BMW Repair: 97 740il misfire and loss of power, valve cover gasket, leaky valve

Question
Hello Tino,

I am sending more info as you have asked me. I am hopping you can help me more.

Picked up the car from the mechanic today.
As I started her up and revved the engine she was pretty rough. I let her warm up and I rev the engine again, it was running much better. Just quick update, I had all spark plugs changed and the Valve cover gasket both banks also changed, she was leaking oil into the spark plugs. Engine its still not running right. I had to take her to 5000 rpm in order to get the engine light to come back on. Codes I am getting are P0300, P0306 and P0308. After the engine light comes on the car runs really really  bad. Once I clear the codes engines runs much better but still not as it should.
Thank you so much
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The text above is a follow-up to ...

-----Question-----
Good day.
I bought this car with 87,000 and I have some problems.
I have an aftermarket Obd2 code reader and I have received random multiple misfire plus specific misfires to cyl 1 2 5 6 and 8. Also, one time only I got a code for Throttle control malfunction which I can not duplicate again. The car idles perfect, but when I rev it up, u can tell its running in 7 or even 6 cyl. Also, after I clear the codes, in order to get the engine light back on, I have to take the car to high rpm's, and that is when u can feel the loss of power and engine starts staling. I took the car to a local mechanic and he said we should start with the basics since even his professional code reader was no help. So, we changed the spark plugs. Than, we realized we have a leaky valve cover, which was leaking oil in the spark plug and he was sure that was the problem. --NOT --- We even swapped coils between them. Misfire at same cyl, 6 and 8. My mechanic thinks it’s the computer that is bad. Some other mechanic thinks it’s the catalytic converter. I am lost, please help. I can not afford changing stuff with no result,
Regards
Chip Neiculescu
(623) 866 2900

Phoenix

-----Answer-----
Hi Chip!
Please tell me exactly what codes you got previously.
And can't you get any fault codes to set now? Then remove the sparkplugs at cylinders 6, 7 & 8. see if there is oil in the plugs. If there is much oil you will have misfires probably due to bad piston rings or usually due to a leaking crankcase vent valve(more on this later). And if there is no spark at 6 and 8, you will also have a misfire, so check for a healthy spark at the plugs (if there is no spark, then the engine computer/DME is not sending the signal for the coil to fire or the wire is bad).
Other things that you or your mechanic can do is to look for vacuum leaks at the vicinity of the crankcase vent valve at the rear of the engine. Also check the throttle valve gaskets for vacuum leaks. Then open the engine oil filler cap very slowly and see if there is a healthy amount of vacuum/pressure. My guess is that the crankcase vent valve is bad but this still has to be confirmed before replacement. It can still be due to other defects, that is why you have to eliminate/minimize the possibilities.
Send me more info as you get it.
Tino


Answer
Hi Chip,
Nothing has changed in what you are telling me now and what you already told me before (except that the faults have come back).
Fault 300 is for multiple misfires, 306 for cylinder 6 misfire, and 308 for cylinder 8 misfire.
So my answer to this is still the same as before. Check for sparks in plugs 6 & 8. Check for oil inside the plugs of 6, 7 & 8. And check the other things that I have already told you. Otherwise, I can not help you any further.
Tino