RB26 overboosting issue. Vacuum line plumbing help?

[Kanaric]

I got a R33 GTR and it has a Link G4 and N1 turbos with a MAC boost solenoid. 

After picking it up I noticed it was overboosting and brought it home and saw that vacuum lines had come off with split ends so i'm trying to figure out wtf they were doing here. 

The solenoid had only 1 line going in and i'm trying to figure out where the other one is. They have the stock route for the one that comes out of the top of the wastegate and that seems connected correctly. The line that comes out of the intake pipe next to that is blocked off with a rubber stopped, same on the other side, so they are using some alternative setup.

I found two vacuum lines that were not connected obviously i'm assuming more can be f**ked up. One of them comes from the back of the intake plenum. There are two large hoses and then a small 3rd one in the rear. It looks like it could go into the boost solenoid but any diagram I see it's supposed to go into some "water tube" that I can't seem to find. 

 

Here's some images to help out.

 

 

In this image the green is as stock and the blue is missing.

In this image the 'water tube' line is the vacuum line I think they may have been using for the boost solenoid

 

The other thing that wasn't attached goes to some T connector because they were zip tied together that made it obvious. This isn't on that diagram but it looks like it comes out of the 'bottom collector boost source' that is on the upper right of that top image.

 

I'm just trying to figure this shit out and whatever info or tips someone can provide would be useful but i'm wondering, does that 'water tube' vacuum line make sense as an alternative thing to use for a boost solenoid? And what is the 'bottom of the collector - boost source' thing on the upper right of the image used for and any guesses on why some T connector would be put on it? 

Anything i've touched before this was simple af dumbass straight forward shit and I don't really know these GTRs that well so any help is appreciated. My last car was a simple stock mount turbo upgrade gtst setup lol. 

 

edit: I tried that plenum line for the solenoid and it overboosts. It's like as if the wastegate is getting no indication of boost pressure. Just will go until fuel cut if I let it so something is not connected right. 

Edited November 17, 2021 by Kanaric

[Duncan]

When you say "picked it up" do you mean it is new to you or it is back from a shop that changed something?

The required plumbing is pretty simple, anything else that is open needs to be blocked off.

You need a post throttle body pressure source to the boost controller (in this case the Link). There is a nipple on the back of the engine which is suitable, the stock line from there runs to the standard boost gauge sender on the firewall (you would also T to this for any boost gauge/logger etc). It is mentioned but not marked on your first diagram

You then need a plenum (pre throttle) boost source which goes to the boost control valve inlet, and then the boost control valve outlet is attached to both wastegates (the green line). An alternative (much shorter) option is to take this source from any point after the 2 turbo outlets merge and run straight to the boost control valve mounted on the passenger side, then back to both wastegates, this can reduce spiking due to the shorter runs

There are separate vacuum feeds to the carbon canister, fuel pressure regulator, BOVs, brake/clutch boosters and also PCV, AAC etc, none of them should need to be touched. But any vacuum line that is open to atmosphere needs to be blocked. The "water tube" has separate lines with both water and air so it is a misleading name.

[Kanaric]

On 11/17/2021 at 1:02 PM, Duncan said:

When you say "picked it up" do you mean it is new to you or it is back from a shop that changed something?

The required plumbing is pretty simple, anything else that is open needs to be blocked off.

You need a post throttle body pressure source to the boost controller (in this case the Link). There is a nipple on the back of the engine which is suitable, the stock line from there runs to the standard boost gauge sender on the firewall (you would also T to this for any boost gauge/logger etc). It is mentioned but not marked on your first diagram

You then need a plenum (pre throttle) boost source which goes to the boost control valve inlet, and then the boost control valve outlet is attached to both wastegates (the green line). An alternative (much shorter) option is to take this source from any point after the 2 turbo outlets merge and run straight to the boost control valve mounted on the passenger side, then back to both wastegates, this can reduce spiking due to the shorter runs

There are separate vacuum feeds to the carbon canister, fuel pressure regulator, BOVs, brake/clutch boosters and also PCV, AAC etc, none of them should need to be touched. But any vacuum line that is open to atmosphere needs to be blocked. The "water tube" has separate lines with both water and air so it is a misleading name.

