Vehicle belongs to a youngster at work. This began as a symptom of misfire under load. He replaced all spark plugs and COPs which seemed to rectify the issue for months. He notified me that the vehicle had began to “run funny” for him and I agreed to analyze the vehicle over the weekend.
A road test was carried out and the vehicle misfired intermittently, more so under load. A hard-acceleration resulted in full engine cut-out! Fuel trim behaved erratically and didn't seem to follow a particular engine operating range. A scan for DTCs revealed :
P0103 “MAF HIGH”
P0102 “MAF LOW”
P0507 “IDLE HIGH”
P0850 “P/N SWITCH CIRCUIT”
Because of the easy ignition system configuration (consisting of accessible 2-wire COPs) 4 traces of my EScope PRO were used to conduct primary-ignition analysis. I like the EScopePRO Because I can trigger all for traces to update superimposed in different colors. I can easy scroll back and search frame by frame It makes for very easy detection of which cylinder is suspect. My analysis revealed all four secondary ignition events shorting through carbon at the time the engine cut out.
Removal of spark plugs for inspection revealed all four were heavily carbon fouled. Spark plugs were replaced as to eliminate the ignition-misfire and to promote a more accurate further-analysis for the root-cause.
After replacement of all four spark plugs, my next test regarded measured air rate vs fuel trim. I wanted to see how the MAF was reporting and if correction to fuel delivery was required. At an idle of about 1200 rpm , this MAF reported 5v/186gps. A direct measurement was made at the signal wire of the MAF and it reflected source voltage (>12v) whether the MAF Connector was mated or not. If the harness was slightly moved, the engine stumbled and almost stalled.
I then used my scope to monitor the signal over a 20 second sweep, while manipulating the engine harness. This allows me to pinpoint the location of the short without initial surgery.
The technique places me in the suspect-area in less than a minute and the harness was split open in a small, 6-inch section. There, I found the COPPER. The MAF signal wire was exposed to a source voltage wire along the the wire that fed the P/N switch.
I insulated the wires, repeated the scan tool analysis of the MAF SIGNAL at idle and a road test confirmed the repair. I have two short videos but those .MOV formats are not yet accepted. Will post those when possible.
Great Job Brandon that is great diag and great way finding the actual source
Great job and great write up as always. I am going to start writing up repairs like this, even if I don't share them.lol
Great write up . Can u elaborate more on the waveform for the plugs shorting to carbon
An ignition coil is nothing more than a step up transformer. Magnetism and electricity go hand-in-hand with in an ignition coil. It is very good device for transferring back-and-forth between these two forms of energy. In this case, as the PCM provides a ground path for each coil, the respective coil begins to dwell and take on energy. This energy produces a magnetic field around the primary winding of the ignition coil. At the appropriate time, the PCM removes the ground path, opening the circuit for the primary windings.
When the primary circuit is open, current ceases to flow and the magnetic field that built up upon the primary winding, begins to collapse rapidly. As the magnetic field collapses back into the primary winding, it’s transformed into electrical energy. That energy is multiplied many times over, by the step up transformer and a Limited amount of higher energy is induced into the secondary windings of the coil.
As we know, electricity is always trying to find it easiest path to ground and that built-up energy within the coil secondary windings has to dissipate. The initial “spike “ We see is measured in kilovolts, or thousands of volts. This inductive kick represents the amount of energy required to overcome all of the resistance in the secondary winding circuit (electrical resistance/physical resistance). For example:
-secondary cable resistance
-spark plug gap
-Air fuel ratio
All of the above play a role in determining resistance of the secondary circuit. Large gaps under cylinder pressure within a leaner air fuel mixture are the hardest to overcome and use the most energy. Scenarios like this, force the firing KV very high. Now I mentioned earlier, that the amount of energy stored within the coil is limited. The more energy that is used to overcome the secondary circuit, the less energy is left over to maintain the spark throughout the combustion process.
There is a Enough heat energy to form a plasma channel between the spark plug electrodes. The significance of plasma is that it is a very highly conductive region, almost a “tunnel of low resistance” . When that plasma channel is formed, the electrons can flow nicely between the spark plug gap and there is plenty of energy to maintain the burn throughout the combustion process. When that plasma channeled can’t be established, the energy from the fire and KV tries to make its way to ground but found a different, easier path. In this case, because Carbon is still conductive, the electron flow went through the carbon rather than through the spark plug gap. This is because, without the plasma channel, the spark plug gap is more difficult to carry current flow then is the carbon.
As I had stated earlier, the kilovolt level represents the amount of energy required to overcome resistance . The higher the voltage, the more resistance it has to overcome is represented. The initial KV spike leads to the formation of that plasma channel. When that occurs, we have that formation begin at a much lower voltage level… This is where the burn line begins, where it meets the firing line and takes that 90° turn that we are also used to seeing on a hand drawn waveform.
