Hi All,

Need some help. I have an '04 R53 w/ ~80k miles (noteworthy mods include DDM CAI, 15% pulley and Borla catback exhaust). So TLDR, the car keeps throwing a P0304 (4th cylinder misfire) upon cold starts. I've tried a whole bunch of stuff to troubleshoot with no solution yet (see below). There are a couple signs that point to potential head damage, but MOST of the classic real signs of a head problem are not present. Aside from the misfire on start and an occasional rough idle, the car feels and pulls great. Anyway here are the details:

• The misfire ONLY happens on cold start - I don't recall it ever happening while driving or once the car is warmed up
• When misfire happens CES will flash and the car will sputter a bit and experience a pretty rough idle (the DSC light also turns on...not sure why but I know this happens alongside misfires?)
• Almost always, if I recycle the ignition once or twice, the car starts right up no problem and experience no future issues until next time the car has sat for an hour+
• While driving, I have not experienced any power loss at all - car pulls like a champ
• Occasionally, I'll be sitting in traffic and it will feel like the car is idling a bit rougher than normal, but nothing like when it misfires upon start
• Car is not leaking coolant or oil externally (when replacing the clutch last year, I shotgun'd all the common leaky bits and she's been tight as a drum since)
• Twice in the last 6mo/5k miles, the car did start to overheat (coolant temp light came on) and both times, the overflow tank was close to empty (the first time, was close to when I did the clutch and figured I just didn't put enough back in the system and/or hadn't bled fully; the second time happened ~4k miles later and was right around the time the CES came on for P0304
• I'm not sure if the coolant disappearing is a coincidence or not, bc topping off the overflow tank is not a new thing for this car over the years, but usually it comes along with visual evidence of leaking coolant from one of the many places these cars do that
• Oil seems fine - no chocolate milkshake found in overflow tank or via dipstick
• No white smoke

Things I've done so far:

• Pulled plugs (changed during clutch job last year so they are pretty much new); no issues visible on plugs or in tubes; I also switched plugs 1 and 4 to see if the code traveled and it did not
• Swapped plugs and coil pack from younger '06 JCW and CES didnt go away -- I also ended up purchasing new pack and plugs from WMW and even with the new parts the CES has persisted
• Took the car last week to local mini shop to help further diagnose....
• Performed both wet and dry compression tests (multiple times) and all showed equal compression across all 4 cylinders of ~110 (note: my understanding is that that is low but the more important thing is that its equal and that plus the fact that the car doesn't experience any power loss while driving makes it seem fine)
• Performed leak down down test (multiple times) showing ~10% leakage which i'm told is not perfect but fine
• Coolant system pressure tested showing no leaks
• I held a dollar bill to the exhaust and it definitely "flutters" compared to my stock JCW which has a consistent stream of air coming out, but again, there is no power loss, no smell and no white smoke... borla exaust maybe to blame?
• I'm continuing to DD the car (~15-20 miles a day) to see if I can make it worse .... so I can make it better

OK experts... what's next?? My mechanic suggested I try swapping the 1 and 4 injectors which I'll do this weekend, but he did say they all seemed to be in working order. We're both a bit stumped and as much as I like working on this car, I don't wanna pull the head until I have no other options. Thanks in advance for your help!

-Matt

 

Was having a similar issue with my mini r50. Turned out throttle body was bad.

 

Sounds a bit like my problem, but on cylinder 1. Will act up on cold start, but will be fine if I shut it down and restart it. There's one difference with mine tho, when it's warm and I drive at a steady highway speed, it will start misfiring real bad, but without the light flashing. I do hear some fluctuation in the exhaust when I drive it, it doesn't sound like it used to. I'm suspecting either valves or head gasket, ignition was done at 200,000 km and it stayed the same. I know for a fact that something funky is going on because the valve stems are clean on cylinder 1, actually looking a bit rusty..., but 2,3 and 4 are crusty like "normal". I wish I could help you and show what I found, but in my case, I decided to leave it at that for now (I have another car), I'll dig deeper once I have a proper place to work on all my projects. If I pull the head, it means that the MINI is holding the tiny garage hostage.

 

what led you there? How would I check that/confirm a bad TB?

 

UPDATE:

swapped injectors 1 and 4 - no change :( still getting P0304 code on cold start...

Cmone folks what else should I check?

 

Was having an issue of my mini stumbling when I would brake hard. Like when I left off gas quickly. So I replaced the throttle body with a new one. The next day I started my mini in the morning. It stumbled an bucked like mad. The DSC light came on an would not go off. I shutters car off an sit for a minute it started fine. Yet dsc light still on. Check codes an it started miss fire on #4. So I did the same things. Plugs,wires, coil pack an compression test. Everything ok an normal. So I cleared the code. Did ok for a while an started doing it again. So I was like I did not do this before I changed out throttle body. So I swapped out for another one an poof. Runs perfect. No more miss fire codes. No DSC light. Been running for a week with no issues.

 

Still no luck. Before I swap the throttle body as a blind “why not” approach I’m gonna try adding bg44 to clean out the injectors... saw a few threads that where this solved rough idle at startup.