Losing fuel pressure when engine not running

[mikinoz]

So I have found that the car is a little hard to start. Doing a little investigation it appears that the fuel pressure across the fuel rail is dropping immediately once the fuel pump turns off after the initial prime.

I only know this because the FPR shows 3BAR under prime and then goes down to 0PSI in about 3 second.

The car will start if you prime, turn the key back to off and then start it on the key again - it will fire, otherwise you seem to be winding the thing over and over to no avail.

So, why does a system lose pressure? I was under the impression that the EFI system should always remain under pressure even when the pump is not on.

I have not done a great deal of research, but I am assuming that there are a few places you can lose pressure, but was wondering if I can get some pointers from the more knowledgeable peeps on here.

FWIW it is for MKII GTI 16V Digifant engine.

Cheers.

[Jmac]

That system should like most others have a residual pressure, which shold be at least 1 bar which is 14.5 psi after 20 mins. Fuel pressure drop is at the injectors, cold start injector, fuel pump check valve ( which is VERY common) or fuel pressure reg.Of course a leaky fuel line would also cause a pressure drop. Quick sanity check is to run the car with fuel pressure gauge then shut it off, immediatly clamp the supply line and watch the pressure , if it drops right away its engine side, if it stays good its pump side, in which case i would be checking the check valve
Quick test and saves a bit of time , at least you will pinpoint if its leaking at the engine or the rear in under 20 mins
Cheers
Jmac

[mikinoz]

That is an ace tip thanks jimmy. Thanks.

[mikinoz]

With only the guage on the fpr this is a bit hard.

I have a suspicion the fpr is the cluprit. If I clamp after the fpr it holds pressure for a lot longer an get to 1bar in 4-5 min not 4-5 seconds when it is not clamped on the return side of fpr. Make sense?

[mikinoz]

So should the FPR be holding this pressure in the system or another component?

[ausgolfer]

I'd try fitting a new accumulator. Is it Digi or Kjet?

[mikinoz]

It is Digi.

I just am not aware of what holds the pressure in the system if it is the FPR or something further down the return line. I can't understand how it could be behind the FPR however as the fuel past that is theoretically only the returned fuel that is over the regulated pressure...

I need a Bentley don't I haha.

[mikinoz]

[Jmac]

Just refresh on what engine you have and the actual delivery system you have mate. Im thinking its ABF cant be sure never was near it enough to know. This has an in tank pump delivery unit with built in non return valve. Ok back to start. Run car so the pressure is up. Switch off and clamp the fuel (feed line) you really must try this. If the pressure does drop then its most likely the reg, but if it doesnt drop mate then itll be the fact you have no or a faulty check valve. Really the only test to verify where in the sytem the leak back is mate. Return line is normally always blue but i did notice to confuse you the return line on that car isnt. Find the feed, then its the one next to it. This will at least give you a firm diagnosis mate, take away the guessing
cheers
Jmac

[mikinoz]

Thanks Jimmy. It is a KR engine converted from K-Jet to Digi II.

So I have some youtube clips to show you what happens when the pump primes and then when the return is clamped. The gauge is the one on the FPR, so I am unsure if that is going to be able to give a correct reading, but I have no other gauge.

Showing the 3BAR when the vacuum line is on, adjustment when it is removed and how it resettles when reattached;

http://www.youtube.com/watch?v=T8iMJYKaTI8

No clamps on return line;

http://www.youtube.com/watch?v=c3SldS5DdTE

And when the return line is clamped it holds better pressure but not altogether, but this may be my clamping assisting as it is hard fuel line to clamp.

http://www.youtube.com/watch?v=hukMU5vB3VM

[Jmac]

Mate just above i have quickly edited the post ,my bad its normally the FEED line you clamp., sorry about that. So prime the pump and system and then clamp the feed line. Normally if the system is equipped with a check valve its on the feed line. So lets say you clamp it, and the gauge stays good then the good side is leak free. If it drops when clamped then the engine side is your fault area mate. If it stays good then the fuel is leaking back pre rail mate, so yeah please try clamping the feed and give results mate
cheers
Jmac