I picked it up from a shop who did bodywork and fully redid the paint, and this is like a state away because I live in the middle of nowhere. They claimed the car ran fine when I picked it up. I drove it and it did but I had to drive 6 hours to get home and it had these problems starting during that trip. So something went wrong during that. 

Whoever did this changes from stock blocked off the stock 'nipples' they were no longer using. 

The inlet on the boost control solenoid is one of what was missing. 

that water tube air line that comes from the plenum, stock, is one that was hanging loose with a cracked end and is exactly long enough so I wasn't sure if they were instead using that for the boost controller. Where is this 'water tube' inlet for that? I wanted to see if that was blocked off or rerouted to confirm but I can't find that. It fits the 'pre throttle boost source' if so. 

The one from the back of the engine was Td up for something, i do not have an extra boost gauge or anything, so i'll trace that out and see if maybe they were using that. 

Thanks for the info. 

 

Quote

An alternative (much shorter) option is to take this source from any point after the 2 turbo outlets merge and run straight to the boost control valve mounted on the passenger side, then back to both wastegates, this can reduce spiking due to the shorter runs

I've been considering doing it like this but I want to make sure I catch if anything is messed up.

 

edit: they put a T on the fuel pressure regulator line for a map sensor. 

Edited November 17, 2021 by Kanaric

[GTSBoy]

On 11/18/2021 at 8:52 AM, Kanaric said:

edit: they put a T on the fuel pressure regulator line for a map sensor.

Not the smartest place to interfere with the vacuum lines because if you manage to have a problem (ie, it falls off/open) you get uncontrolled boost at the same time that you don't get boost referenced fuel pressure. Ideal for rapidly beating the engine to death.

[joshuaho96]

On 11/17/2021 at 2:53 PM, GTSBoy said:

Not the smartest place to interfere with the vacuum lines because if you manage to have a problem (ie, it falls off/open) you get uncontrolled boost at the same time that you don't get boost referenced fuel pressure. Ideal for rapidly beating the engine to death.

I've never understood why people do this when there is a dedicated fitting on the back of the balance tube for the factory boost gauge.

[GTSBoy]

On 11/18/2021 at 10:15 AM, joshuaho96 said:

I've never understood why people do this when there is a dedicated fitting on the back of the balance tube for the factory boost gauge.

It's because it is the easiest to see and touch. Do not underestimate the laziness and stupidity of your typical Skyline modifier.

[joshuaho96]

Regarding this picture:

 

The leftmost hose barb is for boost control. The "water tube assembly" you're talking about has both vacuum lines and coolant pipes. That is the green line pictured on the vacuum diagram labeled as the boost source. The second from the left is the air feed for the cold start air valve, Nissan calls it an intake air regulator. That's a valve with a bimetallic strip in it that closes as it heats up. This piece is right under the intake manifold and is basically impossible to get to without pulling everything else off first. The rightmost is the AAC valve air feed as you have mentioned.

The factory system is super basic bleed-style, when the solenoid opens it is bleeding the green boost source line to the rear turbo pre-compressor inlet, the wastegate actuator sees less boost as a result and doesn't open as far. As for how you should set up your MAC (presumably 3 port) solenoid, I would reference this article which details it pretty well: https://www.onpointdyno.com/boost-control-plumbing-get-it-right-save-money/

My entire intake side of the engine is still in pieces scattered across the garage so I can easily take photos of all this crap if need be, including how the lines should be and what they should look like. Keep in mind there are special orientations for a lot of these hoses, the manual likes to make mention of this multiple times.

Edited November 18, 2021 by joshuaho96