88 735il rough idle;some hesitation

I have a rough idle, and some hesitation on accleration. Im new to a BMW, so i cant quite nail it down. Did a complete tune-up, the works, but no difference. I was reading up on my model, but they dont have
many specs for it. I go to the auto parts store and they can only pull up the 3.4L, not the 3.5L that my car has. Could this be the problem? I got pregapped plugs, but at the time I didnt know there was the two different engine sizes. Or could it be something else? Where can i get the right parts for it?
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Although it was labeled 3.5L, it's only 3430cc, which actually makes it a 3.4L after rounding.
Almost always, on the M30 engine, a rough idle means that you have a vacuum leak somewhere in the system. Make sure that the oil level stick is in all the way. Very carefully check all *all* *ALL* the rubber on the intake side for cracks. You often have to use a smoke source (candle) to find leaks. Check all hoses and lines to make sure they're connected.
FloydR
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wrote

And the oil filler cap is screwed up tight - I had this once!

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wrote

I have the same car with 145k miles. A few months ago, it starts fine but runs rough for a minute then goes okay. It occurs about once a week. It is due for service but I don't know what it is. Here in Florida we stock up on gas for the generator for hurricanes. Each summer, we take the year old gas and toss it in the lawn mower and in the 4 cars we have. It's been okay. The camper had trouble starting after putting too much in, so we stopped doing it. I had put a gallon or two in the BMW but the problem was happening before that. So check for old gas (I know, not likely). I don't know anything about cars so the others can help. Good luck.
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Water in the gas or just evaporation of the higher fractions if your storage tank is not a hermetic seal?
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Bad mass air....

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