Hello all,
I recently bought my 2009 Mini Cooper S and have lobed it since i got it. It has 99,000 miles and hasn't given me any trouble until this week. It started overheating, so in turn I checked the coolant level, noticed it was low and added more to bring it to acceptable ranges, then bled the air out of the sysem using the bleeder screw o top of the thermostat housing. Ran fine for a few days, then the engine light came on, idle got really rough, and overheating returned. Checked the coolant again, when it was cold, and none could be seen in the reservoir. Opened the cap (while the car was cold from sitting overnight) and pressure escaped, allowing coolant to rise up to above the max line. Ok, so am I low on coolant or not? Next i turned my attention to the rough idle. I plugged my handy scanner and came up with misfires on all four cylinders as well as a random misfire code. Knowing that spark plugs should be changed every 60,000 on my mini, I tried that first. The set I replaced weren't in bad shape, but it was probably time for a changing. After the spark plugs went in, and I closed and rebled the coolant system, the car ran great for two hours and the engine light switched off. Now the rough idle and overheating have returned but im only getting misfires on cylinders 2, 3, and 4. I swapped around the coils, but the likelihood off three going bad at once seems farfetched to me, and nothing was affected. What would cause a come and go misfire, and what is my coolant system doing!?!

 

I have a 2009 MCS with 65,000 miles. I had to change the water pump this year. I'd say you have some classic symptoms, especially at your mileage.

Crawl under the car & see if you have any whitish residue on the passenger side control arms or in that general area. If so, you definitely have a water pump leak.

 

Yep, you got a leak. You need to take care of that first before you troubleshoot other problems. Is either the water pump or thermostat housing or both..

 

...and make sure the engine oil hasn't turned milky.

 

Thanks for confirming my suspicions, I thought it might be a leak somewhere, but wasn't sure since there was pressure in the system when I opened it after cooling overnight. After doing some reading, I learned that this is also a symptom of a faulty reservoir cap, and found one at AutoZone for 7 bucks. Figured it was worth a try, so I ordered it and they are overnighting it to me. (Free shipping, thanks to my obsession with rewards programs) My next guess would have been the thermostat, or water pump. It does the classic heater goes cold at stop light, but warms up when driving. Fly'nbrick, my biggest fear is a head gasket or warped head from overheating. So far, there doesn't seem to be any coolant in the oil, or vice versa. The oil was recently changed and is still nice and new looking. Moving back to the rough idle and misfires, I am fairly certain it's not a HPFP issue like I've read about. Every time I've checked the codes, my fuel rail registers a pressure of between 714 and 721 psi, which to my understanding is a normal reading. I need to get the coolant issue fixed first, but I'm thinking of doing a compression test and seeing what's going on inside those cylinders.

 

Any updates with this issue???