If you rev it up the multifire will go away, by design it works that way. One reason is for idle stability. Those paddle's are good for quick checks , If we see something that doesn't look right , I would post the file or image and very likely you will get some direction. He mentions he replaced the ckp , if we think about it could a ckp cause a single cyl, miss? Likely not a trigger wheel yes. Thanks for posting we all learn from these scenarios.
Thank you Geoff, I really did drop the ball on this one. Scott is right, this is classic ford multi-strike. You are absolutely right, I failed to be absolutely thorough in my diagnosis, which I hope people can see that and learn from it. I mistakenly assumed that since it appeared to be slightly pulled down followed by that spark energy, it had to be PCM induced. I've since come to realize that I was following the effect and not the cause. This was a huge lesson for me.
Also recall, Mario, on these "two wire" COP, you can scope all the primary signals right the PCM. Not as fast as a paddle but will avoid the stray signals Ray Y. is talking about.
After being in the business of driveability problem testing for many years, I can feel your pain. The only person that does not make mistakes, is the person that never does anything. To error is human, but when the erasure wears out before the pencil is worn out, you are erroring too much :)
One hard lesson to learn in this business is to learn to trust your test equipment, and not jump to conclusions. Many times gut feelings are just that, and should be proved out with some scope or scan tool testing. In your case, you found a spark plug with a cracked insulator. What would you have done, if you had not been able to see that crack in the insulator?. Using your scope as your eyes would have found the problem, if your scope testing was applied correctly.
In cases like yours, the easiest way to gain access to the ignition coils is with a current probe at the ignition fuse. To get a primary voltage waveform, you can either go to the ignition coil its self, or to any other place you can gain access to that wire to get a voltage signal. I have learned to used voltage and current on these COP systems with great success.
Don't get the idea you can just hook your scope up to a broken car and find the problem quickly. Many times, the use of the scope takes some time and the processes should be learned on known good working cars. Learn what is good, then when the waveform doesn't look right, investigate.
Did you learn from the mistake? If your answer is yes then its not a mistake at all, you have increased your knowledge for the next time. Looking at your waveform, could it be the there is a bit of interference from another ignition coil? looking at the current waveform it looks like there is another cylinder firing, i can only see 6 current ramp pairs there so it is a v6? thats my thought anyway.
Responding to Chad. I learned a ton from this! This is a V6. Thank you very much for the motivating words. My theory is that the coil energy is going to ground and appearing when its the number 1 cylinders turn to fire(since its right next to cylinder 2). I know I could be wrong and would appreciate anyone who could chime in on this, as I have my doubts.
If you want to prove your theory, then hook your scope to the next 5 Ford vehicles that come to your shop, and you will get a feel for what a correct waveform looks like.
This reply was geared toward Albin... I guess I was going at this diagnosis with a pen. I got tunnel vision when I saw that other fire, and ran with it. but this is why I posted this, so that one can avoid making the same mistakes I made. Diagnosing by scopes is an art form, which I admire, and respect. I appreciate the words of knowledge and the encouragement towards a deeper understanding of what the goal is. Thanks for taking the time to share this with me and the rest of us.
Part of the secondary paddle's handle or it's cable, is too close to another COP and it's picking up 2 COP secondaries.
The smaller COP secondary lines up over another COP's amperage in your wave form.
you could be right, although it didn't do that when checking any other coil. but great possibility.
You were trying to place the paddle on a COP, which is under a plenum. I know that if I tried that, I would also be picking up some stray ignition spikes from other COPs.
But you don't worry about it, because your COP current ramps show you which COP secondaries you are picking up and the clearest signal is the COP that the paddle is sitting on.
On a similar subject, one of the big issues is connecting secondary scope leads onto spark plug wires.
If one or more of the scope leads are laying too close to any of the spark plug wires, or if a scope lead is sitting on an ignition coil, you will get a lot of "noise" on the scope screen.
If one of the scope leads is sitting on or too close to a spark plug wire and if that spark plug wire arcs 20kv to the scope lead, the scope lead or the scope could be damaged by the secondary voltage spike
You are spot on Ray, thanks for being persistent. that would perfectly align with my symptoms. I'll admit I can be hard-headed, so thank you for bearing with me!