So over the last four weeks I have been battling a strange issue with my 2007 328xi. It was running fine before I went on vacation for about ten days, when I came back I started it up as it had been sitting for a while. It had a slightly longer start than usual and was clearly miss-firing. I pulled out my Bavarian Technic and started checking what was there. It showed I had miss-fires on cylinder number 4 so I went to Advanced Auto and picked up an ignition coil. I went about taking all the fancy plastic covers out as usual and replaced the coil started it up before putting it all back of course and all seemed well. I put all the plastic stuff back together and left it for the night.
The next morning I went to go to the store and it started bad again and idled rough and I got a service engine soon light. So I pulled out Bavarian Technic again and scaned the car, I found that now it was complaining about the Mass Air Flow system. Naturally I went strait to the MAF and checked all the connections cleared the codes and tried again. Sure enough the problem was still there. I removed the MAF inspected and cleaned it put it all back together and cleared the codes once again. At this point the issue was still there and I needed the car for the next morning to make my 60 mile commute to work and back. So a this point being pressed for time I made a decision in haste and hit the deal for a new MAF which cost $455. I returned home and eagerly put the new sensor in and tested it out and again it still was not right. At this point I was very mad at my self for not taking more time with this issue.
Seeing that this was a bigger issue than a sensor I started looking at my fuel trims which I should have done long before. I was seeing normal fuel trims shortly after raising the RPMs but then at idle the car would hesitate and I would get negative fuel trims around -28 to -30. At this point I was confused as the symptoms were not that of a fuel issue which negative values would cause but were of a vacuum leak and the issue only seemed to affect idle it was fine through the rest of the range, you would never know there was an issue which usually points to a vacuum leak also . It took me many weeks of working here and there on it to find what the culprit was. I took everything apart and examined it, the intake manifold and all hoses and exmined the valve cover but I just couldn’t find it. Finally I took out the fuel injector harness and found a bit of oil pooled up in the number 6 injector area. so I tried following where it was coming from and sure enough in one of the holes on the top of the valve cover that you put the engine cover bolt into was where it was coming from.
I removed the metal clip for the threads and there it was, a great big crack in the valve cover directly under where the bolt tightens in. It seems I lost a washer and that allowed the bolt to tighten too far and bottom out on the valve cover and it caused it to crack. After finding this I attempted to find a way to plug it for now so I can get by. I found I had some RTV silicone sealant AKA Form-A-Gasket and I said what the hell I will try it, I filled the entire crevice tight packed with the RTV and left it for 4 hours to harden a bit. After it hardened I gave it a test and sure enough no more issues my fuel trims were back to normal and idle was smooth again. I hope this may help anyone else with a similar issue and maybe will save them some cash.