2003 Mini Cooper random misfire (I apologise on the length but there needs to be a history to appreciate the end result).

I am posting this thread for anyone that has had an engine code of P300 (random engine misfire) on a Mini Cooper base R50 non turbo. The vehicle is a 2003 model with 109,000 miles from new.

About a year ago I started to get the P300 code. I would clear the code and after a while it would come back. There was no definable interval as when this would happen and no other codes were set and the vehicle ran great.

I went on a long trip and noticed if I reset the code and drove the vehicle the code would stay out and only return after starting the engine once it had been switched off. Near the end of the trip (approx 2,000 km later) the engine started to misfire and the engine light came on. I stopped and checked the code; it set a misfire on number four and a random misfire. Because I was not near home I started the vehicle to drive to a garage, when I did the misfire had gone and it was running fine other than the engine light being on. This happened twice more during the trip but got me home.

During the next few weeks using the vehicle for local trips only the random misfire code was set a number of times, no further engine misses.

I decided to do some checking and maintenance to try and cure the problem. First the usual as the vehicle was at 1000,000 I decided to change the spark plugs, wires and air filter. This had no effect and the P3000 code was still being set. I checked fuel pressure and the intake for air leaks, both checked out ok. I then changed the coil and checked all wiring that fed the coil and injectors, all check ok. It was now fall and time to put the vehicle away for winter without finding the problem.

Spring time came and the vehicle was serviced for use and driven, within a couple of days the same P300 code was set. A compression test was done and apart from a 5% variance from cylinder to cylinder everything was fine, did both wet and dry with no change.

Did some searching (on this site) and noticed some people had problems with valves cracking or burning which gave similar problems. Because of the %5 variance and no other clues I decided to do a valve job which would include a head gasket change should that be part of the problem.

I removed the head and inspected the valves, the exhaust valves had sufficient pitting that they could not be lapped in. I had all valves both inlet and exhaust re-faced. I then lapped them in and re-installed the head, checking the injectors as to operation and spray. A road test produced the same P300 random misfire code.

I had now replaced the plugs, leads, and coil. Checked the fuel pressure/injectors and the intake for leaks, reconditioned the head and replaced the head gasket. This had no effect as to the P300 code being set.

I had been driving the vehicle using a code reader to try to establish a pattern using live data. I notice a slight slow response from the post oxygen sensor. Due to the kilometres (100,000) I decided to change both oxygen sensors, just because I had nowhere else to go. You guessed it no change code P300 set. I did notice that with my new code reader I was able to see pending codes as they came up and the P300 was set as a pending code not long after start up. I think just after the ECM had started operating in closed mode. The P300 code stayed pending until I restarted when it set a hard code of P300 (something I hadn’t motioned up till now is when ever the pending code was set the ASC light would come on).


At this time I had no idea what was causing the code. It was a fortuitous incident that would lead to the cure that happened next.

My wife was driving the car very near our house when the charging light came on, she returned and I found the serpentine belt had shredded and had been thrown.

I went to my local parts supplier (major chain) and asked for a belt. They stated that there were two for this vehicle, an accessory one and an air compressor one. If you know Mini’s you will know there is only one for us in North America. I took the two and went to fit them.

One was a bit shorter than the other. I tried the longer belt first which fit quite nicely, but on closer examination it allowed the belt tensioner to move to almost its fully extended position. This would be fine to begin with until the belt had stretched a little then there would not be the required tension. I fitted the shorter belt which went on great and had the required tension. I made enquiries from the previous owner and he informed me just prior to him selling the vehicle a serpentine belt was replaced with an aftermarket one (had been on the vehicle for approximately 20,000 klm).

After the install I re-set the codes and went for a drive. After 100 klm and no codes, pending or otherwise I realised a serpentine belt was responsible for the P300 random misfire code.

After much thought I can only come to the conclusion that the belt that was fitted (after marked) prior to getting the vehicle was not the correct belt and during its life it had stretched to the point that the automatic tensioner has almost reached its end of travel. This was allowing it to bang on its extreme stop during belt flexing allowing the knock sensor to pick up a noise that when sent to the ECM it translated it as a random misfire.

I would never have thought this would have caused this code and would never have found it had it not thrown the belt. When replacing a belt make sure you have the correct one fitted.

There are many times a misfire can be cured by doing any of the above repairs but I thought if someone has the same circumstances I had this thread would be useful.