For a given fuel injector opening duration, fuel flow through an injector is proportional to differential pressure across the fuel injector. Differential pressure is the fuel rail pressure minus manifold pressure. The inlet of the injector is exposed to fuel rail pressure and outlet of the injector is exposed to manifold vacuum, about 15 inches of Hg or -7.3 psi at idle. At WOT manifold vacuum is nearly zero. The fuel pressure regulator varies fuel rail pressure to maintain a constant 36.3 psi (2.5 bar) difference between fuel rail pressure and manifold pressure.
At idle, fuel rail pressure is reduced via pressure regulator to 29 psi because there is -7.3 psi in the intake manifold, 29-(-7.3) = 36.3 psi differential.
At WOT, fuel rail pressure is 36.3 psi because there is 0 pressure in the intake manifold, 36.3-0 =36.3 psi differential.
This is done so that any injector opening duration gives the same amount of fuel at idle as it gives at WOT or anywhere in between.
Disconnecting the vacuum line raises the rail pressure which increases the differential pressure across the injector and richens the mixture. Sounds like you've got a vacuum leak, a CO adjustment problem or a faulty pressure regulator.
Paul Shovestul
Originally Posted by
phildg2000
Hi all!
Can anyone tell me what the vacuum line on my FPR is for (1989 525i euro M20)? What does it do?
Reason I ask is because my car has developed a hunting idling when cold. I noticed that if I disconnect the vacuum hose from the FPR and plug the open end of the vacuum line with my thumb, the hunting goes away. I want to understand why.
Thanks,
Philip
.....Got to keep the loonies on the paath.
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