[mictang]

Hi everyone,

So my E90 N52K logged a few lean codes that has led me to test the fuel pressure, which showed some pretty weird results.

On a completely cold car, fuel pressure primed to ~3.5 Bar after unlocking the car, and when key was inserted it went up to just under 4.6 Bar.

Left it there for maybe 10 minutes and the drop was less than 0.1 Bar.

Then I started the car, idle pressure dropped but holds at 3.6 Bar, but when I turned it off, the pressure was actually at 4.3 Bar after 5 minutes.

I ran the car again just in case I accidentally touched the key or something, and this time, I watched the fuel pressure slowly creeping up after the engine was off.

If there was only a low fuel pressure, I would have simply replaced the fuel pump, as I have replaced the fuel pressure regulator/filter a few years back due to a bent level sensor (used OE VDO part), the pump itself looked like the original that came with the car.

The car does not have any noticeable starting or power issues, except for a few times if I ran the tank down to the second fuel reserve warning and parked it, the car usually has a long crank or on fewer occasions wouldn't start the next day.

In the latter case, I added gas on two occasions, and the car started right up. Another time I simply used INPA to run the fuel pump for 30 seconds which also allowed the car to start again.

Any ideas? The pressure increase is what really confuses me.

[lookalikehuuh]

i would consider this to be normal behavior as fuel pressure will increase slightly due to injectors no longer firing. Also keep in mind that the car will pre charge the fuel pump as soon as you open the door so I would expect to see higher pressures as the fuel pump primes but no injectors fire to release the pressure.

What I do not consider normal behavior is running your car down to the point where you are needing to prime the pump again with INPA. Its extremely bad for the pump since it is sort of relies on the fuel in the tank to help cool it.

[mictang]

Well, I replaced the pump, and idle pressure is now at 4.8 Bar, or just shy of 70 psi. If I give it a little gas to ~1000 rpm, it reaches the spec of 5 Bar.

The car ran fine on 3.5 Bar before, so there was no noticeable change in drivability/power. Cold start is definitely quicker now.

I guess there is just some general wear in the fuel system due to age. Or is further diagnosis necessary? 0.2 Bar is physically very little but still exceeds the factory tolerance requirement.



As for the original issue, turns out my old pump has a broken guide rod. The "floating" pump is semi-separated from the cap, which can cause the pump not to be able to suck up gas at low fuel levels. The pump action probably has some wear too causing the lower pressure, so the low pressure and long crank/no start at low fuel were actually two different symptoms.