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Thread: fuel pressure bleed down

  1. #1
    Member alanjuggler's Avatar
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    fuel pressure bleed down

    alright chaps,

    been trying to pin down some of my weird problems with the RB2xDETT thing.

    I've put a fuel pressure gauge in between the filter and the rail to see what the fuel pressure is like and it's fine, when the pump is running - just over 3bar before starting the engine and 2.5bar at idle, vac hose off, it returns to just over 3bar. (rb25det is definitely a 3bar regulator - like the SR20)

    the thing I noticed is that the pressure drops fairly quickly when either the fuel pump stops priming or the engine stops running - some times it was within 30 seconds, some times within 10-15 minutes but it was a total loss of pressure.

    I've not got any leaks (or any I can see), the injectors have been tested off the car so I know they hold pressure.

    I took the return line off from the FPR and after the initial flow stops, there's a slow drop (maybe once every 10 seconds or so?) but don't think this would be enough to drop the amount of pressure involved.

    would this suggest a bad check valve on the feed under the car? (or is it possible the fuel pump isn't quite right?) both have been dry for a couple of years - but stored inside.

    any ideas before I start throwing money at it would be lovely.

    cheers

    alan
    white '94 s13 200sx scrapped - mapped to 1.45bar. OS giken box, garrett GT2876R, 950cc injectors, ORC twin plate, nistune. 349bhp/325lbft @ 1.3bar CA18DET
    white '96 s13 180sx - type g with more kouki bits - RB25DET, GTR steel twin turbo conversion, RB26 crank & rods. 2.6L VVT twin turbo, SR20 OSG box, OSG STR twin plate clutch, Z32 ECU w/ nistune.

    current status: 180 a bit broken but to be repaired.

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    I would've thought that's normal isn't it?

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    In my ca18det is the same, with the pump priming the pressure is 2.5 and when it stop the fuel pressure goes to 0 right away. I think it should hold more time, I will try another one when I can.

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    Check valve

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    Quote Originally Posted by voodoo_melon View Post
    I would've thought that's normal isn't it?
    in my hunt for figuring it out, I came across the 300zxTT board in the states, they're talking about a drop from 3bar to 2bar in 24 hours as being a problem. mine definitely is - seems to cause low fuel pressure on the initial crank.

    Quote Originally Posted by shogun
    Check valve
    I figured it would be this - though I was having difficulty finding a nissan one - might just fit one of the generic ones on ebay.

    edit:

    balls, they're saying they're only suitable for carb/low pressure systems - I know 3 bar is relatively low by modern standards, not sure I want to cock something up by fitting a poo part.

    edit #2:

    more reading - the walbro 255 doesn't have an internal check valve either, so the one on the chassis is the only one.
    Last edited by alanjuggler; 06-04-2013 at 20:40.
    white '94 s13 200sx scrapped - mapped to 1.45bar. OS giken box, garrett GT2876R, 950cc injectors, ORC twin plate, nistune. 349bhp/325lbft @ 1.3bar CA18DET
    white '96 s13 180sx - type g with more kouki bits - RB25DET, GTR steel twin turbo conversion, RB26 crank & rods. 2.6L VVT twin turbo, SR20 OSG box, OSG STR twin plate clutch, Z32 ECU w/ nistune.

    current status: 180 a bit broken but to be repaired.

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    Bosch 040 and 044 have check valves.

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    Member alanjuggler's Avatar
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    I'll try nissan first I think - turns out the valve on the CA18 180/200 is different to the one on my kouki 180 - helpfully, the one on the kouki 180 is in common with a fair few later cars, so hoping nissan UK should be able to get it easily enough.

    if that goes badly wrong, I'll consider upgrading the fuel pump for the bosch - tbh, with the RB2xDETT thing wound up to 'proper' boost, I think the walbro might start to struggle anyway, so might be a good investment.

    edit:

    looks like the american sites have them for around 30$ - we'll see how competitive the UK dealers are if it's £40+, I think I'll put the money toward the bosch.
    Last edited by alanjuggler; 06-04-2013 at 21:20.
    white '94 s13 200sx scrapped - mapped to 1.45bar. OS giken box, garrett GT2876R, 950cc injectors, ORC twin plate, nistune. 349bhp/325lbft @ 1.3bar CA18DET
    white '96 s13 180sx - type g with more kouki bits - RB25DET, GTR steel twin turbo conversion, RB26 crank & rods. 2.6L VVT twin turbo, SR20 OSG box, OSG STR twin plate clutch, Z32 ECU w/ nistune.

    current status: 180 a bit broken but to be repaired.

