Low fuel pressure problem

Hi Guys,

I'm so close to getting this darn thing running but it's driving me mad! need a little help/pointer in the right direction

basically, I'm getting a low fuel system pressure by the results of hrs testing - causing no fuel (or at best a light spray from one injector) from the injectors

I'm getting 4 bar (55psi) system pressure when the pump is running, the pump is getting a strong 12v feed

there is plenty of fuel coming out the metering head areas when the nuts are cracked (ooohh errr) but I assume it's not enough

pressure drops down to nothing (0psi) when I turn the pump off - which I believe (again!!   I've replaced this 3 times now) that this is the fuel pump check valve? when I say I've replaced this, each time the old one is removed, it is indeed not working (basically I can blow air back through it so it's no longer closing)

but what could be the cause of the fuel pressure being -10psi to what the injectors need to operate?

Fuel filter? Accumilator? the pump itself? air being sucked into the system as soon as the pump turns off giving a false reading?  :dontknow:

please help, not driven the Mk1 in weeks and fed up walking everywhere  :cry:

accumulator is also responsible for holding system pressure, so it could be down to that. were the non-return valves new?

4 bar isnt enough as you know, so id be looking at changing the pump or checking the lines for debris. some pumps have a mesh basket on the inlet side of the fuel pump, these can get clogged up with debris from tank/filler neck. also, check the 2 big banjo bolts on the metering head feed/return, some of these have a mesh basket built in which if clogged would restrict flow. these should have been removed as part of one of the early sevices, so if fitted try to locate some bolts without the basket in.
If all thats ok, it could be you need to adjust the shims in the metering head pressure regulator.

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well, bit of an update

new fuel filter, accumulator, and injectors fitted… AN EXPENSIVE DAY!

the check valves were all new from vw.. I blasted the current one with compressed air… sorted it out! must of got stuck but doesn't inspire confidence in new parts!

also checked the fuel pump delivery (just over 900cc over 30 seconds which the haynes manual says is good)

so now, it's holding 40psi for 30mins after the pump/ignition is turned off, then slowly starts to tail off  

getting a system pressure of 60psi (just over 4 bar and thats with the valve in the pressure testing equipment I have open and closed to the WUR) so it's still too low for the injectors to operate :cry:

thinking I should shim the pressure relief valve  :dontknow: 1mm will increase it 8psi which should do it

am I right? :?

I haven't messed around with this stuff for a long time but from memory a system pressure of 60 psi is a bit high, I think it should be between 49 psi andr 55 psi.

no thats the control pressure you're thinking off, we're looking at the system pressure which should be from 4.7 to 5.4 bar/68-77psi.

have you got any spare metering heads? could be the spring is worn or something is up inside the metering head. otherwise chuck a 1mm shim in there and that should help a bit

ah! i see you're in berkshire. if so, come over to see me I have a 1.8 k-jet metering head which i dont need, yours for free/beer if you collect. I havent tested it though, but the pin on the bottom moves freely and worst case it has a spare releif valve you could pillage for bits

Yes thats right, I had the figures about right but should have been bar not psi and 4.9 - 5.5, not far out! Have you tried taking the relief valve out and cleaning it, clean the seat carefully aswell. I'm fairly certain Bosch used to do a repair kit for the valve so it must have a reputation for playing up. This is all from memory but I think there's a small 'o' ring on the valve that came as part of that kit, that could have deteriorated.

yep hes right you know, VW sell a repair kit with new seals n shizzle if you need it