Hello, Recently acquired a 09' R57 with the N12 and bought the car with a warm engine misfire. Thought I would try to learn a bit about the R57's and the N12. Owner reports the misfire would happen after driving for 30 or so minutes and #3 seems to be the source of the issue. I ran codes and sure enough #3 came up followed by code for multiple random misfires. No other engine codes present.
- Pulled plugs they were replaced last year as were the Bosch coils. Plugs looked a bit dark but nothing crazy.
- Installed them in a random order and started the engine. Took about 30 or so minutes sitting in the garage before, sure enough misfire started.
- I shut it off and instantly pulled #3 plug and found the plug wet with oil.
- I pulled all the other plugs they were dry and did a compression test and all came back at 190.
- Thought I would hook up the scanner and check engine temp as in my opinion I would have thought the cooling fan should have come on in the time I was running it. What I found was at 212F it would start to misfire on #3.
- I assume something happens potentially with cam timing?? at 212F?
- I ran it up to temp a few more times and sure enough at about 212F #3 and appears wet with oil.
I know these have valve seat issues but the oily plug is bizarre to me. The engine has no clatter or strange noises aside from what I can tell an oil pump that whines. I would somewhat expect noises from a valve seat dropping or dropped. Im good with pulling the cylinder head but thought I would ask the experts beforehand.

 

While the MINI engines get a lot of bad press regarding -- in general -- cam timing (and some of it is warranted to be sure) -- the behavior you are observing does not suggest cam timing to me.

Valve seat issues generally refer to one or more seats coming loose due at least in part to overheating. I did encounter a Subie with a persistent misfire from one cylinder that proved to be a bad (burned) exhaust valve seat. But the cylinder was down on compression but I can't recall by how much.

A bad valve seal of one of the valves at cylinder 3 can account for the condition of the plug. The seal lets too much oil into the valve guide and this excess oil ends up in the cylinder.

But the misfire I believe is tied to temperature and that suggests an electrical problem, like a coil, wire or sensor, acting up from the heat.

I note the coils were replaced about a year ago. That's no guarantee the coils are all good. You shuffled the plugs but you need to move the coil from the #3 cylinder to the #1 cylinder and the coil from the #1 cylinder to the #3 cylinder and see if the misfire follows the coil.

You need to check or monitor battery voltage as the engine runs. It is a bit rare but alternator output can decline after engine start. It just so happens my one and only time I needed to replace an alternator -- with around 1M miles (in total) of driving -- the alternator was fine right after cold start and for about 30 minutes of driving. Then I noticed the battery voltage down from ~14V to closer to 13V. Recognizing what this likely meant I pointed the car towards the dealer (about 10 miles distant) and made it with the battery voltage at ~12V. The engine/car was not manifesting any issues but I have seen other cars with what proved to be a bad alternator manifest warning lights, then the engine begins to run rough, CEL comes on, engine then shortly after dies.

 

I did swap coils and plugs a few times in that cylinder and also tried a spare coil and spark plug I have on hand. My scanner does show voltage and one thing I noticed was changing at 14.3 (probably glanced at it while warming up)seemed a bit high but at least it’s charging. I do agree with the possibility of voltage dropping and spark collapse when the engine and alternator heat up. Will make an effort to look at the charge rate when she reaches close to the 212F mark. I did clean the ground I see at the front of the valve cover just in case. I never was able to get a direct answer, but when should the cooling fan kick in? Yes I did confirm it works using the A/C. Hottest I ran it wile misfiring was 221F and no fan came on. I’ve see no real spec sheet regarding the cooling system in this.

 

Today’s adventure revealed something definitely happens with cam timing at 212F. I ran it up to 212F and off it went misfiring. I disconnected the exhaust cam sensor and it smoothed out perfectly. I also discovered thermostat must be set at 225F because it did a great job keeping that temperature albeit without using the fan. Looks like the next step pull the head and inspect.

 

Update. I removed the cylinder head and stripped it checked for any movement of valve seats, none noticed unless they can drop a bit and then return flush?? Had the head cleaned as there was a lot of carbon buildup, not on the valves per se but rather in the exhaust port. Head cleaned a slight skim and valves lapped. Reassembled everything using new valve stem seals, of course reinstalled cylinder, head reset, adaptations, etc. fired her up and sure enough misfire and wet plug still present on 3 at around 100c…. Remove the cylinder head again and I’m currently sitting here. Scratching my head 😂 one thing I didn’t notice is the lifters as I call them are rockhard all but one which had a bit of cushion. The rock hard ones I could collapse, but had to use a pair of pliers. Not sure what those should be like when new?

 

this is a photo of the crap dripping out of number 3 second time removed. And just to be clear, I bought the car with this misfire. The stuff dripping out of the cylinder is oily in nature, but doesn’t seem like oil and when I bought it and checking the spark plugs, they were all equally dark with no crust on #3 as I would have expected. As mentioned above, she runs perfect no noises until that magic 100c mark. I do notice vacuum buildup when removing oil filler cap at idle. I have checked the PCV diaphragm, and it has no rips.

 

Update - This time I decided to do a trick Ive done in the past to the valve seats found in early 2.0L VW air-cooled heads. I simply peened the valve seats on #3 and re-lapped the valves on 3. Happy to report this did the trick and can now idle the car until the cooling fan kicks in at 108C. Now on to figuring out why I get a idle surge for about 20 seconds on cold start.. Did get a code for intake Vanos movement. On to checking the condition of the non-return back valves at this point. I didn't realize they were there and they were left in the head during the cleaning/surfacing process so could potentially be full of junk.