rapidly surges 1k-2k rpm, replaced multiple parts

[derek_4491]

hi im a noob to bmw and need help. i have a 93 325is that has no codes (performed stomp test) and obviosly no check engine light on. on idle it surges from 1k to 2k rpm rapidly and does not stop even when warm. i have replaced the: iac valve, checked and replace the iac hoses,fuel pressure regulator hose, intake gasket,throttle body gasket, injector o-rings, maf sensor, and inspected all intake ducts/tubing for cracks. and sprayed all around intake with carb clean to see if the rpms changed.. and no luck with any. any help would be greatly appreciated. or have u ever heard of this even???

[derek_4491]

i have now replaced the cam sensor and o2s. still no fix. would a faulty fuel pressure regulator do this? Its just blowing my mind. please help

[witeshark]

Very mysterious, checking the fuel pressure regulator may well be worth a try.

[derek_4491]

ok checked fuel pressure with key on engine off: 50psi which chiltons says spec is 49-51. so ok there.. with engine on its at 45psi at 1k when surges and 41psi at 2k rpm. i also checked vacuum its at 14in.hg at 1k rpm and 18in.hg at 2k rpm dont know spec on this but i figured if it was a vacuum leak when the engine would surge to 2k the vacuum would decrease? and if the fuel pressure increases when the vacuum decreases this would mean the regulator is working properly correct? also disconnected the battery multiple times to reset ecm adapts it would idle at a steady 1300 but would not surge for about 20sec. then the surging would come back. im really trying hard to find correct diagnosis just need pointed in the right direction. please help me i need to have a car to drive any suggestion would help. thanks

[derek_4491]

ok replaced ecm/dme with my other 325i. module number was the same. and no change. any ideas i dont care if its even a guess bc im running out of options right now, this bmw is blowing my mind. and is there any other vacuum lines other than: fp regulator, icv, brake booster, crankcase vent, and throttle body to canister. sorry the vacuum diagram is missing on this car

[witeshark]

It really seems like a vacuum leak, there must be a something overlooked like a loose clamp.

[derek_4491]

ok i looked over the vacuum lines again all look good. and checked vacuum at idle with my other 93' 325 and it was at 14in.hg the same as the problem car.?:

[derek_4491]

ok just trying everything now might have stumbled on something.. i induced a vacuum leak and the idle dropped down and almost died i put my finger over the intake to stop the leak. then i pulled back slightly to regulate the leak with the leak about 1/4 covered it idled at a steady 800rpm(a little miss but idled fine).. what the hell does this mean. Possible high resistance in maf circuit, without a code??

[witeshark]

I don't know enough about vacuum leaks to really help more, but it seems you're getting closer. Testing the MAF might be my next move.

[derek_4491]

ok so instead of pulling the vacuum line off of the brake booster i went ahead and remove the line BEHIND the maf idle stabilized but at 1300rpm. i adjusted the idle set/stop screw to lower idle speed to 700. installed a vacuum filter into intake duct and ziptied the hose(crankvent) to substructure. idles perfect, revs beautifully and idle doesnt surge. and launches great. this totally blows my mind how less vacuum makes it run to spec, it goes beyond everything i kno about cars but it worked for me. o and i checked all circuits from maf to ecm/dme .3-.4 ohms which is great continuity.? problem fixed but looks kinda ghetto on a bmw lol

[witeshark]

Nice going, thanks for sharing the answer!

[caodan2010]

are u using after market stuff or OEM?