OK Seriously
FTE Legend
Joined: Jun 2006
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From: Ottawa, Ontario
You need to measure the fuel pressure and check for a fuel leak from the vacuum port on top of the fuel pressure regulator. These things usually fail in 1 of 2 ways, the begin to leak and the extra fuel is sucked into the motor through the vacuum lines, or they sieze up and hold the fuel pressure artifically high. In both cases this hurts fuel milage.
Lead Driver
Joined: Nov 2002
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From: Concord, NC
The FPR controls the fuel pressure (fuel flow basically) at the injectors. The engine has a vacuum supply line that when full vacuum is applied (~19 ) the fuel regulator is opened to allow excess fuel to flow back to the fuel tank(s). As the engine RPM increases, the Vacuum supply decreases closing the FPR and therefore supplying more fuel to the engine.
FTE Legend
Joined: Jun 2006
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From: Ottawa, Ontario
The FPR is designed to deliver a slightly higher fuel presure when vacuum decreases, this has the effect of an accelerator pump on a carb and delivers a little more fuel while accelerating. A correctly operating FPR will deliver 36.5psi at idle and close to 40psi(on the small V8's) any time the throttle is opened suddenly causing a vacuum drop. Fuel pressure quickly returns to the lower preset level after a throttle change because the vacuum does too.
Lead Driver
Joined: Nov 2002
Posts: 9,555
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From: Concord, NC
I didn't say anything wrong... The fuel pressure regulator does not act as an accelerator pump, that's taken care of by an accel enrich function in the engine computer. What you said otherwise is exactly what I said previously.
Most injectors are flow rated at 3 bar, ~43 psi. Flow rate increases with more pressure and decreases with less. The regulator is designed to maintain this split, which will be the same as the static fuel pressure (fuel pump on, engine off). If manifold pressure drops from 14 atmospheric/WOT to maybe 6 at idle, the fuel rail pressure would ideally drop from 43 to 35 to keep the same 29 psi split across the injector. These numbers are just for example. The fuel pressure regulator is used to keep fuel flow from being influenced by throttle position/manifold vacuum so that the computer can accurately meter fuel flow to the engine. Without the regulator, a 10ms pulsewidth would flow a different amount of fuel at closed throttle as it would at wide open and from an engine calibration standpoint, that's a nightmare.
Most injectors are flow rated at 3 bar, ~43 psi. Flow rate increases with more pressure and decreases with less. The regulator is designed to maintain this split, which will be the same as the static fuel pressure (fuel pump on, engine off). If manifold pressure drops from 14 atmospheric/WOT to maybe 6 at idle, the fuel rail pressure would ideally drop from 43 to 35 to keep the same 29 psi split across the injector. These numbers are just for example. The fuel pressure regulator is used to keep fuel flow from being influenced by throttle position/manifold vacuum so that the computer can accurately meter fuel flow to the engine. Without the regulator, a 10ms pulsewidth would flow a different amount of fuel at closed throttle as it would at wide open and from an engine calibration standpoint, that's a nightmare.
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From: Renton, WA
A little correction needed here for the benefit of all. The FPR is designed to deliver a slightly higher fuel presure when vacuum decreases, this has the effect of an accelerator pump on a carb and delivers a little more fuel while accelerating. A correctly operating FPR will deliver 36.5psi at idle and close to 40psi(on the small V8's) any time the throttle is opened suddenly causing a vacuum drop. Fuel pressure quickly returns to the lower preset level after a throttle change because the vacuum does too.
The FPR is designed to deliver a slightly higher fuel presure when vacuum decreases, this has the effect of an accelerator pump on a carb and delivers a little more fuel while accelerating. A correctly operating FPR will deliver 36.5psi at idle and close to 40psi(on the small V8's) any time the throttle is opened suddenly causing a vacuum drop. Fuel pressure quickly returns to the lower preset level after a throttle change because the vacuum does too.The FPR does NOT act as an accelerator pump, this is rediculus and very wrong. Your fuel pressure numbers are a little silly to, there is no standard fuel pressure for idle, nor is there some preset level or any differance for a givin engine size.
The job of the FPR is to maintain the same relative pressure between the fuel rail and the manifold across the injector. For every inch of increase in vacuum or put another way decrease in relative or absolute pressure, the FPR decreases the fuel pressure by the same amount. This is why the FPR is vacuum modulated NOT throttle position modulated like a accelerator pump.
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