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      08-24-2009, 11:44 PM   #23
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Quote:
Originally Posted by jeremyc74 View Post
The low pressure sensor MONITORS the feed pressure, which is held steady at 5 bar. The low pressure pump is inside the fuel tank, and it's the same thing that's used on the 128i. It's a typical fuel pump that's not variable pressure.

The ECU determines what the pressure should be on the HIGH side of the HPFP, and then adjusts it based on the reading from the HIGH PRESSURE sensor.

You can't control the outlet pressure of a variable PD pump based on it's inlet pressure. The inlet side is a constant 5 bar, and the low pressure sensor is just making sure it's where it's supposed to be.

Again, I can't come up with any logical reason the low pressure sensor would have anything to do with this. It's just there to make sure there's proper pressure feeding the HPFP, not to control anything.



Edit:

Ok, I'm taking another look at this, and now I'm questioning whether or not the low pressure pump is variable speed or not. There's another diagram in the tech document that shows some of sensors, and they're showing a transistor where the electric pump is. They also mention that if the low pressure sensor fails the electic pump will continue to deliever fuel at 100%, which suggests that it's capable of something less than that.

IF that's the case, and the LP sensor were failing in such a way that caused the pump to run at a much slower speed than it was supposed to maybe the HPFP is being starved, which is leading to the repeated failures after a short period of time.
Yes, That's what I was suggesting with this previous post:
"If the low pressure has something to to with overall high pressure, then it could be part of the problem.
I would guess if the low pressure sensor is causing the HPFP to be under supplied with fuel, that would cause a problem in that the HPFP would be working harder to keep fuel pressure up with an insufficient supply."

It's a possibility.
Hopefully, we'll get more answers soon from those doing the tests.
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      08-25-2009, 07:08 AM   #24
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Quote:
The fuel is delivered from the fuel tank by the electric fuel pump via the feed line (5) at a “feed” pressure of 5 bar to the high pressure pump. The feed pressure is monitored by
the low-pressure sensor (6). The fuel is delivered by the electric fuel pump in line with
demand. If this sensor fails, the electric fuel pump continues to run at 100% delivery with terminal 15 ON.

This is my understanding of why Eric@AMS and CP-E suspect the issue might be related to the low pressure sensor. Please tell me if I've got it wrong...

The low pressure sensor monitors the feed pressure of fuel from the in-tank pump to the HPFP to make sure it's at 5 bar. If the low pressure sensor fails, then the in-tank pump runs at 100% capacity regardless of actual demand by the HPFP. Their thinking is that this could be having some adverse affect on the HPFP... possibly overloading it and making it wear out at an extreme rate (I think this is why Eric@AMS also mentioned that CP-E is looking at making a more robust HPFP replacement, one that might not wear out if overloaded by a failed low pressure sensor). They are also looking into the possibility that the low pressure sensor could be failing without being detected and are trying to determine if it should also be replaced whenever a HPFP fails. Is this correct?
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      08-25-2009, 07:38 AM   #25
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Quote:
Originally Posted by Absorber View Post
This is my understanding of why Eric@AMS and CP-E suspect the issue might be related to the low pressure sensor. Please tell me if I've got it wrong...

The low pressure sensor monitors the feed pressure of fuel from the in-tank pump to the HPFP to make sure it's at 5 bar. If the low pressure sensor fails, then the in-tank pump runs at 100% capacity regardless of actual demand by the HPFP. Their thinking is that this could be having some adverse affect on the HPFP... possibly overloading it and making it wear out at an extreme rate (I think this is why Eric@AMS also mentioned that CP-E is looking at making a more robust HPFP replacement, one that might not wear out if overloaded by a failed low pressure sensor). They are also looking into the possibility that the low pressure sensor could be failing without being detected and are trying to determine if it should also be replaced whenever a HPFP fails. Is this correct?

If running the in-tank pump at 100% causes damage to the HPFP, it's a MAJOR design defect on BMW's part. It's hard to imagine exactly what damage it could be doing, but there are a lot of small parts on the inlet side of the pump, so maybe it's causing fuel to blow by a seal.

