Jump to content
SAU Community

Recommended Posts

Hey, is there a known restriction in the R34 GTT fuel return? I tested my fuel pressure with a Walbro 255, and have 52 psi at idle, with the regulator vacuum line on and off.

The only time I get 43 psi is if I fully disconnect the return line off the regulator and let it flow out into a bottle, so it seems to be something after the reg causing it.

I've disconnected both ends of the return line and I can blow through it. I went under the car and don't see any dents or kinks in the hard lines. I blew air into the return port of the fuel hat and it's harder to blow through, but I'm guessing that's because of that tank transfer valve attached to it. I’ve replaced the return lines from the reg to the hard lines as well

Any ideas? Thanks

Link to comment
https://www.sau.com.au/forums/topic/484962-er34-fuel-return/
Share on other sites

29 minutes ago, Dose Pipe Sutututu said:

Yeah the venturi/siphon inside the fuel tank lid.

Either you drill it out or replace it with a radium venturi/siphon or go tits out get a FFG fuel tank lid and replace the OEM unit.

Makes sense, I was thinking it had to be that thing. I will try modifying it first and see if I can get it to work. Is it drilling out the fittings on the hat, or drilling out the actual siphon pump?
IMG_1401.thumb.jpeg.327d0ab370bf04413efc17ce4d74b4e8.jpegIMG_1402.thumb.jpeg.91eee0fd4e0a5c42a6f8735e27d1ad9d.jpeg

2 hours ago, tylink720 said:

drilling out the actual siphon pump

It's a nozzle used to generate the suction. It's sized to produce enough suction with the stock fuel flow driving it. If you upsize the pump then you have to squeeze that increase through there. And you get LOTS of suction.

I don't know how much to drill it out. Remember that cross sectional area increases with the square of diameter, so you probably don't want to drill it out a lot, for "only" a 255L/h pump. You're up about 25-30% on stock capacity, so you probably only need about a 10-15% diameter increase.

  • Like 1
  • 2 weeks later...

I've drilled out the siphon pump to a little larger than stock, I think I used a 5/16 bit. Put it back together and still had the 50psi fuel pressure. Took it apart again and ended up breaking one of the fittings on my fuel hat which sucked, but luckily I was able to get a new one within a week. Fit the new hat, and tested the pressure with the siphon pump removed so it just pumps straight back into the tank, still 50psi. 

I have to assume now it's my return line that's the restriction. I was hoping it would be easy to replace the rear fuel lines that connect to the tank, but it looks like I'd have to lower or drop the tank which looks like a crap job. I will try using an air compressor first to blast out the return line and see if it does anything

24 minutes ago, tylink720 said:

I've drilled out the siphon pump to a little larger than stock, I think I used a 5/16 bit. Put it back together and still had the 50psi fuel pressure. Took it apart again and ended up breaking one of the fittings on my fuel hat which sucked, but luckily I was able to get a new one within a week. Fit the new hat, and tested the pressure with the siphon pump removed so it just pumps straight back into the tank, still 50psi. 

I have to assume now it's my return line that's the restriction. I was hoping it would be easy to replace the rear fuel lines that connect to the tank, but it looks like I'd have to lower or drop the tank which looks like a crap job. I will try using an air compressor first to blast out the return line and see if it does anything

You had the same idea I did. If that doesn't do it start measuring pressure at various points in the system, the pressure drops at each point will tell you where the restriction is. Could be an FPR too. If you've hardwired your fuel pump to run full blast all the time the FPR is almost certainly your problem at idle. Nissan used the FPCM to slow down the pump at idle.

Edited by joshuaho96
  • Like 1

Try bypass the fuel dampener to see if there's still an issue.

R34 GT-t uses a couple of them instead of 1x like the R33.

If that solves the issue, then delete the OEM ones and install an aftermarket one like a Radium one.

 

In the past, on S15s (same fuel tank hat/lid as the R34) I enlarge the siphon/venturi and it fixes the issue. Was hoping that was enough for you.

  • Like 1
On 10/4/2023 at 2:35 PM, joshuaho96 said:Could be an FPR too. If you've hardwired your fuel pump to run full blast all the time the FPR is almost certainly your problem at idle. Nissan used the FPCM to slow down the pump at idle.

I don’t think it’s the FPR as I’ve run a short hose into a bottle straight off the reg, and the pressure drops down. Maybe a new reg is a good idea anyway though. I will try doing what you said and test down the line. Although the only spot I can measure is between the hard line and tank, but it’s cramped as in there. I’ll have to lower the tank to get in there. 

On 10/4/2023 at 5:47 PM, Dose Pipe Sutututu said:Try bypass the fuel dampener to see if there's still an issue.

