Hey all,

i just joined the forum after scouring the web trying to find solutions for my poor car's CEL. I've been to my mechanic a few times and it seems like I'm on the right track for figuring things out on my own. If anyone has any insight towards my issue I'm all ears! Let's start from the beginning.

I bought my car in November and subsequently drove it up to northern California in December (I live in LA atm). No problems! I was elated! ...Until I did get a CEL for a P0300, but I swapped the spark plugs for some NGK's and that was solid all the way up until last week (about a month).

I was just casually driving home after work, nothing demanding. I always warm up my car for ~3 minutes before heading out. On the freeway just 5 minutes after leaving work, I got the light back on. I carefully drove it towards home, exiting the freeway when I stopped at the light and it was very rough and chugging along. However after the freeway, it only needed about 5 minutes until it readjusted itself? And it drove fine.

I ordered some spark plug wires and didn't drive it until I replaced them. Things seemed fine until I revved up entering the freeway (maybe 5k rpm) and dingdingding, CEL is back.

After some more reading, I decided to try replacing the timing chain tensioner. Easy enough job, with no beneficial result however. Just as I was curiously about to try a used ignition coil pack, I instead drive over to my mechanic who reassessed my steps and confirmed what I had done thus far. They ended up changing the coil pack as I surmised and everything seemed fine for 2 days.

Obviously I'm writing this so that cursed CEO came back after I was in sport mode on a highway going about 50mph and slowing down for a red light.

I'm pretty distraught at this point as I'm new to fixing up cars since this is my first. After begrudgingly looking for answers online again, this is what I've compiled for potential random/multiple cylinder misfires:

X vacuum leak
X Dirty fuel injectors
X low fuel pressure
X dirty MAF sensor
X air/fuel is lean
X idle air control system
X distributor failure
X compression
X timing chain issues

My mechanic checked compression, so it seems that's fine. The spark plugs, wires, and coil pack are not the culprit, nor the timing chain tensioner. Oil is full with 5w30 synthetic Pennzoil. I only fill up with Shell 91, but I'm thinking to try something, anything different.

In December, I did use one fuel additive that was supposed to clean my injectors. Not sure if that has any bearing but i figure it's worth mentioning.
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I've had an engine mount replaced in December and the transmission oil pan dropped and changed this week.

Other tidbits of info - on cold start my engine does make some obnoxious higher pitched noise that seems to come from the driver's side of the engine until about 1-2 minutes of idle then it disappears. After that when it's in gear and idling, it makes some slightly different light jingling noise.

I bought a Bluetooth OBD2 scan tool which can tell me real time sensor data like STFT, etc. I noticed sometimes that stft graph in specific will spike up and down consistently and other times it will be fairly smooth.

Okay that exhausted every bit of info I can write. Sorry for the long post. I'm quite desperate to get my car in good condition since I drive it to work and school 6 days a week, 30-75 minutes each way.

Again, any help or discussion is highly appreciated.

 

My 2005 R50 does EXACTLY the same thing only not as frequently as yours. Mine will start misfiring, then after a period of idling for just a few minutes, the misfire will go away and it'll drive fine. I have dug a bit deeper than you in an attempt to resolve it. The first few times it happened, I diagnosed a burned exhaust valve and replaced the cylinder head (it had 2 burned valves). The car ran fine for a few months and then had a misfire episode. I swapped plug wires, coil pack, and spark plugs. I then had it misfire on me in two separate instances a few days apart. My last attempt was to pull the throttle body and cleaned it thoroughly. I also pulled the MAP sensor and cleaned it as well, both were very dirty. It has been 3 weeks and no issues so I don't know if that was related but so far, it seems to be working.

 

Hey David, thanks a lot for the insight! It gives me some much needed motivation to get at it again after this rainstorm in LA lets up. I know nothing about exhaust valves, throttle bodies, or even MAP sensors (I only know it's mass air flow) but I enjoy spending the time learning.

How did you diagnose those? I have heard of dirty MAP sensor, but not the others yet.

Cheers to hoping your CEL doesn't creep back up on ya!

 

The R50 and R53 use a MAP sensor instead of a MAF. MAP sensors tend to be more resilient against random vacuum leaks so even if you have a small vacuum leak, the computer should just up the fuel to keep it from leaning out. My next attempt will be to pull the crank sensor and try cleaning it off and swapping the o-ring.

As for the initial diagnosis, my car had a rough idle when warmed up. A compression test showed lower than normal compression in cylinders 2 and 3, a leak down test confirmed exhaust valves. As for the throttle body, our cars don't have a standard Idle Air Control valve (IAC). It uses the throttle plate to regulate idle. I read one random post here on NAM where someone mentioned cleaning it and the MAP sensor so that was what I tried since they were so easy to get to. I think I spent about an hour removing, cleaning, and reinstalling the components.

If I run across a solution, I'll be sure to post about it

 

I did read about checking the vacuum pressure but didn't see any diagnostic tools at my local O'reily or Autozone so kind of dismissed that for now. Especially after I tried checking the fuel pressure with an Autozone tool and ended up almost getting fuel in my eyes (eye protection, d'oh).

I ordered some cleaner from Amazon, the MAP sensor looks so easy to get to, I'll give that one a shot first.
And Autozone has leakdown test kit, I might need to educate myself on using that one if necessary after cleaning.

 

If you had a mechanic perform a compression test and the numbers came back good then I wouldn't worry about a leakdown test.

I used MAF cleaner on the MAP sensor. Also used it on the throttle body. It had some nasty caked up junk right behind the throttle plate. As mentioned before, it is behaving. I guess I'll find out if that solved it in a couple of more weeks as it typically strikes once a month or so.
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All these months later, I'm 95% done with essentially rebuilding the engine akin to a new head gasket change. I have lapped the valves and cleaned the whole head to a sparkle!

This was after i brought my car into a Mini dealership and paid for an inspection. They suggested $2500 for cleaning out carbon buildup on valves.

I dug my heels in and started learning to prepare for it. It has gone well so far! However I obviously snapped a few brittle pieces of my 14yo Mini.

Can anyone help me figure out what this line is in the photo?

It rests right above the engine mount, near the crankcase vent valve parallel to two other tubes. They run back towards the passenger seat then down into the engine bay, lined up neatly along guides. The other side runs off along the intake manifold and ends vertically plugged into something nearby the airbox.

My uncle says it's a vacuum tube, I'm having trouble finding the exact part though.

 

It is a vacuum tube, it connects to the throttle body and also the fuel pressure regulator on the fuel rail. I'm not sure what it connects to when it runs over to the passenger side of the car.

 

Thanks David! You're always giving great feedback That's the last part i need, time to order and get this finished!

 

Hey all, I just wanted to wrap this thread up since I finished my repair. To recap for anyone checking this in the future:
Around 100k miles I started to get a P0300 code from my CEL. After trying many simple fixes, such as ignition coil pack, wires, and spark plugs, nothing was helping. So I brought it to the dealership and they told me it was a carbon build up on the valves, and that the head would need to be removed and the valves lapped.

Thankfully on an R50, removing the head is a fairly straightforward repair! I have never, ever repaired cars before and with some very careful planning I managed to finish the job with a few hiccups of old, brittle parts breaking (they needed replacing anyway!).

If anyone has any questions, feel free to post and I'll poke back in to try to help :D