The other thing that makes this unlikely IMO is that the ECU HAS to know that the sensor has failed to cause the pump to run at 100%, and they should have realized thats' what's causing the damage. If the sensor has anything to do with this problem, I'd say it's more likely that it's causing low feed pressures and starving the pump, rather than overloading it.
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      08-25-2009, 08:02 AM   #26
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my vote goes to starving the pump too, most pumps require the liquid they pump for lubrication and cooling

I would love to see some pictures of what actually fails on these pumps
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      08-25-2009, 08:14 AM   #27
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Quote:
Originally Posted by imported_mega View Post
I would love to see some pictures of what actually fails on these pumps

Same here.

If you look at that cutaway, at the bottom you can see a slanted disc, which is what drives the pistons up and down. The fuel would have to lubricate the cups on the bottom of the pistons, so there's one wear item for sure. The piston seals would be another obvious one.
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      08-25-2009, 08:22 AM   #28
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Quote:
Originally Posted by jeremyc74 View Post
The other thing that makes this unlikely IMO is that the ECU HAS to know that the sensor has failed to cause the pump to run at 100%, and they should have realized thats' what's causing the damage. If the sensor has anything to do with this problem, I'd say it's more likely that it's causing low feed pressures and starving the pump, rather than overloading it.

So is it possible that the low pressure sensor is failing and the ECU and in-tank pump aren't being told that the low pressure sensor failed... causing the in-tank pump to pump less than 5 bar of pressure... starving the HPFP?
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      08-25-2009, 08:30 AM   #29
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Quote:
Originally Posted by Absorber View Post
So is it possible that the low pressure sensor is failing and the ECU and in-tank pump aren't being told that the low pressure sensor failed... causing the in-tank pump to pump less than 5 bar of pressure... starving the HPFP?

That's a question I can't really answer.

I work with sensors and controls like this all the time on an industrial level, and it would be an extremely rare case for that to happen....BUT...the stuff I'm working with is higher quality (3 or 4 times more expensive than what BMW is selling this part for), and operates at a different voltage and current range.

If the ECU is programmed to look for it, I'd say the answer is "No, it's not possible" because if it were done right you'd run a little self test on it from time to time to make sure you're not getting false readings. In other words, if I were doing it I'd write a little piece of code that tells the pump to ramp up by a certain % and check the pressure change, then ramp down by the same amount and check it again, then subtract the two and compare it to a known good pressure change. You could do that on a periodic basis without effecting the engine, and if it's not working properly you won't see the change and you can set a fault, but we don't know if they're actually doing that.
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      08-25-2009, 11:25 AM   #30
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I had my low pressure sensor replaced before the last HPFP failure. They noted on the work order that the sensor was reading 6.8Bar while in actuality it was 5.2Bar. So, in effect, it was reading high, and supply was lower than it thought. If that could reduce the low pressure pump supply, it would seem to make sense that the HPFP would be recieving a lower than required pressure.

The down side here is that I still had a failure of my 881 HPFP 2000 miles AFTER this replacement. So, the jury is still out in my opinion.
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      08-25-2009, 11:36 AM   #31
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Quote:
Originally Posted by isitspringyet View Post
I had my low pressure sensor replaced before the last HPFP failure. They noted on the work order that the sensor was reading 6.8Bar while in actuality it was 5.2Bar. So, in effect, it was reading high, and supply was lower than it thought. If that could reduce the low pressure pump supply, it would seem to make sense that the HPFP would be recieving a lower than required pressure.

The down side here is that I still had a failure of my 881 HPFP 2000 miles AFTER this replacement. So, the jury is still out in my opinion.

If it were reading high, that will certainly cause the low pressure pump to slow down, if it is in fact a variable speed pump.

Is this the sequence of events here?
Pump failure
New HPFP (881)
More problems
Sensor replacement
More problems
New HPFP

If that's how it went, I guess it's possible the bad sensor could have already damaged the new pump. How long did it run between the pump replacement and the sensor replacement?

Very interesting! The plot thickens!
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      08-25-2009, 12:40 PM   #32
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Yep! I had already thought that it was the HPFP based on my previous experiences, but the codes did not give them the HPFP, so they only changed the low pressure sensor as described. The slow starts never went away, and finally the HPFP totally crapped out, 2,000 miles later. I do suspect that the pump was already damaged, since I originally took it in based on symptoms that I am way too familiar with! It does appear that the 881 HPFP will last longer with a problem, my first and second pumps failed soon after the first symptoms showed up.
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      08-26-2009, 12:26 AM   #33
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a light at the end of the tunnel????
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