Electrical Cylinder 4 Misfire


Jun 7, 2018
Dublin, OH
Hello all,

I have a 2000 Mustang GT Coupe with a manual transmission, and I am having trouble diagnosing what is making my cylinder 4 misfire, there's a lot of stuff that has happened with the car, as well things I've tried in order to troubleshoot this misfire. The car has long tube BBK headers, an offroad H-pipe (no cats), Flowmaster mufflers and tail pipes, as well as a cold air intake, and the only codes in the system were 3 o2 sensor codes.

Note: Cylinder number 4's fuel injector has the only different connector, it doesn't fit very snugly, but it does fit.

After replacing my transmission, I accidentally left a coolant hose unclamped, and when I took it on the road with the hood still off, when I gave it some gas, the hose popped off spraying coolant all over the car at about 20 miles an hour (car was about at operating temp). I clamped the hose, drove it home, and it was running fine and strong on all 8. I took it out a few times over two days, and I noticed it first starting to miss at the 1500-2200 RPM area, and didn't pay much attention to it since the battery was disconnected recently (PCM reset).

Then on the third day it got really bad, I could feel it shaking just sitting idling, and I decided to park it until the next day so I could work on it. I got out stethoscope, and tried to listen to everything, the most apparent thing was when I could hear 7 of the 8 injectors pulsing, number 4 I could not hear making much noise at all. I couldn't hear anything from the coil packs as expected. The spark plug looked good, I cleaned it though, but I noticed some coolant sitting near the coil pack as well as around the injector O-ring for cylinder 4. I decided to replace the fuel injector thinking it had gone bad, but when I did nothing changed. Fuel came out of the fuel rail without resistance where I pulled the injector off, so I could tell the rail wasn't clogged. After replacing the fuel injector I decided to swap the coil pack, I picked up a new coil pack, swapped it in, and nothing changed. So then I swapped spark plugs with cylinder 5, and nothing changed. I decided to start trouble shooting wiring.

I verified that the female spades were making good contact with the male spades on the injector, as well as the wire. I did the same with the coil pack plug. I opened the large plug on the passenger side firewall corner with the 10mm bolt on top (Figure 1), and when looking at it with the sun glistening upon it, it appeared to have some spots of moisture inside the engine side of the connector plug. I used some QD electrical cleaner, let it dry, reassembled it, and started the car. I couldn't tell for sure, but I think it may have run a little better after I did this, but as soon as I pulled out of the driveway the miss was prevalent. And at this point the car throw the cylinder 4 misfire code. I tried to use the cleaner on it 2 more times with the same strategy of letting it dry, and nothing changed.

Figure 1​

I disassembled the connectors for the fuel injectors on cylinder 1 (control cylinder) and 4 (misfire cylinder), and verified that their input wire had a good connection with the large connector for the engine harness in the corner previously mentioned. I put the leads directly onto the fuel injector from the connector, to verify that they were sending good power to the injector, and nothing changed. Then I went after the coil wires, I verified the output wires of both cylinder one and four, but could only verify a connection on the large connector with cylinder number 1's input. I tested every pin and did not any continuity with cylinder 4's input wire.

I then went and started to open the loom for the large plug, but could not actually find the dark green/violet wire that is on cylinder 4's fuel injector connector (Figures 2,3,4) although I found cylinder 1's. I couldn't trace the harness except for one spot near the battery but I couldn't tell what is was or where it went (figure 5). I then went to the car's main PCM, disconnected the three connectors in front of it on their own plate, and then pull off the main connector, and began testing those receptacles with the injector connectors. I'm not 100% this is a reliable test, because I found a pin diagram online that listed both coil input wires, but could not get continuity on either of them, I tried first with just the probe (which was tested and verified), and then with the copper wire wrapped around the probe tip (also tested and verified).

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Figures 2,3,4,5​

This is the point where I'm at with this issue, I couldn't trace the wiring harness past where is pictured in figure , so I don't really know what to do next. All suggestions, input, as well as troubleshooting steps are welcome. Here are some more images.

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I ended up finding the fix semi on accident. The cylinder's coil pack plug wasn't latching/clipping onto the coil pack properly so it was sending signal properly. I tested it by putting the connector's leads directly onto the coil pack and then it ran fine.