328i N51 unsteady idle potential fix
For the longest time I had a rough idle on my 2008 SULEV 328i, N51B30. I looked into all kinds of possible issues but wasn't able to diagnose the issue. If you're driving along and come to a stop, the idle would settle in, but the rpm would bounce by 2-300 rpm randomly and intermittently. Needless to say, there were unpleasant vibrations and noises when this happened. This was happening when the car was still under warranty at 30000 mi, and the dealer tried a few things but nothing worked (there was no fault code or SES light). They finally ended up increasing the idle speed to 650 rpm at my request so that the unpleasant vibrations would at least be minimized, but the problem continued. I contemplated getting the intake valves blasted but realized that would be useless on a Port Fuel Injected engine like the N51/N52.
Fast forward 1.5 years and 20000 mi (2 miles after my car ticked over 50000 mi to be precise), and I'm sitting in the car, idling outside a restaurant, buckling down and getting ready to leave when the whole car starts shaking like a mutha-f*** and the SES light comes on. I curse the curse of the rough idle, drive to Autozone and pull codes. P00BC (MAF sensor too low) and P0305 (Cyl 5 misfire). I proceed to clear codes to see if it will return. I decide to attack the Cyl 5 misfire first, so I get home and pull the #5 coil. Check static DC resistances on all 6 coils, and they're all similar, #5 does not stand out. Before I pull the trigger on plugs or coils, I decide to give it one more chance. So I put a little bit of dielectric grease on all boots, interchange #1 and #5 coils and put everything back nicey-tightey.
After about 250 miles of driving since, my idle has not been smoother ever. The car purrs when idling at 650 rpm every single time and idle speed is mostly rock-solid. There's a slight movement occasionally but that's normal due to idle air compensation.
So if any of you guys/gals is getting a premature misfire fault or experiencing bad idle at low miles on the N51/N52 motors without the SES, check and lubricate your coil boots. Might or might not be the issue, but could be a potential fix..
Last edited by sammk; 12-05-2012 at 12:01 PM.