hi all, i ahve a 2004 mcs, w/112,000 miles under its belt. I recently have been experienceing a rough idle, its not terrible it just kind of feels like it misses every couple of seconds and then its back to normal, hardly noticable in rpm changes according to the tach, but i can feel it. So i figured i would start with the spark plugs/plug wires, and air filter. with all of those changed and still having the same symptom, i thought i might try to run some techron through it to see if that might help cure my problem. No luck on that either. My exhaust has been sounding a tiny bit louder and i am noticing a little bit of a difference in performance while accelorating. Do you think this could be my o2 sensor going bad? i dont have any check lights on and my gas mileage still seems to be right up where it has been. Any thooughts on this would be greatly appreciated, no one lives around me w/experience with Mini's so i need some help!!

also i am concidering changing the fluid in my transmission. I talked with my father who is an "in the old days" mechanic, he suggested that I not cahnge the fluid since many of them have a detergent in them and after a while it stops working as well, stuff starts to build up and the transmission gets used to that built up gunk. Then he says when you put in the new fluid, it cleans all that gunk out, but then the transmission fails due to the gunk missing?! is this true? is anyone noticing a failed transmission after changing the fluid or is this somehting that i should absolutely do as i'm sure it hasn't been done before! thanks a lot!

 

'03 R50 w/ almost 120k miles... changed the fluid about 7k ago when i redid my clutch... no problemos. fluid was the original.

in my experience... o2 sensors don't start going bad... they are or they aren't. a bad o2 sensor would definitely cause the service engine light to come on.

did you happen to inspect the exhaust system?? a small hole or two in your pipes can cause it to sound weird and lose power, and maybe idle oddly.

my ride's idle was effed up when the A/C compressor kicked in... sometimes it would even stall out if it kicked in while i was at a light.. changed plugs (to BOSCH platinum 4s) & ignition wires (to jackson racing) last week and haven't had an issue since..

 

Actually O2 sensors do degrade over time. It's a combination of heat exposure and carbon buildup. The more buildup, the less oxygen it can sense. You would also notice a slightly lower MPG because the computer is adding fuel to compensate for the difference in the A/F ratio. However, you have to compare the same driving conditions. If you're a leadfoot, this won't work. Once the sensor/computer is no longer able to compensate to attempt to create a balanced ratio, the light comes on.

The manual says replace the oxygen sensors every 100K miles. You can get away with only changing the pre-cat sensor, as the post-cat one is exposed to less emissions.

The miss could also be due to corroded coil terminals. Make sure they are not black in appearance. Terminal #3 is a prime culprit. Clean them with STP carb spray cleaner and a rag or use a brass brush. Only other possibility is the coil pack itself is going bad. If you have to replace it, don't bother with an aftermarket one. Get a stock replacement.

Maybe consider running Seafoam or STP treatment through a tank of gas. There's lots of threads on this in NAM.

Changing the fluid in the transmission is a good idea. Most newer manual ones use ATF, not gear oil.

 

I agree on changing the tranny fluid. I've done it on other cars with 150k miles and higher.

I went through the same thing on my wife's 03 cooper (102k mi.). It eventually did throw an intermittent CEL and nobody could diagnose what was wrong with the car until it melted three intake valves. Turns out the intake gasket was bad and was letting in unmetered air, causing the motor to run lean....thus the melted hardware.

I don't know how you would test for a leaky intake gasket. But my mechanic was adamant that this is an issue that would affect many, if not all higher mileage Minis. Although, I'm not sure if this would affect an S.

 

I've seen it mentioned in other threads how to detect a leaky head gasket. Two methods come to mind. One is to use a length of hose held up to your ear and to the head/block area and listen for leaks. The other involves using a smoke agent around the block and look for air movement.

I'm sure you could find these and others if you do more searching in the threads.