fuel pressure question
 

hello to all
i have couple of question
1=how do you determine a softfail fuel pump
2=is the fuel pressure suppose to drop a tad when vacuum is applied under load
my fuel pressure is 42 with key on and 50 to 52 running when checking fuel pressure regulator if i applied vacuum it drops down to 42

the reason i'm asking because it seems i'm running out of fuel when down to 1/4 tank

Fuel pressure rises when there is little or no vacuum to the fuel pressure regulator. This is because at Wide Open Throttle (WOT) there is very little manifold vacuum but the engine needs more gas since it is at WOT.

Search for bad transfer pump. Sounds like you have a bad transfer pump. Either fix it or keep the fuel above 1/4 tank.

Isn't the fuel pressure supposed to stay 50~52 PSI while running, regardless of the load applied?

To me, dropping to 42 PSI means a faulty/softfail fuel pump/transfer pump.

Good Luck!

4 things to check....

1) Rate of rise. The fuel rail should reach peak pressure within 2 or 3 seconds after key is turned to "run" position.

2) Not running, the pressure should read 51psi (+/- 3 psi)

3) Running (idle), the pressure should read 36 to 46psi.

4) Remove vacuum line from regulator (simulates open throttle), running pressure should increase 6 to 10psi.

If these 4 items check out, pump and regulator are functioning properly.

meaning to test regulator i applied direct vaccum to see if it is working then i put it back were it was

Applying direct vacuum like you did will indicate a high vacuum to the regulator which signal to the regulator that the throttle is closed so there will be less demand for fuel so the pressure will drop back down to idle pressure.

If we are talking about the 3.0l in the E53, then everyone needs to be VERY careful about what they are stating here.

I do not know the E53 platform like I know others, but if this is the same M54 engine configuration that is in the E46, the fuel pressure IS NOT vacuum controlled! BTW, if you attempt to "Test" the FPR by applying vacuum, you WILL see the pressure change, BUT the M54 DOES NOT USE manifold vacuum to change the fuel pressure. The M54 should have a static or fixed fuel pressure. Injector pulse width is changed in the M54 for more precise fuel delivery.

LeiZ seems to understand the M54 fuel system, however, others are comparing the M54 to earlier engines that did have variable/vacuum controlled fuel pressure. The M54 does not use this system. Just note where the fuel pressure regulator vent hose is connected, I believe it is connected to a 90 degree fitting, #10 in the following diagram, which is in the lower intake boot, AHEAD or IN FRONT of the throttle plate.

RealOEM.com - Online BMW Parts Catalog

So be very careful what information you are handing out. The M52 did have a vacuum controlled fuel pressure system, but the M54 had a fixed fuel pressure system. There is a "vacuum" hose attached to the fuel pressure regulator, HOWEVER, this hose does not have vacuum applied, it is forward of the throttle valve, so there is NO intake manifold vacuum present at this point. This hose is nothing more than a "filtered" atmospheric reference.

You CLEARLY have a soft failed fuel pump if you are running out of fuel at 1/4 tank. Use the Hidden OBC Menu to monitor the fuel level in each side of the fuel tank. You should NEVER have more fuel in the drivers/left side of the tank then the passenger/right side of the tank.

The passive "sucking jet pump" has no moving parts and rarely fails, if there is a problem it is usually due to someone messing around inside of the tank.

I would not even attempt to check fuel pressure, its foolish. Your vehicle is over 15 years old with the original fuel pump, IT NEEDS to be replaced. Why wait for the pump to totally fail and leave you stranded on the side of the road somewhere or even in your driveway when you have to be somewhere. If you change the pump on YOUR schedule, YOU can control the cost. If you wait and have to change the pump on the VEHICLES schedule, then you have very little control of the cost if you have to have the vehicle towed and/or have to overnight or pay list price for a pump.

The pumps become soft due the brushes and commutator wearing and the pump speed and power drops off because often one or more windings on the armature are not being used. Once the pump stops on the bad winding or is subject to very cold weather, you will have a no start situation.

Look up the pump part number of BMW Parts Catalog then plug the part number into the BMWPNPC tool (BMW Part Number Price Comparison for Windows & Mac) to find the best price. At least for the E46 Siemens/VDO fuel pump BMA Auto Parts has the best price using the discount code SPECE30 and they offer quick and free shipping on $75+ orders as I recall. They may not have the best price on the E53 pump, but check out the bmwpnpc tool.

If the fuel pressure is not vacuum controlled then why is the fuel pressure regulator attach to the intake boot with a vacuum line? Incoming air through the intake boot does create a vacuum (pressure differential) when it passes over the small opening of the vacuum line connector. This is Bernouili principle of fluid dynamic.

The key here is the fuel pressure regulator is attached to the "intake boot". There is no manifold vacuum in front of the throttle valve. This is NOTHING more than a filtered atmospheric reference.

The same FPR behavior can and will occur if you remove the hose at the FPR, BUT if the open end of the FPR gets clogged by dirt, bugs nesting or water intrusion and possible freezing, then the atmospheric reference will be lost. Just ask Mazda about the spiders building nests in the gas tank vent lines and causing problems that led to a recall!

So BMW has connected the FPR to the intake air path between the air filter and throttle valve where there is NO manifold vacuum but there is little to no way for water, dirt or bugs to plug the FPR atmospheric reference.