"What does the vacuum connection to the FPR affect, higher engine loads?"

The vacuum connection affects the FPR at all engine loads.

At idle, the vacuum is at it's highest, and the FPR regulates the LH 2.0/2.2 pressure to about 26.2 psi. At maximum load (Wide Open Throttle), the FPR "feels" atmospheric pressure and raises fuel pressure by about 9 psi to 36.2 psi. The higher LH 2.4 pressure ranges between 34.5 and 43.5 psi.

Running with the vacuum hose detached as you were, would result in higher fuel pressure. And the open vac hose would be seen by the O2 sensor as a significant vacuum leak, and reported to the ECU. So the net result be rich running due 1) the higher fuel pressure, and 2) ECU response to the "lean mixture" seen by the O2 sensor.

See Bentley page 241-9 for LH fuel pressure testing and specifications.


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Bruce Young, '93 940-NA (current), 240s (one V8), 140s, 122s, since '63.