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Go Back   Bimmerfest - BMW Forums > BMW Model Discussions > 5 Series > E39 (1997 - 2003)

E39 (1997 - 2003)
The BMW 5-Series (E39 chassis) was introduced in the United States as a 1997 model year car and lasted until the 2004 when the E60 chassis was released. The United States saw several variations including the 525i, 528i, 530i and 540i. -- View the E39 Wiki

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  #1  
Old 05-03-2015, 05:50 AM
gmak2012 gmak2012 is offline
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Exclamation Running Rich Saga: Fuel pressure too high - need some advice

1. Fuel pressure for a cold car, engine off is about 35 - 40 psi
2. Fuel pressure at idle is 65 psi
3. Fuel pressure when revving engine up to 2000 rpm stays at 65 psi
4. Fuel pressure when take vacuum off of Fuel Pressure Regulator (FPR) goes up to 75 psi

The FPR is working - but maybe not fully (??). i.e. Diaphragm /spring inside don't relax enough to let fuel pressure fall. Could a faulty fuel pump cause the pressure to be too high? Can it be adjusted?

I thought that I would replace the FPR with a $34 Bosch one since the one on there is true-blue BMW (and only 3 years old on my car - who knows how long it sat in a parts warehouse before that - or if it isn't some cheap knock-off (it's been a while and I don't remember who I ordered from).

Is there any other advice? The car runs just fine apart from the high fuel pressure (with silightly lower mpg that I would otherwise get) - but I'm worried that at some point the DME won't be able to compensate and I will be looking at a CE - not to mention the slow but steady damage to the cats. I'm also worried that perhaps it will reach a point where there is difficulty starting as things progress. And yet, it's been like this for at least 3 years that I know of....

Again, any advice would be greatly appreciated because I don't want to start throwing parts at the fuel pump if that is not the way to go. I'm hoping for a simple solution to this (fingers crossed) and maybe a new FPR will do the trick.

Cheers.
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  #2  
Old 05-03-2015, 06:17 AM
pshovest pshovest is offline
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Mein Auto: '10 535iA, '00 528iTM
Are you sure gage is accurate?
No CEL, what prompted you to check fuel pressure?
Cat damage is not likely. If pressure is actually high, DME will compensate by reducing fuel trims. You can see fuel trims with INPA and many inexpensive OBD2 code readers like Actron 9145.
Bad FPR or blocked return line downstream of FPR.
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Old 05-03-2015, 06:27 AM
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johnstern johnstern is offline
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I disagree with you that your fuel pressure regulator is working. I don't know what the exact fuel pressure spec is for the fuel pump but I'm sure 75 psi is fine.

It is the job of the FPR in our cars to regulate FP so it is 3.5 Bar or about 50 PSI at idle, not 65 psi.

I totally agree with pshovest's above. After you make sure that your Fuel Pressure gauge is correct and the fuel return line is clear, replace your FPR. IMHO it is worth buying an OE FPR (not an OEM one).
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Old 05-03-2015, 07:05 AM
gmak2012 gmak2012 is offline
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Thanks guys.

@pshovest: I've had negative trims for a while. FPR was the last element to investigate, outside of looking for leaks or O2 sensors not in correctly on the exhaust side.
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Old 05-03-2015, 07:13 AM
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540iman 540iman is online now
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Location: N.W. Indiana
 
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Mein Auto: 2002 540i sport
Quote:
Originally Posted by pshovest View Post
Are you sure gage is accurate?
No CEL, what prompted you to check fuel pressure?
Cat damage is not likely. If pressure is actually high, DME will compensate by reducing fuel trims. You can see fuel trims with INPA and many inexpensive OBD2 code readers like Actron 9145.
Bad FPR or blocked return line downstream of FPR.
The FPR is manufactured the way it is so that the DME, through feedback from the O2 sensors by the action of the fuel trim adjustment may be at a minimal amount to be able to hold the mixture at close to 12.5 to 1. There is, of course, a value for the FPR that becomes the manufacturing tolerance- a high and a low value to assist the O2 sensors with sending to the DME an air/fuel ratio needing fairly close to "zero" adjustment (fuel trim) ideally. BMW, knowing that the FPR is simply a diaphragm and a spring will have (or could have) quite a bit of process variability when the fuel pressure filter/regulator is made. Ideally it would keep the pressure between 45-50 PSI which will make for a very easy trim value across the rpm/load range. It sounds like your FPR was just at the high end of the manufacturing process, but within the capability of the values the DME will need to store in your trim table. I agree with the OP who said if your fuel pressure becomes more than the trim table can adjust for, YOU WILL get a SES error. You just need to have the confidence that your O2 sensors are good and that they will take-over from here. If you were having what you *thought*was a rich condition, then the O2 sensors would be the correct suspect. If they are operating correctly, their output would swing rapidly about and below .5VDC but they would "swing" between about .2V and .8 V trying to be centered at about .5VDC. If your tail pipe were to be cleaned and then rapidly turned sooty with unburned fuel QUICKLY, then I would check your pre-cat 02s.

Last edited by 540iman; 05-03-2015 at 07:18 AM.
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  #6  
Old 05-03-2015, 07:30 AM
gmak2012 gmak2012 is offline
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Mein Auto: 2006-530xi; 1998-528i
Really good information in this thread. I never knew that about the FPR. I'll clean the tail pipe (it's the scuba tank with tip turned down) and see what happens. Meanwhile, I think an OE (Bosch) FPR might be a cheap test.

O2 sensors are 2 years old.

Last edited by gmak2012; 05-06-2015 at 04:25 PM.
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