Apologies for yet another cylinder 3 misfire thread, but we have done some troubleshooting already and are at a roadblock and we need help please. 2013 Mini Cooper S, 55k miles.

To summarize, a friend's car stalled in the middle of an intersection, was tower to dealership, they quoted a crazy amount to have fuel injectors, spark plugs, brakes and oil changed.

So we decided to do it ourselves. Everything replaced that the dealer was going to do themselves, and the car runs smoother than before, but still misfiring on 3.

Swapped coil pack from 3 to 2, cleared code, code came back still cylinder 3 misfire. Kept car running, went down the line and unplugged coils from all cylinders to try to notice a difference in how the car ran, 1,2, and 4 did as expected, car ran worse when unplugged.
Again, we took a known good coil (one that when unplugged made car run worse), put it in cylinder 3, then plugged 3 while car was running, and it made no difference.

We are now thinking it may be the wiring harness going to the ECU from the coils because the actual connector on 3 does not look corroded at all. We peeled back some of the mesh around the wiring on the harness where it visually looked the most scuffed up, expected a short of some sort, but the wires looked fine.

 

Do this first:
Compression check
Vital to do before anything else.

It is possible the wiring is bad, but nearly unheard of in my experience on a low miles car, unless mice have been nesting under the hood.

Do this second: Check the spark with a plug out of the engine. Be very careful to keep the plug grounded as an ungrounded coil can fry the coil driver in the ECU. It is OK to remove the connector from a coil, no risk to the ECU.

 

Is it best practice to do these two things at the same time?

Unplug fuel pump relay and unplug all coils from spark plugs while putting a compression tester in 3 and have the spark plug from 3 in the coil and out of the car and check it for spark?

As far as checking it for spark when it's out of the engine but still in the coil pack, what's the safest way to keep it grounded whilst doing so?

 

Compression test came back to about 190 psi consistent between all cylinders.

I took the spark plug out with the coil and i couldn't see a spark when turning over the engine, but i could've been doing it totally wrong. Coil packs were replaced before performing this test.

Any help would be appreciated!

 

Best to do this when dark. Just to establish a baseline do the spark test on a known good coil/cylinder.

Have an assistant crank the engine.

Coil connected to harness, plug in coil, threads of plug firmly in contact with a metal part of the head.

A known good plug/coil should deliver a visible spark

 

Also, congrats on the compression test! Good to know the engine is really healthy.