Fuel pressure when engine is off

[tan_rich]

Hi all, first off I’m not sure if this is an issue but it’s something I noticed with the car and wanting to learn how the fuel system properly works.

After installing a bucket less stage 2 lpfp i’ve notice I have been getting sometime rough cranks/starts.

When I turn off the car after a few minutes my fuel pressure also dropped below 72 psi. To line 60 almost and to 50-30 sometimes. Sometimes when I turn on the car but not the engine after a few hours it’ll be 50-30 psi and a somewhat rough start. Sputter almost.

On WOT, my fuel pressure stays between 65-85 ish give or take. But isn’t steady, it’s jump back and forth a lot.

I don’t think the issue would be on the fuel pump itself since the fuel pressure regulator should be holding the pressure on the low side. Correct me if I’m wrong since I’m thinking the fpr is the problem.

Questions:
What should my fuel pressure be after 10-15 minutes after the car is off?
What should my fuel pressure be after a Few hours?
Should my cruising fuel pressure be irrational light that?
Should my WOT fuel pressure be fairly stable or being a jumping a lot normal?
How is the fuel pressure regulator controlled in our car?
Is it normal to see fuel pressure go above 100 psi sometimes from cruising or something. I assume it’s just happens when you give it a lot of load and the fuel pump reacts accordingly but the FPR should be good enough to bleed it out.


2013 335is DCT 90k miles, FBO for like 60k miles.
Mods:
Downpipe
Intercooler
Intake
Chargepipe w/ forge diverters
MHD flex fuel
VS stage 2 bucketless
Running e30-35 for about 9 months
Past 2 weeks have been e50
Thanks all

 

[Twisted Tuning]

Fuel pressure is going to bleed off slowly with the car off. So what you are experiencing in that effect is normal. not sure how long it takes to bleed completely off, but a few hours later still having 30-50psi still in the lines seems pretty damn good to me.

honestly not sure on the 100psi, could be the LPFP sensor being aged, or like you said just the EKP reacting to high load momentarily. I'd be more concerned if it was staying steady at that pressure for long periods of time.

 

[STE92]

The book say it should hold pressure 72 +/- .
I'm having similar issues. I think the pressure regulator/check valve is bad. Mine when cold sits overnight and it the am will crank and crank no start. Second time u try starts right up. Car has new inj, hpfp, lpfp.

 

[tan_rich]

The book say it should hold pressure 72 +/- .
I'm having similar issues. I think the pressure regulator/check valve is bad. Mine when cold sits overnight and it the am will crank and crank no start. Second time u try starts right up. Car has new inj, hpfp, lpfp.

I read about that as well. I do recall checking before that my fuel pressure held around 72 but that was about 30 mins after I turned the engine off. But now it dropped within 5 minutes.

I’ll keep and eye on it if my wot and cruising pressures look off. 150$ for a new regulator is peace of mind if that is the actual issue.

 

[rac]

Stock regulators hold pressure pretty good for a while. In any case the lpfp primes before you crank so you should be back to ~72psi when your ready to crank.
The pressure is being read in the engine bay and the regulator is at the tank, so at high flow rates and tranisent conditions there will also be a difference between what the sensor sees and the pressure at the regulator.

If the pressure regulator is partially blocked / not functioning properly than it will exacerbate pressure control obviously. Your observation after installing new pump with higher capacity is because the pump control system (base tables within dme) aren't matched to the higher output pump so your getting more fuel than you need and the regulator cant keep up. If the dme does closed loop feedback (I don't know but I assume it does) it probably overcompensates causing the oscillations. This should not affect start up - log start up and confirm you have good pressure while cranking.

Regarding rough starts, if you went from E30 to E50 at the same time as the pump swap then that is the most likely explanation for your rough starts (the ethanol content). Go back to E30 and see if it starts like before. If so, you need look at enriching your cold start fuel amounts for E50.

 

[tisdrew]

When people upgrade lpfp it's usually to run e85 mixes. Your only real symptom posted is long cranks when cold. Starting with e85 in a cold environment is a universal problem--not just a n54 one. I live in florida so haven't really had that issue after I changed my hpfp. Another common reason for long cranks is a dying hpfp but it would do it on any fuel mix, 93/e30/e85.