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Robbie's Guide To The Low Pressure Fuel System
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Robbie
 


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Robbie's Guide To The Low Pressure Fuel System

Robbie's Guide To The Low Pressure Fuel System

Introduction

1. I've received a few PMs requesting information on how to test the Low Pressure Fuel Pump (LPFP) and I have seen quite a few tired LP pumps recently as the older ones are nearing the end of their operational lives. Unfortunately some inadequate diagnostics have led to a few good pumps being replaced whilst misunderstandings with the system have led to failing pumps being left in situ and other parts getting the blame for the symptoms.

2. There are some great guides on the forum explaining how to replace the LP Fuel Pump, so that information will not be repeated here. Jakeboy produced a brace of good photos recently that show the tank installation, removal, disassembly and the pump itself. The photos can be found here:

http://www.disco4.com/gallery/thumbnails.php?album=41000

3. When discussing the LPFP it has to be considered in the context of the tank and fuel line system and the High Pressure Fuel System it serves. This is especially true for diagnostics as the LPFP is not actively monitored by the engine ECU. It has no fault codes of its own but can trigger fault codes on other systems, including the HP fuel system. Care must be taken to distinguish between symptom and cause before the parts cannon is fired. Low rail pressures, injector issues, drivability issues and a brace of fault codes can all be caused by the LPFP. They are all traps for the unwary, as is changing a fuel filter to temporarily recover the fuel pressure. It may improve things for a while but is unlikely to be the true cause.

4. The LPFP is a simple DC pump immersed in a baffled compartment in the rear of the fuel tank. It is energised by its own relay, has its own fuse with power wires running down the right hand side of the vehicle, via the wheelarch connectors. The fuel from the pump is driven around the system via a pressure reducing regulator, set at 0.5 bar or 7.25 psi, then via the fuel filter and either passes through to the HPFP or returns back to the fuel tank via either the fuel cooler or via the filter again to warm it up. Once returned to the tank more fuel is drawn from the front pick-up, sent around the swirl pot and back to the pressure regulator again then on to the filter and so the loop continues.

Click image to enlarge


5. It is important to note that almost all the fuel pumped by the LP pump is delivered back to the fuel tank. The same is true of the HPFP as the majority of its fuel goes back to the LP system too, with only a fraction being used by the injectors. The book gives a minimum flow rate of around 1.7 litres per min being pumped, but a good pump will flow at over 3 litres per min. Put another way, your full tank of fuel is circulated through the system and to the engine and back every 20 minutes. The fuel gets warm (which is handy in winter), keeps the filter warm (also handy) and can be cooled by its own cooler when it gets too hot.

Testing - Input

6. There are a few trains of thought when it comes to testing the LPFP. As I like to start with inputs before outputs I tend to look at power first. Power and grounds should be checked, initially at the battery junction box (BJB) via Fuse 1 (25 amp). If the wiring is suspect then you can check power and grounds at the wheel arch connector and at the pump connector if needed:



7. The LPFP relay, located in the BJB, is another potential source of failure (it can also be jumped to empty the fuel tank if needed). Fuse 1 is the top left mini-fuse in the BJB (called the EJB in the D4) pictured below:

Click image to enlarge


8. To understand the power being used by the pump, which has a relationship with the actual work the pump is doing, we need to look at current draw. There are 3 main ways of doing this: via the fuse test points and a bit of ohms law maths; via a DC current clamp or direct to a digital multimeter (DMM). As a good pump can draw 10 amps or more in cold conditions and the fuse is set at 25 amps using a DC current clamp is preferable to blowing the typical 10 amp DMM fuse:

Click image to enlarge


9. The above pump is drawing 8.8 amps and was tested in the recent milder weather. The same LPFP drew 9.9 amps in the colder weather when the diesel was a little thicker. As this is a safe value for DMMs it is possible to plug in a regular meter and check for close to zero amps when ignition is off as well as recording the full current draw:

Click image to enlarge


Click image to enlarge


10. Testing the input current can tell you a great deal about the health of the pump. A good pump will draw sufficient current for all conditions and will be reliant on the pressure reducing valve to bring its output down to 0.5 bar (7.25 psi) on the tank side of the fuel filter. From experience a current draw of 4 amps should be good enough to produce sufficient system pressure, but effectively the pump is running at or close to capacity and the pressure reducing valve has little to do. I have noticed that D4 pumps appear to draw slightly more current than a D3 pump; the pump innards appear very similar so this is not fully explained.

Testing - Output

11. Having covered the power input we can consider testing the output side armed with the knowledge of the state of the wiring and the LP pump power. The LP side of the fuel system is fitted with a test port fitted with a standard (ie tyre valve size) schrader valve. To find it first remove the oil cap, engine cover and remember to replace the oil cap immediately to avoid anything dropping into your engine.

