No Pressure at Fuel Rail-Fuel Pump Works (or does it)

[Explorerlyon]

I have zero pressure at the fuel rail. Zero pressure when the key turned on. Zero pressure when the vehicle started. The vehicle does start and run rough, but as soon as you press on the gas pedal the vehicle dies.

I removed the hose at the fuel pump under the rear seat. Attached a hose section to a bottle. When I turned the key on, zero gas. When I try to start the car, gas flows. My understanding is the pump is supposed to turn on when the key is turned to the on position to get fuel into the rail prior to starting.

What do I diagnose next since the pump is at least pumping fuel when the vehicle is started? Just not pressure at the fuel rail.

Edit: No codes, checked with INPA and ISTA

[80stech]

The pump only runs for a few seconds when the key is first turned on. You need to check your fuel pressure gauge and connection to the rail.

[Explorerlyon]

Originally Posted by 80stech

The pump only runs for a few seconds when the key is first turned on. You need to check your fuel pressure gauge and connection to the rail.

When I turn the key, no fuel pumps out. I only get fuel out of the pump with the engine started. I disconnected the vehicle fuel line from the top of the pump and connected a test line to determine when fuel flows.

I have tried two different pressure gauges at the fuel rail, both show zero. With the engine running, I pressed in the Schrader valve, fuel flowed out with very minimal pressure. Did the same with the key on and a dribble comes out.

[80stech]

A lot doesn't make sense with this and I get a feeling there is more to the story. Can you get fuel pressure (not just flow at zero pressure) at the pump? What happened leading into this ? Maybe make sure the pump is getting a full 12volts while you have access to it.

[Explorerlyon]

Originally Posted by 80stech

A lot doesn't make sense with this and I get a feeling there is more to the story. Can you get fuel pressure (not just flow at zero pressure) at the pump? What happened leading into this ? Maybe make sure the pump is getting a full 12volts while you have access to it.

Thanks for the help and guidance.

The vehicle ran perfectly. 5 minutes into a drive the other morning it lost all power, sputtered and would not drive. The engine still starts right up but idle is rough. Applying gas pedal bogs down the engine and it dies.

With my test hose connected to the pump, I get good flow. I have not figured out a way to measure psi on my test hose. I will see if I can rig something up. You may be on to something....I have flow but not pressure causing me to "think" pump is good.

I was going to pull the fuel filter this evening and see if I can determine flow into the filter and out of the filter. I don't know if a clogged filter could reduce pressure down to zero though.

[80stech]

By checking the flow and the pressure at the pump you are eliminating the filter so I wouldn't bother with that yet. The filter is not a small job and unlikely to plug instantly like you are describing.

Again, check the power to the fuel pump.

[Henn28]

Can you hear the pump running, or trying to run when you pull the seat and turn the key on? They are actually pretty quiet, so you may need to put your ear close to it while someone turns the key to the second position. I’m betting it’s not running, or is intermittent to the point of almost no pressure. In which case I’m with 80stech: is it getting steady power, or is it simply bad?

My money is on a bad pump. How many miles are on it? There is a good thread on the board about pump lifespan (approx 4k to 6k hours), and estimating hours on your pump. I just installed my third pump in my 2002 e53 at 155k because I have a new filter in the car but the fuel rail pressure was dropping very quickly when I turned the key off. I looked at my records and the second one went in at about 80k. Low miles maybe, but the car was on the road 3+ hours per day in socal for 5 years so lots of hours on the pump.

I can throw parts at a my cars with the best of ‘em, and fuel pumps aren’t crazy expensive, considering how long they generally last.

[andrewwynn]

My scanner will command the pump on. Without that, the DIY solution is to jumper the relay or just apply 12v directly to the pump.

You should have ≈60 psi at the pump so the FPR has enough overhead to work with.

I think the e53 with the 4.4 engine don't run the pump until you hit start unlike the 3.0 like I had that will run 6 seconds to prime at key on. (based on the similar vintage 540 I've worked on).

Hook up a fuel pressure gauge to the rail and prime the pump then turn off the key.

the pressure should drop quickly to ≈40 but very very slowly drop. It should have ≈10 psi overnight. If it drops quickly the o-ring in the FPR has failed. You can disassemble and replace the o-ring for 49¢ or replace the whole filter/FPR for about $65.

If pressure doesn't reach 50 but stays after power off it's the fuel pump.

[Explorerlyon]

I put in a "T" near the pump to measure the pressure. I get 5psi with key on and 18psi with vehicle running. I am also getting 18psi at the fuel rail.

I had 12.4v at pump for a "blip" with key on. Then drops to 11.8 during starting then about 13+ with engine running.

I think I have to conclude bad pump. I have flow but not pressure at the pump.

[80stech]