Overnight Cold Start Rough Idle N62 4.8iS

[mr_robot]

Ok, been trying to diagnose a cold start rough idle I've been experiencing ever since it has been getting cold here in New York.

It has new coil plugs and spark plugs from FCP's ignition service kit about a year ago.

Just got the VSS and all corresponding gaskets including the vanos seals done and at the same time rechecked the spark plugs for any fouling. Everything was good.

DME adaptation was reset after job was done as well. Car drives perfectly normal and strong otherwise.

I'm aware of the intermediate lever SIB and probably it is the issue but just want to understand something.

I get rough idle and sometime the CEL flashing with misfire codes P0300 and random cylinders, sometimes its cyl 2, 4, 6,7 or just 6 or 8, it's never consistent.

With that said it only happens after the car has been sitting over night (over 12 hours) next day going to work.

If it's my I.L. shouldn't it happen all the time for cold starts? For example, after I drive to work and come back (usually 6-7 hours later) it does not show these symptoms. It only seems to happen over a certain amount of time car has been sitting.

So if that is the case it could be a software/setting issue? Like what is different about 6 hours and 12 hours, what changes does the DME make to the valvetronics to go into a certain lift value?

I feel like I can bandaid fix this until I eventually get to do my I.L. if it's something I can change in the settings.

Thanks, any input is appreciated!

[80stech]

Check for vacuum leaks at the brake booster. Also between that and if the aux air pump isn't working can cause exactly what you are describing.

[mr_robot]

Originally Posted by 80stech

Check for vacuum leaks at the brake booster.

Also performed a smoke test (with the snap-on machine) prior to doing valve stem seal for any potential leaks and was good. The seal that mounts to the timing chain cover was replaced when doing the VSS job as well.

[80stech]

smoke test won't show booster leak. Both my X5 and X3 had a similar problem. They both lost booster vacuum overnight and replacing the vacuum valve and grommet was the fix. Both boosters themselves where ok. The X5 was the worst (miss and codes) and also needed the aux air valve and pump relay which cured some of it, but took the vacuum valve and grommet to totally fix it. I would have a hard time believing some of it if I didn't go through it myself. Mind you I am talking about M54's and I ruled out the more common stuff first.

[mr_robot]

That pump is just for the brakes so wouldn't I have some brake pedal feel issues?


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[80stech]

No, the aux air pump that pumps air into the exhaust manifold.

[mr_robot]

As far as I know the N62 in the 4.8is doesn't have a secondary air pump. Just the vacuum pump on the right side of the timing cover.

Or am I not following you?


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[Emory39]

My X5 had a bad vacuum valve but never did what you are describing. That said if I am waiting for a traffic light the RPM's jump a little bit. Like 50-100 rpm it does not cause the rpm gauge to fluctuate. I guess wrote this because I want to know what is the I.L. and could it be causing this.

[80stech]

Yes, I guess lots of differences with the V8. I'll leave this to the V8 guys.

[mr_robot]

Intermediate levers (I.L.) are the rocker arms that changes the amount of lift on the intake valves via the eccentric shaft. Basically a 3rd camshaft to control the intake valves aka valvrtronics. BMW released a SIB for N62 motors for out of tolerance rockers (IL) to be the cause of random misfires on cold starts.

Should not affect once warmed up. So the symptoms also aren't there on warmer climates so maybe that is why there was no recall just on "customer complaint basis"


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