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Fuel pressure problems

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pudding:
It's a tricky one because a bad HPFP, or relief valve, bleeding off fuel pressure abruptly could cause a misdiagnosed LPFP!  Similarly, a bad LPFP can also cause fuel pressure faults, so the two are heavily interlinked.

That's the trouble with modern engines and fault codes. A component flagged as faulty could be the result of something else failing.  Bad grounds or a bad CAN gateway for example can light up a dash like a Christmas tree, and people start throwing new modules at the car, which doesn't fix the root cause  :grin:

Given the LPFP and HPFP are similarly priced (new oem), an element of parts cannon firing could be required.  The used LPFP you fitted, was it the exact same part number of your original one?  They have been updated by VW a couple of times over the years.  The early ones were prone to overheating and causing fuel cuts.  VW's fix was to simply remove the lid from the pump housing  :grin:

Given the symptoms, I suspect it's the  low pressure side.  It sounds like it's running out of puff as the load increases because it's old and tired perhaps.  The crazy 90% duty kind of hints at that.

And as last minute thought, have you checked the HPFP cam follower hasn't worn through and the pump's piston isn't riding on the cam?  That severely limits fuel volume!

Uncleben:
Think I may have to admit defeat, the car needs a lot spending on it as is (cambelt due, noisy chain, noisy flywheel, rusty sills and front wing) mot has expired too, car has done 193k and its showing now!
I've had nearly 6 yrs problem free from it, so now I think everything is going wrong at once! don't think it is worth spending more money, especially as I've already spent on replacing all the low pressure components, and as that is where the problem still seems to be then do I just replace it all again but with genuine new parts?
may just sell as is and cut my losses!
The lpfp I bought off eBay was the exact same, then I tried one out of a 2010 octavia which was the same part number apart from the last letter, still to no avail.
I will inspect the cam follower just to rule it out as it has only been about 25k since I had it changed last, but you never know!
I appreciate all your help regarding this issue, but, as you say, trying to diagnose problems without seeing the car is a pain.
Cheers.

pudding:
I can totally understand that. Maybe it's time to upgrade to a newer model, but it sounds like yours has done extremely well given the mileage!

Uncleben:
You were supposed to talk me in to keeping and fixing it!
While I'm pondering my options I decided to take the original lift pump apart out of curiosity, all I noticed was a lot of dirt/sediment in the bottom of the bowl, is that normal? there is a superfine mesh filter on the bottom so not sure how it got through, but even so, the fuel filter would have prevented anything reaching the engine. :thinking:

pudding:
But that would make me a hypocrite as I gave up on mine in November  :grin:  It ran fine though, so not really the same situation, it was just time to move on to something more relaxing in traffic.

There shouldn't be a lot of dirt inside the bowl but the pre-filter is generally always filthy.  You could try a mechanical fuel pressure gauge to verify the sensors aren't lying?

I think you can plumb it into the Schrader valve on the HPFP.

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