Hello!
I’ve got a ‘02 MCS that recently developed an interesting misfire problem.

The symptoms: At highway speeds, (50+ mph) it’s getting a misfire in cylinder 4 only. I’m reliably able to repeat the misfire under wide open throttle/near wide open throttle under load regardless of gear selection besides 1st & 2nd. When underload at speed, it’ll make a pop, throw a flashing CEL, and go into limp mode. Upon shutting the engine off and starting it again, it runs just fine as if nothing happened. Only OBD code showing is P0304, nothing else.

My theory:
Spark plugs/wires (replaced C4 plug and all 4 wires, still misfires) plugs are the NGK BR7EIX
ignition coil
fuel injector
Fuel pump?
Pre-detonation

History of the car: Bought it off a local guy who was 3rd owner and garage kept it/left it stock. He didnt know much else about it. With me I discovered the head was blown.
Parts replaced: Head gasket
timing chain/gears/tensioner
Plugs

Parts added:
15% SC pulley
DDM cold air intake

Otherwise the car is bone stock engine wise and remarkably clean. Does anyone have any ideas/ran into this problem before?


Edit from the future: To the poor b*****d that comes across this thread in the future, check your spark plug threads for damage and torque down properly! Cylinder 2 threads were potentially damaged prior to my ownership and I never caught it.

sorry to hear it.

I guess it could be many things.

maybe start with the basics. leak down test, pressure test, fuel pressure test

or before that, swap the plugs on 2 and 4. see if it follows the plug. or swap the wires. confirm this first, but I think you can swap two of the cylinder wires and the engine will still run, as a test. NOT SURE WHICH ONES SO CONFIRM IT BEFORE TRYING

I had this same misfire condition about 7 years ago. The connectors on the coil pack were in really bad shape and I had to replace the unit.

Check the 3 & 4 connectors on the coil pack thoroughly, because of their orientation behind the hood scoop, they tend to capture moisture. If you pull the plug off and see any rust (especially on the underside of the connector) that may be your problem. Brush it clean with a scouring pad or a wire bristle brush, dab a little die-electric grease on there, then take a q-tip and clean the inner connectors on both wires. Before snapping it back together, I recommend adding an o-ring (approx 15mm) onto the coil pack housing for both 3 & 4, then after popping the wires on, roll those o-rings back as close to the wire connector housing as possible. This seems to work really well in deflecting the moisture that comes flying in through the hood scoop at speed. I've been checking those two frequently ever since and haven't seen any signs of corrosion so far.

So I took both of your advice and neither solution yielded different/better results. So I just replaced all 4 plugs/wires and the coil pack, yet still getting the C4 misfire under the same conditions. Could this be injector or ECU related due to the increased boost over stock?
Edit: Went and started it this morning and now it misfires at idle with a P1692 code
Edit to edit: It’s because I removed the ignition relay for a compression test

I had a P0301 - misfire on cyl 1. The press fit for the Cat was not sealing anymore. Had it welded. There is a thread on this.

So small update and rather anti-climactic. I did end up finding 2 small leaks before and after the cat along with the cat-back gasket leaking. But while test driving it shot the #2 plug straight out of the head taking the threads with it. Suspicious part is the OBD code is still cylinder 4 despite cylinder 2 being dead. So turns out i was chasing the wrong cylinder the whole time and needing a reman head. I dont have the trust or patience to be messing with helicoil/time-serting the head. :,)

bummer to hear. let us know how you get on with the new head. are you going to do a full rebuild?

I’m going to do top-end only for now. The motor itself only has 108k miles and the cylinder walls look remarkably good as well as the valve train when I last saw it. No evidence of metal in oil, oil burn, or blow by so far. I also replaced the timing chain/gears while I was in there the first time so hopefully after replacing the head I wont have to crack the motor open for a while at least.

sounds right to me. look forward to seeing the refresh

Figured I’d give an update, but I removed the old cylinder head and lo and behold, the old head was cracked into all 4 valve seats on cylinder 2. Ordered a reman head and the install went well, BUT now I’m getting a p0300 code so the fun continues!

E, I've dealt with misfires on gen 1 and 2 MINIs for years. I've seen misfires caused by faulty sparks plugs, fuel injectors, dropped valve seats, burn through on exhaust valves, insufficient fuel pressure etc. I'd recommend you take the car to a good INDY shop for a misfire analysis. A good INDY shop will use a lab scope to capture an ignition wave form. Proper analysis of the wave form can point you in the right direction. Also, be careful about throwing parts at the car. Even with OEM supplier parts I've see bad parts right out of the box which makes isolating the root cause of your misfire problem even harder.

lol, I actually figured it out a few days ago. It ended up being the TMAP/MAP sensors in conjunction with the signal wires into the coil pack having their insulation rubbed off somehow conducting with the fiberglass heat wrap. Repaired those wires and runs beautifully.

Yep ... I've see that too. So how did you determine it was the MAP sensor causing the problem?