ER34 Fuel Return

[tylink720]

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

[Dose Pipe Sutututu]

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.

[tylink720]

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?

[GTSBoy]

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.

[tylink720]

3 hours ago, GTSBoy said:

You're up about 25-30% on stock capacity, so you probably only need about a 10-15% diameter increase.

Sounds good, I’ll give this a try. Thanks for the help

[tylink720]

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

[joshuaho96]

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 October 4, 2023 by joshuaho96

[Dose Pipe Sutututu]

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.

[Kinkstaah]

TIL

(I have an adjustable fpr...)

[tylink720]

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. 

[Dose Pipe Sutututu]

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

[tylink720]

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

[Dose Pipe Sutututu]

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.

[tylink720]

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

[joshuaho96]

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. 

[Dose Pipe Sutututu]

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.

[tylink720]

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

[tylink720]

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

[GTSBoy]

https://gfb.com.au/products/fuel-management-products/

?

 

[Dose Pipe Sutututu]

Avoid the Tomei junk, GFB like GTSboy recommended or Turbosmart options are good.