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30 sec. rough idle on cold start


I have 1998 bmw 528i, striaght 6 cyclinder, with 217k on the speedo.

The problem. after the car has sat over night or for more than 4 hrs, on start up, i will have a 30 sec. rough idle.

So i will start the car, the rpm’s shoot up to 12oo, and then slowly back down to 7-800 rpms, and then studders for 30 secs., like it’s not getting enough fuel.

what i have done so far to fix it.

replace these items.

precat 02 sensors, just about all vaccum lines, secondary air pump and the contorl valve for it, spark plugs, and coil pack boots, maf sensor,and just last weekend a complete intake manifold gasket rebuild, which included, both intake gaskets, this car has two throttle bodies, so both gaskets got replaced, and the air temp sensor gasket.

now since i did the intake gasket rebuild the cars power is much more responsive, but the cold start seems to be more voilent, still 30 secs. but now it seem that the car shakes a little harder.

the reason why i did the intake rebuild was to try to fix the shakes, but i also keept getting a po170 code. which in turn was killing my bank 1 precat sensor. two have died. and all the research i did was leading to a intake manifold leak.

So i’m left to belive that’s a fuel problem maybe. i have replaced the fuel filter a 1yr ago. so now i’m thinking maybe a fuel presure regulator, or possible coil packs, they both are orginal. like i said, on start up, it’s smooth up to 1200, and all the back down until it gets to 700 rpm’s. after that the car runs strong, and will not do it agian unless it’s sits 4 hrs or longer.

thanks for any leads. i’m also a member on a bmw forum, and if i fix this i would be a bmw god, maybe. no one seems to be able to figure this out.