  8. #8
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    This page has some excellent information about fuel systems

    http://forums.vwvortex.com/showthrea...es-Fueling-FAQ

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    My fuel system has 3 Bosch pumps.

    Intank pump, swirl pot, filter, dual pumps, filter, rail/injectors, high flow fpr, return to pot and tank.

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    shogun, I don't quite think I need the level of setup you have, I'm not aiming for high G track work - my goals for the RB thing is just a very driveable 450ish horsepower - or really, whatever is generated by these steel rebuilt turbos at 1bar.

    I have read though that the 044 isn't supposed to be used as an intank lift pump - what do you use?

    edit again:

    also been reading the manual for the normal S13 - it only confirms that there's a check valve on the vent line for the tank - not the main feel line.

    I honestly can't remember if a normal car has one on the feed too
    Last edited by alanjuggler; 07-04-2013 at 08:57.
    white '94 s13 200sx scrapped - mapped to 1.45bar. OS giken box, garrett GT2876R, 950cc injectors, ORC twin plate, nistune. 349bhp/325lbft @ 1.3bar CA18DET
    white '96 s13 180sx - type g with more kouki bits - RB25DET, GTR steel twin turbo conversion, RB26 crank & rods. 2.6L VVT twin turbo, SR20 OSG box, OSG STR twin plate clutch, Z32 ECU w/ nistune.

    current status: 180 a bit broken but to be repaired.

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    If there is a check valve it's in the pump.

    Can't you just make sure you turn to "start" at about 4 secs into the 5 sec prime? Even if you have to switch off and on again.

    Fuel is incompressible. The only "give" in the system is the reinforced rubber hoses. So a few drops is lots of pressure.

  12. #12
    Member alanjuggler's Avatar
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    suspect it's probably a combination of problems then, age of walbro (must be 5 years old by now, even though not heavily used), lack of check valve, age of FPR, not fully sealing - now to find a decent FPR for the RB25 - wonder if they're similar to the SR20. will check.

    will probably replace the walbro while I'm fiddling. not sure whether to use the bosch 044 or not - it does 'seem' like a much higher quality pump.

    edit:

    aha, not in common with the SR but in common with the KA & VQ from the maxima. will check the american pages (& ring nissan for a laugh )

    edit:

    by the way, skyshack, I know I could compensate for it (and I would have done, with the old car) but trying to avoid that sort of idiosyncrasy with this one.
    Last edited by alanjuggler; 07-04-2013 at 11:22.
    white '94 s13 200sx scrapped - mapped to 1.45bar. OS giken box, garrett GT2876R, 950cc injectors, ORC twin plate, nistune. 349bhp/325lbft @ 1.3bar CA18DET
    white '96 s13 180sx - type g with more kouki bits - RB25DET, GTR steel twin turbo conversion, RB26 crank & rods. 2.6L VVT twin turbo, SR20 OSG box, OSG STR twin plate clutch, Z32 ECU w/ nistune.

    current status: 180 a bit broken but to be repaired.

  13. #13
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    both stock FRP and also adjustable aftermarket Malpassi FPR acts same, fuel pressure drops quite fast after pump switch off, this is completely right with ca18det with stock/walbro fuel pump.

  14. #14
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    how fast is fast?

    the Z32 forums seem to consider it a problem (and I am using a Z32 ECU, which is why I mention it). There's also mention of it over on SR20forums, NicoClub and a blog that mentioned it specifically (Eric Hsu's blog (tuner in america), writing about an R32GTR.)

    I'm not saying the CA doesn't bleed off quickly or isn't meant to - but I suspect in my case I want it to hold fuel pressure longer than the minute or so it is right now.

