Went in to limp mode with a JB4 G4 on the car. I replaced my coil packs and plugs with bosh parts, also did a new valve cover gasket and added an oil cooler "car did not come with one". Car still runs bad getting miss fires and ever code you can get with the N54 engine .
My car has 154000 miles.
My car has an automatic transmission.
One common failure on the BMW sixes is the intake manifold gasket. I can come off in chunks and leave air leaks going into the engine causing misfires. Check carefully the flange where the intake manifold bolts to the cylinder head. Check also all of the vacuum lines and fittings. Any bad connection can let enough air into the engine to cause a misfire. Otherwise I would be very suspicious of your JB4 Upgrade. I’ve seen forums where others have reported problems. A skilled technician with a Digital scope should be able to read the patterns coming out of the ECU and make the call. If you want help with this you can call Your Mechanic to have a technician come to your home or office and check out your misfire problem and let you know what can be done for it.
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