Engine Need help confirming misfire diagnosis

Good evening, all - troubleshooting the P0316 (Engine Misfire Detected on Startup) and P0306 (cylinder 6 misfire) DTCs in my son's 2002 mustang and need a sounding board to confirm my tests and resulting observations. Typical of misfires, the car is running rough and exhaust smells of gas, but will not die.

Spark:
- New plugs/wires installed 21mos/19k miles ago. Pulled plug wire #6 (and later the others) from coil pack and could see the spark arcing to the wire. Car seemed to run noticeably worse, leading me to (maybe wrongly) think it was working when connected.
- connected a spark plug tester to the end of #6 wire and saw arcing there as well.
- pulled #6 spark plug with wire connected and turned engine over - saw plug end arcing.
- swapped #4 & #6 plugs and wires and cleared DTCs. No change in rough idle and both DTCs (still indicating #6) returned after a few minutes

Fuel:
- CCRM, Fuel pump & filter replaced 12mos/15k miles ago. OBD2 reader shows consistent 38-45 psi pressure in fuel rail
- All new Motorcraft injectors installed 18mos/18k miles ago. Checking with an automotive stethoscope, I can hear the distinct ticking of each one (including #6) as it fires.

Compression:
- trouble here: try as I might, could not get the compression tester fitting to screw into spark plug hole (it's threads don't seem to be tapered).

Observation:
I think the spark and fuel tests above are enough to at least isolate the issue to the cylinder itself (valves, head gasket, piston or rings) but want other opinions. Is seeing a strong spark arcing from all six spots on the coil pack enough to rule it out? Is hearing the distinct ticking of the injector (and smelling fuel) enough to rule it out?

While I search for a tapered fitting, I am open to suggestions for further testing. A couple of (fairly) easy ones that just came to mind are using my HF laser thermometer to check the exhaust manifolds, and removing the valve cover to check #6's valve springs. Maybe I'll get lucky and find one broken.

Thanks for any guidance.

@wmburns - I'll double check the routing and use of stand-offs, but I'm certain the right coil position/wire was connected to the right plug, as the engine ran great (with no DTCs) for the 19k miles since the ignition parts were replaced a couple of years ago. I'll also rerun the EM temp tests.

I should have mentioned that I'd read about the failed alternator diode signal issue and tested mine as well. Value on VOM registered only in one direction as expected.

As for the cam sensor, I did replace it as well, as I also initially found a p0340 DTC. It resolved the p0340, but the cylinder 6-specific and first 1k rev misfire codes remained and returned when cleared.

@mattbaker302 - I see what you're saying, but can't get past the fact that the DTC keeps returning to #6 after swapping plugs and wires & clearing codes.

Other advice I've gotten is that the lower intake manifold is prone to cracking and leaking coolant in the spark plugs, but I see nothing unusual about the plug when I remove it and there is no unusual smoke in the the exhaust (only the smell of unburned gas).

Thanks a ton for the feedback!

wmburns said:

Just wondering......How many diode(s) did you actually tests? Just one? Or did you test all (8)! How did you isolate the other diodes in the bridge?

Click to expand...

Good point - I only conducted a basic test - with the engine off, one lead on alternator battery positive (B+) connector and the other on the alternator case itself, then switched the leads and rechecked. A reading in one direction only. Given the ease of access to the alternator, I may look at removing it to have it tested. What is the ripple test you mentioned?

I was finally able to run compression tests on the driver-side cylinders last night. Results were #4 ±142 PSI, #5 ±145 PSI and #6 (with the DTC) ±138 PSI. Does being within 5% of the highest one lessen compression being a potential factor?

<Edit - I found a post that listed maximum-to-minimum compression ratios for Ford 3.8s. The minimum for a high reading of 145 PSI is 109 PSI, and the #6 result is well above that.>