F25 X3 Rough idle, no codes, runs great otherwise

**thomn8r** · 10-14-2020, 10:27 PM

My wife's 2013 X3 N20 started idling rough a few days ago - it's a very discernible shake that you feel in the seat and in the steering wheel. There is no difference in engine sound with the vibration.

I checked for codes, got nothing. No check engine lights. It runs great except for the strong wobble at idle.

The car has 107k on it.

She took it to a local BMW shop. They didn't find any codes, but noticed the motor mounts were worn, so they put new motor mounts in it, which basically made it worse because it transfers the vibration into the cabin more efficiently.

I took it back to the shop and more thoroughly demonstrated the problem. They've tried swapping plugs, coil packs, and fuel injectors to no effect. They did leakdown test and said it's at 80% on #3. Today they pulled off the valve cover to check for a loose rocker arm or broken spring - nothing. Next they want to remove the intake and check for carbon buildup. This is starting to get expensive. The next step after that is to remove the head, for which they quoted 20 hours.

I've done some googling, and found all sorts of threads on similar issues, but no definitive fix. One thread said it was an exhaust issue. Others mention "walnut blasting" but I'm not sure what it is they're blasting, exactly.

I'm at a loss as to what to do next other than to throw some more money at it, and the wife isn't happy that she has to drive my M3 (she finds it too low, too loud, and too stiff)

**jclausen** · 10-24-2020, 11:39 PM

Have them use a bore scope on #3, in the old days when adjusting valves on alfa romeos you would tap the top of the valve to break odd carbon that might collect

**dukedkt442** · 10-25-2020, 08:26 AM

Walnut blasting *might* help as the valves get coked up on DI engines with no fuel running across the intake valves; the EA888 in my mom's Tiguan just had to have it done at 90k. The mechanic took before and after pics, and man I couldn't believe the different! Her car was suffering from occasional random misfires on all cylinders. Look at your fuel trim numbers to see where they're at; *should* be at 0%, but anything +/- 10% and you'll start seeing some issues. High + indicates a vacuum leak somewhere, high -indicates a fuel supply issue. The N52 uses an eccentric shaft to adjust valve lift and keep an idle rather than an IAC, and I assume that the N20 is the same. IF it does, locate and inspect the eccentric shaft sensor (under the valve cover towards the front of the engine) for oil. The seals leak, and of the sensor gets wet with oil, the computer can't read the eccentric shaft and can't keep a smooth proper idle.

The other thing (and I don't want to scare you) about the N20 is they are known to eat their timing chains when the guides wear/break, so if you have slack in the chain, you'll get a rough idle and other driveability issues. If the guides/chain fail, you have a boat anchor under the hood; the engine is toast. IF what you're seeing is a warning sign of impending doom, you may have lucked out. Ask them to check the timing components.

**thomn8r** · 10-25-2020, 09:35 AM

They borescoped it, and there was some carbon, it wasn't horrible; walnut blasting didn't change the leakdown numbers. Monday they're going to take off the valve cover and go from there.