After working on my car, a 2004 545i, I am having misfires across all cylinders, very rough idle, and a dramatic loss of power, not to mention several codes. I have been trying to diagnose the cause but I'm at a loss.

I was attempting to inspect the secondary air system, to deal with a code saying that one of the secondary air valves was not working. I tried removing the intake manifold and after disconnecting a dozen electrical connections and unbolting it, I was unable to remove it on my own so I decided to put the whole thing back together and try to get help later.

With everything back together and connected I started it up and immediately the idle was off, bouncing erratically from 500-800 rpm, stalling, and throwing codes for misfires. The throttle was not responding reasonably, with the engine dying with a light press and revving quickly with a slightly firmer press. I rechecked everything, looked for loose or missed connections and couldn't find any.

Today it is slightly better, but not much. The throttle seems to work normally but the idle is still weak and erratic, I'm getting misfires on several cylinders (2, 4, 5, 7, and 8), and the car is very low on power when I try to move it. With the clutch in, at RPM over 1500 it smooths out and seems to run okay, but under load it struggles.

Any help is appreciated,

 

Did you mess with the cam shaft sensor. Was the battery disconnected when you were pulling plugs. What codes are being thrown? I would disconnect the battery and go through the process again making sure every connector snaps in all the way. If it wasnt doing it before then it is something you missed putting it back together. post what you find

 

It's most likely the main cable connector from the DME box believe it is called the valvetronics wires that connector is a PIA to reconnect, happened to me when I disconnected those, that connector appeared to be seated but was not, had same symptoms as U described here.
Make sure you unlock it and re lock that lever locking the connector in place.

 

Turns out that I had not gotten a good seal with the intake onto the heads, creating a massive vacuum issue. With the intake on correctly, it worked fine.

 

there may be no problem at all actually. my dad has a 745i and had all the same symptoms you had. if you let your car sit for a few months with cheap gas this will happen (no 93 here for me, only 91 octane) since it goes stale. flushed the old gas out and filled it back up with fresh gas and all the check engine lights, emissions and misfires all went away. its worth an easy shot