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Strange fuel pressure Issue

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Old 02-18-17, 04:11 PM
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Strange fuel pressure Issue

Not sure where best to post this. So might drop it in a couple of places. Hope mods don't mind. I need some help!
So I have a 20b converted FD3 with single turbo. I went to dyno a few months back and had no fuel pressure issues that we noticed. I have since changed my primary injectors for smaller more manageable ones and the car had been run in the workshop a few times setting up the ECU. Nothing else has changed. The rest of the fuel system is as follows. New AN6 nylon braided fuel lines all the way from tank. AN6 inline cartridge filter. Extreme Rotaries fuel rails, SARD FPR. Quite possibly copy. Walbro 450lph pump. Fuel system tested up to 80psi static with no issues.
So car drove 10miles to dyno doing some road mapping on the way. Seemed fine but kept it off boost anyway.
first 30mins on dyno it ran fine. Then suddenly started missing and going lean with even light load coming on throttle. I noticed the fuel pressure was dropping off to basically zero as the throttle was opened. Shorter run to try and suss it of course. Tried pulling off vacuum line and putting to a other point on side of the plenum. No difference. Tried with the mine off all together so vac could not effect FPR still dropped off as soon as throttle started to open. With little. Other options I decided to pull the filter off and check it. It had a bit of crap in there. Maybe debris from new lines. Blew it out and put it all back. Fired up and all appeared perfect? Ran for about another 15mins and seemed okay. Fuel pressure increasing with boost. All good, then had to call it a day.
pulling off the dyno it seemed a bit off again and i. The way home it went from bad to worse and was lucky to make it back driving on light throttle and no load as much as possible.
I wondered if it might be pump going weak after a while or best effecting FPR as it is mounted to back of engine. Bearing in mind we had it really dine any power runs just setting up the matrix table on the ECU up to 5500rpm or so and no mega heavy load yet?
I went out this morning and fired it up. Ran like a bag of crap and fuel pressure was off just how it ended yesterday. I whipped of the filter and had a look. Seemed clean. Tiny, tiny bit of stuff there. Blew it out and put it back. I initially it seemed a bit better but then clearly started to drop almost immediately and revving the engine the pressure was dropping off to 20psi or less. Then decided to turn engine off and wind up FPR pressure. Went up to 75psi no worries. Then started the car same fuel pressure and it seemed to hold fine. But when you rev it drops more than it should. Was much better with the pressure wound up but maybe that is to be expected? So backed pressure back down to 43psi idle and same story. Pressure dropping way off. So big question is what the hell is it? Pump messing about? Seems unlikely if it can push the static and idle fuel pressure okay? Dicky FPR with a mind of its own. Or a lump of crap in the fuel line blocking the filter then going back down the line when I disconnect it?
Let me know your thoughts.
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Old 03-20-17, 01:30 PM
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premix, for f's sake
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Have you checked the pumps inlet or power supply?
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Old 03-22-17, 12:10 AM
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What fuel filter are you running?
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Old 03-22-17, 03:45 PM
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I have not looked into this further yet as engine is out being rebuilt so it has dropped down the to do list. I plan to check voltage. I presume you mean at the plug that goes through the fuel hatch to the pump. I took power direct from battery to a new relay for the pump so it should be good. But will check it to rule it out. Will also check the filter sock is not caked in crap or something silly.
I have the walbro sock on the inlet that comes with the 450lph pump. Then at the engine bay I have one of those billet ally filters with a sintered filter supposed to flow well over the lph required.
all clean inside as well.
Other possibility I wondered was could the fuel tank breather be blocked essentiality causing the tank to go into a vacuum over time and hence why it seems to get progressively worse? Is there a one way valve on the breather which could have been switched around while the tank was off and new lines were being run?
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Old 03-22-17, 07:45 PM
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Start with electrical. Try to see whether fuel pumps are receiving full voltage when it's acting badly.
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Old 03-23-17, 04:33 PM
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Thanks for info.. I have heard this before. just not been able to check yet.. I will have to start with other things such as check filter and lines clear before I fire up the rebuilt engine on it! Where should I take the voltage reading from.. I am not sure there is any wiring near the pump when it is in the tank and running? What would be a cause of dropping voltage at the pump anyway? in case I can visually look for something in the meantime?

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