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Old 01-20-2013, 12:06 AM   #1
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Join Date: Jan 2005
Location: Chicago
Posts: 232
My Ride: 325it ESS TX-2
Fuel Pressure Leak Down Test - Strange Results...

So, I am chasing another stupid fuel trim issue that prevents me from passing the emissions test. This time it's rich (Peake E3, E4 and P0175, P0172, long term fuel trims -8 to -10% both banks), car runs just fine, though. I've read about what people say about the fuel pressure, that it should not drop more than 0.5 bar (~7 psi) over 20 min., etc. So, I do a leak test and find the stangest thing: After shutting the engine off, the pressure RISES for the first 10 min., then slowly (probably a bit too fast) bleeds off. What is causing the initial rise? I am not hallucinating, I did this twice, and I am not looking at a single outlier data point. This is a nice, smooth curve that shows some slow pressure equalization. But, why? I really don't understand this. Immediately after the engine is shut off, pressure is 41 psig. There's got to be some area of the fuel system that at that point is still pressurized to 46 psig, which then bleeds into the fuel rail...

Steps I went through:
1) Engine off, turn key to pos. 2, fuel pump primes for about 2 sec.: 50 psig (that seems OK)
2) Fuel pump shuts off automatically (after 2 sec.): instant drop to 46 psig (was expecting a bit less of the drop-off here - is check valve OK?)
3) Start engine, let idle for a few minutes: 45-46 psig (gauge needle vibrates a bit, but is steady at any RPM higher than idle)
4) Engine shut off (t=0): instant drop to 41 psig
5) Record pressure over time.
Here's the strange thing: after two minutes pressure is already at 44 psig, it keeps rising until 10 min., maxxes out at 46.5 psig, then falls slowly over time.

Anyone got an idea? Is this normal?
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Freude am Fahren
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Old 01-23-2013, 11:34 PM   #2
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Join Date: Jan 2005
Location: Chicago
Posts: 232
My Ride: 325it ESS TX-2
Anybody? There's got to be someone here who has taken a leak down curve before and knows if this is normal or not. Please?
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Old 02-15-2013, 07:14 PM   #3
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Join Date: Jul 2010
Location: Kansas
Posts: 7
My Ride: 2002 M3 Coupe MT
Sounds like you need to swap out the fuel pressure regulator. What model car is it? You should be able to google the required pressure. I have an M3 so the regulator is set at 5 bar.
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Old 02-20-2013, 02:58 PM   #4
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Location: Brentwood, CA
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My Ride: 99 328 5spd
50 psi (3.5 bar) is spec fuel pressure on 325.
-the drop you are encountering is normal
If your vehicle is running rich then you need to consider the following:
-Is the car reaching operating temperature quickly? (running cold will yield rich fuel mixture)

The FP regulator is built into the fuel filter anyhow, so if you are worried about a bad fp regulator then replace the fuel filter. Make sure it's OE or Mahle.
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