    I think a new FPR and bosch 044 with a check valve in is probably the best way to sort it out.

    the problem is with that I can't fit an adjustable one in my rail - I have the FPR mounted the other way around for the sake of plumbing. might have to change that if I go for a nismo/tomei FPR.
    Last edited by alanjuggler; 08-04-2013 at 08:20.
    white '94 s13 200sx scrapped - mapped to 1.45bar. OS giken box, garrett GT2876R, 950cc injectors, ORC twin plate, nistune. 349bhp/325lbft @ 1.3bar CA18DET
    white '96 s13 180sx - type g with more kouki bits - RB25DET, GTR steel twin turbo conversion, RB26 crank & rods. 2.6L VVT twin turbo, SR20 OSG box, OSG STR twin plate clutch, Z32 ECU w/ nistune.

    current status: 180 a bit broken but to be repaired.

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    as soon as fuel pump stops, pressure become to fall down and let say in 1-2 minutes it is almost at 0... I dont see any reason why it should hold pressure while pump is off (unles pump doesnt have checkvalve), as ECU will prime it anyway at ign on...

  16. #16
    Member alanjuggler's Avatar
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    I don't want a car that relies on cranking it at a certain time - i.e. when the pump is priming, I want to it to be like a 'normal' car.

    in my case, the ECU primes the fuel pump and as soon as it's stopped, it drops at least 1bar of pressure.
    white '94 s13 200sx scrapped - mapped to 1.45bar. OS giken box, garrett GT2876R, 950cc injectors, ORC twin plate, nistune. 349bhp/325lbft @ 1.3bar CA18DET
    white '96 s13 180sx - type g with more kouki bits - RB25DET, GTR steel twin turbo conversion, RB26 crank & rods. 2.6L VVT twin turbo, SR20 OSG box, OSG STR twin plate clutch, Z32 ECU w/ nistune.

    current status: 180 a bit broken but to be repaired.

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    Quote Originally Posted by alanjuggler View Post
    I don't want a car that relies on cranking it at a certain time - i.e. when the pump is priming, I want to it to be like a 'normal' car.

    in my case, the ECU primes the fuel pump and as soon as it's stopped, it drops at least 1bar of pressure.
    but pump will prime when ign on and also when ign start, i.e. cranking, so I dont see your point. you cannot find situation when engine is cranking and fuel pump is off. this is how any modern car using fuel pump... in reality you even dont need any fuel pump prime prior cranking, as at the moment of first starter turn is fuel pressure ready...

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    Quote Originally Posted by alanjuggler View Post
    I don't want a car that relies on cranking it at a certain time - i.e. when the pump is priming, I want to it to be like a 'normal' car.

    in my case, the ECU primes the fuel pump and as soon as it's stopped, it drops at least 1bar of pressure.
    But CA and SRs drop pressure, they don't require any special treatment. Just stick the key in and go.

  19. #19
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    the pressure doesn't return quickly enough when you start cranking (it's the 1 second or so it takes as with priming to flick up to full pressure) and seems to cause the sluggish to start symptoms. I don't want that - my CA was enough of a bitch to start when tuned that I'm not looking for that anymore.

    there's a fair few threads on zilvia & sr20forums about it.

    the R32 GTR manual goes for 'when ignition switch is on' - should be 3bar - also states if the engine does not start, check fuel presure after 5 seconds. suggests that they would expect it to hold pressure at least for a while after the pump has stopped priming. (I'm referencing the R32GTR manual because the R33 ones for the RB25 are poo.)

    I suppose the other way to check would be to run a live to the fuel pump, keep the fuel pressure high and see if it changes anything.

    the funny thing is, I broaded my search just to the general topic and there's a fair few tuned car forums talking about it and a fairly common theme seems to be the walbro 255

    there's also talk of them wearing fairly quickly and dropping flow, which would explain it causing more of a problem for me than for some people. it is an old pump.

    I know I'm being a little bit of a fairy about this but I left my CA with this kind of problem and it ended up being a bag of toss overall. really fast and torquey when running but a pain to live with.
    white '94 s13 200sx scrapped - mapped to 1.45bar. OS giken box, garrett GT2876R, 950cc injectors, ORC twin plate, nistune. 349bhp/325lbft @ 1.3bar CA18DET
    white '96 s13 180sx - type g with more kouki bits - RB25DET, GTR steel twin turbo conversion, RB26 crank & rods. 2.6L VVT twin turbo, SR20 OSG box, OSG STR twin plate clutch, Z32 ECU w/ nistune.

    current status: 180 a bit broken but to be repaired.

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