What should rail pressure be at startup

[335i_nick]

I am trying to diag a no/low rail pressure situation, cant get this car to start

When cranking I see like 80-85 psi on the LPFP, but less than 100psi at the rail. I crack open a fuel line and there's nothing coming out. I am thinking its the HPFP.

Wondering what should rail pressure show upon start up?

 

[Tiron]

At idle I believe its around 800psi, it might take 1/2sec of cranking to actually reach that though. I've never checked it during cranking specifically. If you're cranking for very long and only seeing 100psi you've got something going on the the HPFP.

 

[335i_nick]

At idle I believe its around 800psi, it might take 1/2sec of cranking to actually reach that though. I've never checked it during cranking specifically. If you're cranking for very long and only seeing 100psi you've got something going on the the HPFP.

That's what I am thinking but HPFP was completely fine before I swapped a new engine in this car, thats why it's so "weird" to me. Also trying to rule out the regulator.

 

[doublespaces]

Has the car ran since the swap? Keep in mind the hpfp can fail due to electronic reasons and not just mechanical.

I had one hpfp that was 'new' and it worked for less than one mile. I swapped out multiple hpfps and ruled out any chance it was install error etc.

I don't recall the fuel pressure when cranking but it should be around what you're seeing. The car can limp at that fuel pressure so it should be able to start.

Check the cable going to the hpfp and make sure it has a pwm signal going to it.

 

[Asbjorn]

FWIW here's a cold start log from my car

Hide legend Reset zoom

datazap.me | Asbjoern | cold start

datazap.me

And here's a log of how the rail pressure drops to 800ish psi after half a minute or so

datazap.me | Asbjoern | cold start

datazap.me

No lpfp pressure sensor on Z4s unfortunately.