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Aftermarket Cams and Vacuum Pressures

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    Aftermarket Cams and Vacuum Pressures

    Alright here goes. I recently built a 2.7i. Regular eta bottom end (freshly rebuilt), Bimmerheads 885 head w/ 274 regrind cam, M50 injectors, headers, Sssquid tune. Engine has about 350 miles on it since the build.

    I’m chasing a slight mixture issue at low RPM I think. Car runs great but if left to idle for longer than a minute it will pop the CEL on (1221 code), but will go away if the throttle is blipped. Also, at idle if I quickly stab the throttle, it stumbles over itself a bit before revving up. Exhaust will also pop on upshifts occasionally, usually when shifting in the 2-3k range. All this said, the car pulls hard all the way to 7k.

    Sounds like a vacuum leak, right? Hooked a vacuum gauge up the FPR line and it read 10-11 psi at idle. Boom, smoking gun. Took it to a shop and had it smoke tested. No leaks. (Might be relevant that at this time the car had a completely dead O2 sensor, CEL on all the time, and was running pretty bad).

    Other noteworthy things. New Bosch O2 and blue temp sensor. Compression dead even at 150 psi across (I think my gauge reads a little low though). Valves perfectly within spec.

    So... can anyone running a larger cam (especially a BH 274) comment on their idle vacuum pressures?

    And... are there any potential vacuum leak areas that a smoke test doesn’t reveal?




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    Build Thread

    #2
    It sounds like your engine is running fine. Chip tunes aren't ever perfect and a bigger cam will generate less vacuum at idle. The M30 does that on the stock cam; it's why BMW went with hydroboost in the e28.

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