Hello,
I recently replaced the cylinder head on my ‘02 MCS after discovering the previous owner did a thread repair job on the cylinder 2 sparkplug which resulted in the plug being ejected and cracking the head. I replaced it with a reman head and after buttoning everything up and test driving it, I’m feeling a random misfire at idle, and eventually the computer will catch up and give me a P0300 code. Looking for some help in diagnosing what it could be. I have connected my scan tool and looked at live monitors for the misfire and it seems isolated to cylinders 2&3. The misfire itself is also extremely intermittent. For example I drove it about 60 miles one day with no issues, parked it, started it about an hour later and it was misfiring. Similar scenario today, I started it and drove it about another 80 miles and idled just fine up until the end of the drive where it started misfiring at a stop. The only other behavior its exhibiting is the battery was oddly dead in between those drives. (1 y/o battery)

*edit with new symptoms: Went to crank the motor and nothing happened, no click, nothing. Cycled ignition off/on again and started just fine. Drive it maybe 1 mile from home and started misfiring again and accelerated from 15mph to approx 30mph in 2nd gear and it outright shut itself off. Pulled over, cycled ignition off/on, no crank. Cycled 2nd time, cranked, stumbled and stalled. Cycled a 3rd time, it cranked and ran fine. Absolutely 0 codes current or pending. Im at a loss.

I did do a smoke leak test on the vacuum system and I didnt see any leaks. I did read a thread about it possibly being the map/tmap sensor? Or could this be a bad battery causing voltage issues?

The work I”ve done:
Reman cylinder head
Timing chain, sprockets, and tensioner
SC oil change
New Plugs, wires, coil pack
15% pulley (installed prior to new head)
DDM intake (installed prior to new head)

Edit 01/21/26: Things I’ve done: New battery, swapped ignition coil/wires/plugs again, changed upstream O2 sensor, checked bypass valve spring, checked intercooler boots for tears/leaks/improper seating, checked compression, checked for vacuum leaks, replaced gas cap. I’ve still turned up with nothing to even begin to fix/improve the issue.
Codes: P0300, P0171, P2301. All happened independently of each other.

Edit 01/21/26 in the afternoon: TMAP & MAP sensors arrived, replaced them both and now the car runs fine…

***tentatively considering it fixed***

The thread I read: https://www.northamericanmotoring.co...m_content=post

 

You might try moving your #2 and #3 injectors to positions #4 and #5 (and vice versa) to see if the misfires move cylinders.

 

I'm no match for many of the well informed Mini mechanics that contribute their knowledge here on NAM BUT from one of my similar intermittent start/no start scenario - turned out to be my battery. At the time I found it hard to believe since my battery was only 2wo years old.
The car would start, drive fine, voltage gauge read 14 volts driving & idle (alternator good). Parked the car, went out to start, headlights strobed, no crank - battery dead. Bought and installed new battery. All was right with the world again. Hard to believe - but it has often been said that Mini's are rediculously particular about battery health.

Just a thought.

 

how is compression after the head replacement?

 

Compression was actually better after the head replacement, I suspect the valves in the old head were on their way out anyway.

 

Ill give that a shot, as I suspect the no start is probably the battery as well, but do you think it could also be causing the random misfire?

 

I'm not sure about cylinder 5 as mentioned above, but I would agree to the idea of moving injectors and coils to see if the misfire travels.

 

Hmm I wouldn't rule out electrical gremlins because it is intermittent and you had the click but no starter motor.

Make sure the ground wire from engine to body is not corroded and check for wet floor carpet on driver and passenger side.

Also, just to be sure, check the terminals on the ignition coil if they are not corroded.

 

After talking to other professional mechanic friends and further contemplating, I would agree. I’m most suspect of the battery not holding a charge while doing low speed, low rpm driving causing the voltage to gradually drop just enough to wreak havoc on the electrical system. Volt meter shows 14ish volts so alternator is working as it should. But I’ll do the due diligence of checking grounds.

For clarification I wasn’t getting a click at all upon attempting to crank despite battery voltage being 12.3 something which is the same symptom it had exhibited prior to any of the head work causing me to replace the battery the first time. Just without the misfire symptoms.

 

you should get over 14 volts at the battery, with the car running, if your alternator is working right and no wiring issues

 

Just curious; Did you disconnect any wires from the starter when you replaced the head?
Again, maybe a dumb question...but just curious.

 

Yep, I’d get over 14 volts, (14.2-3 or something). I did end up replacing the battery and so far no misfiring/no crank problems. I’m thinking it was due to it being a cheap battery/sitting for too long.

Edit: Wasn’t the battery. It was driving fine for a few hundred miles and had a pending P0170 code, so I replaced the upstream O2 sensor with a NTK O2 sensor and upon starting it after replacing that, it began misfiring again.

 

I did not, but I did think about that lol

 

I went and checked all major grounds/power connections and none were corroded/loose. No wet floor either