In the case of the carbon path, the firing KV overcomes resistance but cannot establish a plasma channel and the burn line begins very very high, using a lot of energy. The electrons flow through the carbon and because they have found a ground path, the burn voltage begins to descend as time goes by. This is represented by the downward sloping burn line, and a key indicator to me as of what just occurred within the cylinder. The misfire occurs because the spark has not bridged the gap of the spark plug and the air fuel mixture did not chemically change, nor did it release any energy. (No “bang”)
I am going to post an article below that I’d like you to read. I learned a lot from this man👇 let me know if I’ve helped you in anyway, Adrian
Was able to view the videos in One Drive. Just used the back button to return to your post. Thanks Brandon.
Why did you go after ignition first, when the MAF was clearly not doing its job and clearly had a circuit related issue? Asking to learn, I would have gone at the MAF first based on codes, probably finding and isolating the circuits so I could drive it. Maybe the pugs would have cleaned up maybe not, then I would have sold the job as a wire repair and if the plugs didn't recover, sold plugs too.
Would you have approached differently if it was a customer car and not just a buddy? I ask because from a customer satisfaction standpoint selling "plugs" and then selling the MAF wire repair next is confusing and makes it seem like the tech is guessing at a repair. Whether they are or not doesn't matter, when customers think we are guessing it is not good. More direct diagnostic strategies enhance customer confidence.
I chose the path I did because , during the misfire condition, fuel trim did not support improper AF ratio. The misfire was prevalent and easy to replicate. The cut out was prevalant and easy to replicate as well. On many occasions I have seen two wire COP‘s, during high firing KV conditions, goof ECU functionality and cause strange issues like throttle angle limitation and loss of ignition control., loss of injector output, PCM reset, etc.
I guess my point is, the misfire is the symptom. I had to rectify the symptom to drive the vehicle and give it a fair analysis without being steered into a rabbit hole. Driving the vehicle under the current condition let me to feel the heavy misfires. It was hard to distinguish between chicken or egg syndrome. The MAF input was rarely skewed during the Road test. I happened to catch it on the PID list at the time of the failure occurring. At times, VMA have reported normally. This explains why my fuel trim rarely deviated from a normal range, during the road test.
I chose to monitor the ignition waveforms because they were low hanging fruit and I wouldn’t know without any doubt, the colors of the misfire. Looking back, I would’ve approached the vehicle in the same way I did. I did not feel it the right course of action, to chase a wiring issue, find it/repair it and then Road test the vehicle to find misfires occurring. I would have simply in the laminated my DTC’s, not my drivability concern.
It's an interesting approach. :o)
So, you've seen high Kv cause circuit codes before? On MAF sensors?
FYI I wouldn't even have looked at MAF PID in this scenario, the misfire is a result, it's not the actual symptom (so there is no chicken/egg confusion here, not to me anyhow).
I wouln't chase a misfire without knowing the ECM is happy with the MAF (after seeing MAF circuit codes).
Anyhow, thanks for sharing. :o)
I apologize as perhaps I was not very clear. There were M AF codes However at times there were no MAF input issues at all. In other words, this was not a hard-fault. I happened to see this scan data at the time the faultwas present. A blink of an eye later, the fault had vanished and MAF reported normally. The misfire/cut out is what I was chasing… Plain and simple. The MAF input was simply information on the side that I didn’t care about either way. I was chasing the misfire/cut out.
I have not seen high firing KV cause any MAF circuit codes. I have seen high firing KV cause all sorts of weird ECU issues. My thought process was, fix the misfire and find out the root cause of the misfire. The cause of the misfire was the Carbon-fouled spark plugs. The cause of the carbon fouled spark plugs was the Intermittently-skewed MAF input .
I guess therein lies the magic question; chase symptoms or chase causes? :o)
BTW no need to apologize, I'm not knocking ya and I don't feel slighted in the least, just pointing out a possible way to fix the car and not have to call a client twice. Even well performed diagnostics often could be improved on.
I know your not, good-buddy. Always great conversation with you. I appreciate the volley
In this case I would have gone after ignition misfire also. A miss that can be induced by loading the engine is a classic case of ignition breakdown. Now if I had seen that the fuel trims were railed during the misfire I would have gone in a different direction but Brandon indicated the trims were inconclusive.
In hindsight you might say he should have fixed the maf fault first but a faulty ignition coil can induce high voltage back into the ecm causing false codes. There was no way to know initially that there was a harness fault but he was able to duplicate the misfire. That's a hard fault worth checking out first.
When faced with a car with multiple faults it's hard to say which path to start on is best. For me, part of the calculation is what is easiest to do first. He said the ignition was low hanging fruit so why not start there?
Did you happen to scope the primary after replacing the spark plugs? I ask that because - is it possible the fault you found in the primary was a result of the fuel control response to the erratic MAF?
i apologize as I didn’t save a capture Of the post-fix ignition waveforms. Let me explain it like this;
If the misfire was directly related to “overfueling” rather then the result of overfueling (carbon), the ignition traces would short directly to ground rather than indicating a resistive-path Was chosen. Carbon is conductive but offers a higher resistance than a wet plug. The waveform would present differently .
I can certainly post a capture but you can even see during the failure-capture, the igntion event through the carbon path was Instantaneous and not progressive. A wet plug wouldn’t come-n-go from event to event. Does this make sense, Robby?
If your like, I will post a “fix” capture tomorrow?