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
Absolutely , 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
Attempt to open these links below… They are links to my one drive account and show the movies I attempted to post in the initial post. 1drv.ms/v/s!Ap187_WRGm… 1drv.ms/v/s!Ap187_WRGm…
I will attempt to attach one Drive links to the two videos I mentioned in the original post. Bear with me as I’m not sure if it is going to work out as planned: 1drv.ms/v/s!Ap187_WRGm… 1drv.ms/v/s!Ap187_WRGm…
Was able to view the videos in One Drive. Just used the back button to return to your post. Thanks Brandon.
Charlie , my pleasure! Thanks for the feed back
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
Hi Pete, 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
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…
Pete, 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
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
Bob, This was my perspective exactly! Thanks for your Input
Roger that Bob!! Totally agree!! Start with what you know, not what you don’t!!!
Hi Brandon, 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?
Robby, 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