Rather dusty D4 engine bay:

Click image to enlarge


12. Under the cover towards the back of the engine is the LP pipework and the schrader valve, in this case with a grey plastic cap:

Click image to enlarge


Close-up shot:

Click image to enlarge


13. The exact position of the valve varies a bit between the earlier 2.7L engine, the later EU4 variant and the 3.0L shown here. With the cap removed a regular low fuel pressure tester or a combination gauge can be screwed on:

Click image to enlarge


14. With the key off for a protracted period there should be no pressure in the system. If the system has been primed recently there will be a small amount of residual pressure:

Click image to enlarge


15. At idle with a good pump and an ok fuel filter (as this will drop the pressure slightly from the regulated 0.5 bar / 7.25 psi on the tank side of the filter) the pressure should be over 0.4 bar:

Click image to enlarge


What you do not want to see is a waving needle or a stupidly low pressure:

Click image to enlarge


16. Ideally the pressure will be closer to 7 psi. If you know the pump current is good and the pressure is down but reading steady then perhaps a new fuel filter is due. If the pressure is above the 0.5 bar / 7.25 psi then there may be an issue with the pressure regulating valve. A good output looks like this:

Click image to enlarge


17. If the needle is unsteady then the pump may be struggling or even missing some impellers. If the pressure drops significantly when the throttle is pressed then you know the LP system is finding it difficult to cope with the demands of the HPFP, which runs at scary pressures:

Click image to enlarge


Summary

18. The testing of power, grounds, current draw and LP fuel pressure are complementary and should give you a good insight into the health of the LP fuel system. The LP pump is not cheap so meaningful diagnostics are required before firing money at the problem. The LP system provides the circulation and fuel temperature management that the high pressure system needs. If it cannot provide the fuel required the HP system will suffer, so it is vital that the health of the LP system is established before investigating issues at either the HPFP, fuel rails or the injectors. Along with battery, glowplug and injector issues, low fuel pressure is also a common cause for poor cold weather starting.

19. I've bound to have missed some top tips along the way, so please feel free to add your own thoughts and experience. Testing LP fuel pumps is cheap and easy to do, so give it a go.

Click image to enlarge


Regards to all.

Robbie

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Post #148133031st May 2015 7:05 pm
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grzesiul
 


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Post #148133231st May 2015 7:08 pm
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tayaste
 


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Well, I've read all through that and understood all of it. Even a doughnut like me Bow down

Great stuff and thanks for your time Thumbs Up Thumbs Up

Only question I have now is where do I buy that pressure gauge from Laughing

Bow down Bow down
   
Post #148133631st May 2015 7:17 pm
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yogi972
 


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Top work Robbie😁
  
Post #148137331st May 2015 7:47 pm
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Flack
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Nice write up Robbie

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Post #148137831st May 2015 8:01 pm
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DN
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That's a brilliant piece of work Robbie, thanks for taking the time to do it. Bow down Bow down ....... Now that pressure gauge.......Must look into one of those Laughing
  
Post #148138131st May 2015 8:03 pm
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Gareth
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Added to Wiki. Cheers Robbie. Thumbs Up
  
Post #148147131st May 2015 11:02 pm
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Landie71
 


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Great writeup Thumbs Up
  
Post #14814961st Jun 2015 6:26 am
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maplecottage
 


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Excellent post Robbie

DN wrote:
That's a brilliant piece of work Robbie, thanks for taking the time to do it. Bow down Bow down ....... Now that pressure gauge.......Must look into one of those Laughing


Draper or Sealy do a fuel / vac test meter which would meet your needs Whistle Only about £12 AIRC

Steve
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Last edited by maplecottage on 1st Jun 2015 8:50 am. Edited 1 time in total 
Post #14815131st Jun 2015 7:48 am
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Cellerdweller
 


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In a word excellent Thumbs Up
Thank you for taking the time to produce and share many will benefit from that .
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Post #14815191st Jun 2015 8:07 am
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DN
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maplecottage wrote:
DN wrote:
That's a brilliant piece of work Robbie, thanks for taking the time to do it. Bow down Bow down ....... Now that pressure gauge.......Must look into one of those Laughing


Draper or Sealy do a fuel / vac test meter which would meet your needs Whistle Only about £12 AIRC

Steve
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. Thanks Steve, I have already ordered the same as Robbies, which I think is a Sealey VSE 952. Thumbs Up  
Post #14815241st Jun 2015 8:30 am
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Disgoeshere
 


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A Band of Brother's forum this truly is,and another amazing and invaluable guide to compliment it.
Thank you Robbie Bow down
  
Post #14816621st Jun 2015 2:41 pm
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Russell
 


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Great write up, on the first drawing there are a number of numbered items, have you a key to all of these please, sme are obvious but some are not so. Thumbs Up
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Last edited by Russell on 1st Jun 2015 6:47 pm. Edited 1 time in total 
Post #14816941st Jun 2015 3:49 pm
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Robbie
 


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Thanks guys, the kind words are appreciated.

DN wrote:
I have already ordered the same as Robbies, which I think is a Sealey VSE 952. Thumbs Up


It is a Sealey, bought for a tenner a fair while back to be used as a vacuum gauge. I actually ordered the version without the plastic case as it was much cheaper, but it came with the case anyway. The case is a bit pants as it does not hold the gauge with the hose attached. It does not come with a schrader connection either, but accepts a barbed one with ease (about £2 on the bay). Or just hack a hose off an old pump, as Geoff suggested.

It's not actually my regular fuel pressure gauge, but I have lost the brass adaptor on my big set that changes a large schrader to the smaller tyre size schrader. If anyone knows where to get one as a replacement….

Russel, the decode:

Click image to enlarge


Oh and to answer one PM, the old technique of banging on the tank to get a stalled pump to run still works!

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Post #14817841st Jun 2015 6:26 pm
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DN
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Thanks for the info Robbie, when it arrives, I'm sure it'll do the job. I have an old foot pump hose with a schrader connector on too, so should be ok. Thumbs Up
  
Post #14818381st Jun 2015 7:23 pm
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