Case studies

Justin Miller - JM Diagnostics - Herriman, Utah

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2004 Ford F-150 5.4L

I'd like to share a routine repair

 I was called to a new shop today. First time providing service for a young couple that opened a new shop in town within the last year.


This shop I visited while traveling through a part of town I grew up in about six months ago. The new shop stuck out like a sore thumb to me. Like whoa, that’s new in a town that doesn’t change. When I stopped in the wife of the couple was the only one there. He was off driving a car or something. I left my information and explained to the wife what kinds of help I can provide if they got in a jamb. Honestly never expected to hear again from them. They are trying to get the industry figured out and just don’t know what they don’t know. That’s what I provide a chance to get out of a jamb when the technology is beyond their understanding.

We can talk about how they are in a bad business model another time.


2004 Ford F-150 5.4L. See I told you routine repair. History is it came in for a 4wd failure. He repaired the hub actuators and replaced a non-functioning vacuum solenoid as well as the vacuum harness that deteriorated in too many places. Customer says “while it’s there, mind doing a tune up” plugs are pulled and broken plugs removed with no problems to note. New motorcraft plugs are installed as well as new coil boots. After the maintenance is done it developed a misfire.


You already know the situation. The complaint is a misfire felt, codes P0351 and then while trying to fix it P0174 set. New motorcraft coil is installed. Second coil is replaced as a “warranty” Shop is now full of heavy line work and owner doesn’t even know how to go about diagnosing the misfire. I show up prepared with several tests planned to begin the diagnostic process. First is a scan of codes with Snapon Verus. Those that know me know I despise that tool. But in this situation, it is the right tool to grab first. Confirmed P0351 and P0174. Check mode 6. All misfire tests are reset and have not been completed.


Grab IDS and VCM2 for a power balance test. No VCI detected and can’t get it to be recognized. Great, restart laptop after unplugging everything. Start it all back up, still no coms. I’m on a mobile call and I know other ways to get this answer I’m looking for. The question is, “is cylinder 1 the misfire or is it another cylinder.” To confirm this I connected 1 low amp clamp on the red/light green wire at connector C139. This connector is the black in line connector attached to the ECM connector harness right in front of the ECM. You will also note that Mitchel doesn’t think that C139 is important information. Motor diagrams do think it is important. I agree. Second low amp clamp (both Pico TA019 clamps) on the red/light green at the cylinder 1 coil only. Vacuum waveform for possible need later. But the problem sticks out. It shows cylinder 8 with a serious issue. When the ECM releases the ground on the coil all hell breaks loose. This proves the ground is controlled by the ECM as well as the coil is capable of being charged so the connection is good to the coil.

I tap into cylinder 8 primary to watch the primary pattern. Same information there, only more. The spark line is where the problem occurred. That means it’s after the coil on the secondary side. We know we have new parts there, factory brand, so we move the coil at this time. It doesn’t change. It didn’t move with the coil and boot. That leaves plug. We pull the plug expecting to find a crack in the porcelain or tracking. None found. We know the plug is bad so we installed the new plug that showed up. New plug installed and rescoped to confirm repair.

 At some point before the plug and coil parts changing on cylinder 8 attempted to use the VCM2 again. Still nothing. Called a friend to ask for direction repairing my tool. He said he was having issues with his VCM as well. Miraculously my VCM started working while speaking to him. The power balance test was then run. I forgot to save the screen before unplugging my vcm. Trust me, the line is flat under that prompt window.

The miss is no longer felt, the pattern is clean and the power balance is showing a flat line. I clear codes and monitor fuel trims. The longer it ran the closer to 0 they went. I called it good when they hit 6. Falling from 23 on bank 2 and 12.5 on bank 1.

Customer is very pleased and sees the value in the service. In summary, I went expecting a coil/pigtail issue or even a failed coil driver in the computer. I let the test direct me to the right answer. We need to remember the code number doesn’t tell the truth. Only testing and proving the problem does. I also had to be prepared to adapt my test procedure to a secondary one when the first test couldn’t be completed. Granted, the next step after the power balance would have sent me to the scope anyway. It would have saved me 1 step as I would have started with everything in cylinder 8 had I had the direction from the power balance.

I would like to thank Brandon Steckler for taking a moment to review this with me today. Also Keith DeFazio for answering his phone when I called. He’s so good my vcm knew then game was over and it was time to start working. That’s talent.