fuel delivery issues

[kavitzy]

Fuel pressure drops off as soon as you turn the key, but does this intermittently, about 50%. The drop off is sudden, it goes from 6 to 0 in less than a second. I've replaced the pump along with the regulator but the problem still persists. I'm ready to buy all 8 injectors but I don't see how a stuck open injector could cause that fast of a drop. My guess its a check valve but I thought the pump or the regulator have one but those are brand new.
Car has 150k miles

[Audibot]

Have you replaced the fuel filter? I know I used to see pretty big dips while driving, though not what you're seeing. Just another cog in the system though, and one that seems to be overlooked a lot.

[kavitzy]

Yup, I've replaced the filter. I should mention that to start the car I have to abuse the starter and crank for a good 3 seconds to pressurize the lines. I'm wondering if I got a bad fuel filter from factory.

[ven0m]

Does the S4 have the same N80 fuel regulator valve as the RS4?
But honestly this issue sounds like an electrical gremlin and not mechanical (was troubleshooting a similar issue on a friends A5 3.0). Check all power connectors (wiggle them a bit) and check the fuel pump relay, it looks like a faulty coil in the relay itself that "lets loose" every now and then.

Troubleshooting suggestion: Try to replicate the problem and have somebody at the back with a Volt-meter (multi-meter) and measure if the +12V to the pump falls to 0V and comes back. That will confirm if the issue is electrically related.

[my_RS4]

Replace fuel pump relays and check airbox flap... Might be stuck.



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[Grigor@Bestiale]

Indeed it’s very unlikely that injectors are the problem, all of them have to be faulty at the same time intermittently for this to happen. Have you scanned the ECU/what are the codes?


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[s4ski]

FWIW, I've been having continuous intermittent starting issues. Starts the first time every time in the morning. Then thereafter, about 75% of the time it cranks for about 3-6 seconds before it sputters to life. Idles a bit rough at first. My fuel pump was recently replaced, but we think the pump is faulty and am waiting for a replacement.