E55 Rough Idle and no power.
 

I have a new to me 2000 E55. I took it out for a drive today and while getting on an on ramp I pressed the accelerator and all of the sudden the car would not go beyond 3000 RPM and there was a drop in power. I took the next exit off the highway and headed back home. When I came to a stop I could really feel the engine idling roughly. So bad that I put the transmission in neutral at stops because I was worried about a stall. I wasn't far from home and in city traffic much less power. I went up a steep hill and could not get past 3000 RPM and 30 MPH with the throttle to the floor. No CEL came on during the whole time. I got it back home, turned it off, checked fluid levels (fine). I started it and the car seems to be running normally again. I have done some reading on the blogs and I'm thinking that the Mass Air Flow sensor could be going bad. Another thing I saw is the Throttle Position Sensor could cause this. I know that the oil breather covers are leaking and I'm sure the breather hoses need to be replaced so I'm not sure if that could cause this problem. I plan on doing the breather covers and valve covers in a few weeks.

Any insight on what that issue could have been?

Put it on a STAR diagnostic system and you will have your answer in under five minutes (plus a list of everything that has gone wrong with the car since the last time codes were cleared). A standard OBD2 code reader connected under the dash will give you any codes stored in the ME (Motor Electronics) module; you must, however, use STAR and the 38-pin connector under the hood to access all modules, i.e. TCU, SRS, ESP, Audio, etc.
If you do not do the above, you are just throwing money and parts at the car.

check the CPS

Connected the OBD2 to the car and hit the fault code jackpot! These were stored codes but the check engine light never came on.

P0304 cylinder 4 misfire
P0303 cylinder 3 misfire
P0301 cylinder 1 misfire
P1542 Accelerator Position Sensor Signal
P0302 cylinder 2 misfire
P0422 Main catalyst efficiency below threshold Bank 1
P0432 Main catalyst efficiency below threshold Bank 2
P0170 Fuel trim malfunction Bank 1
P0300 cylinder misfire random

And one pending code...
P0153 O2 sensor circuit slow response bank 2 sensor 1

Given all the codes, it would seem the Star diagnostic is good advice to try to chase down exactly what's causing this rather than try to replace more parts than needed.