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Fuel pressure Regulator

 Nathan Harris  writes,

>I have a nagging question,
>Shouldn't the engines idle change if I remove the vacuum hose to the
FPR(whilst
>plugging the inlet to the manifold) and I suck/blow on the hose to the FPR?
My
>understanding is that it should, however mine doesn't change. Should I
replace
>this part?

No, do not replace this part based on this test. The idle speed should not
change if your car is working properly. Disconnecting the vacuum line to the
fuel pressure regulator will raise the fuel rail pressure and richen the
mixture to some degree. If your O2 sensor is connected and working, the
Motronic ECU will sense this rich condition and correct (by reducing fuel
injector opening time) to the limit of it's ability. The idle speed will
remain constant because it is regulated independently by the idle control
valve and the idle control unit. Applying a vacuum  to the regulator will
reduce the rail pressure and lean the mixture. You won't notice this for the
same reasons. 

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1986 325
1983 533i
1980 528i
1979 320i ( gone but not forgotten )