R34 GT-t uses a couple of them instead of 1x like the R33

Yep I bypassed the dampener on the fuel return line too but didn’t help. I still have the dampener on the feed. Maybe I’ll test bypassing that one too as I put the gauge before it. 

15 hours ago, Dose Pipe Sutututu said:

Hang on, are you running the stock FPR? If so, get an aftermarket one

Yep stock reg. Ill look into a new reg, just seems strange that it drops the pressure fine when I remove the return line. Also seen posts on here saying the stock reg is good for mild pump upgrades. It is 25 years old I guess

1 hour ago, tylink720 said:

 Also seen posts on here saying the stock reg is good for mild pump upgrades.

Car still might work however the fuel pressure is probably all over the shop like you've just discovered.

1 hour ago, Dose Pipe Sutututu said:

Car still might work however the fuel pressure is probably all over the shop like you've just discovered.

Alright, I've started looking for an aftermarket reg. This cars been a bottomless pit so far, and I haven't even started the proper power mods yet lol.

thanks

5 hours ago, tylink720 said:

Yep stock reg. Ill look into a new reg, just seems strange that it drops the pressure fine when I remove the return line. Also seen posts on here saying the stock reg is good for mild pump upgrades. It is 25 years old I guess

If you want to run the stock regulator you need to keep some method of making sure you aren’t trying to flow 255lph of fuel at idle. Otherwise you will experience what you’re talking about. 

  • Like 1
2 hours ago, tylink720 said:

 This cars been a bottomless pit so far, and I haven't even started the proper power mods yet lol.

Welcome to Skyline ownership, cheaper and more reliable buying a BMW M2 Comp, but here we all are lol.

1 hour ago, Dose Pipe Sutututu said:

Welcome to Skyline ownership, cheaper and more reliable buying a BMW M2 Comp, but here we all are lol.

Yep lol, half the fun is working on these piles. My brother has a newer BMW and it’s super quick and reliable

Any current recs for a good budget reg? I don’t want to buy some cheap eBay stuff, but also don’t want to drop $500 on just a regulator. I’ve seen some for about $140-$170 like Tomei, P2M, NRG, AEM, ect

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now


  • Similar Content

  • Latest Posts

    • @Haggerty this is your red flag. In MAP based ECU's the Manifold pressure X RPM calculation is how the engine knows it is actually...running/going through ANY load. You are confusing the term 'base map' with your base VE/Fuel table. When most people say 'base map' they mean the stock entire tune shipped with the ECU, hopefully aimed at a specific car/setup to use as a base for beginning to tune your specific car. Haltech has a lot of documentation (or at least they used to, I expect it to be better now). Read it voraciously.
    • I saw you mention this earlier and it raised a red flag, but I couldn't believe it was real. Yes, the vacuum signal should vary. It is the one and only load signal from the engine to the ECU, and it MUST vary. It is either not connected or is badly f**ked up in some way.
    • @Haggerty you still haven't answered my question.  Many things you are saying do not make sense for someone who can tune, yet I would not expect someone who cannot tune to be playing with the things in the ECU that you are.  This process would be a lot quicker to figure out if we can remove user error from the equation. 
    • If as it's stalling, the fuel pressure rises, it's saying there's less vacuum in the intake manifold. This is pretty typical of an engine that is slowing down.   While typically is agree it sounds fuel related, it really sounds fuel/air mixture related. Since the whole system has been refurbished, including injectors, pump, etc, it's likely we've altered how well the system is delivering fuel. If someone before you has messed with the IACV because it needed fiddling with as the fuel system was dieing out, we need to readjust it back. Getting things back to factory spec everywhere, is what's going to help the entire system. So if it idles at 400rpm with no IACV, that needs raising. Getting factory air flow back to normal will help us get everything back in spec, and likely help chase down any other issues. Back on IACV, if the base idle (no IACV plugged in) is too far out, it's a lot harder for the ECU to control idle. The IACV duty cycle causes non linear variations in reality. When I've tuned the idle valves in the past, you need to keep it in a relatively narrow window on aftermarket ecus to stop them doing wild dances. It also means if your base idle is too low, the valve needs to open too much, and then the smallest % change ends up being a huge variation.
    • I guess one thing that might be wrong is the manifold pressure.  It is a constant -5.9 and never moves even under 100% throttle and load.  I would expect it to atleast go to 0 correct?  It's doing this with the OEM MAP as well as the ECU vacuum sensor. When trying to tune the base map under load the crosshairs only climb vertically with RPM, but always in the -5.9 column.
×
×
  • Create New...