Another "where do these lines and plugs go" RB25DET question

[thatjawncrazy]

if anyone has a stock rB25DET and can trace these lines and wires and show me what yours connect to I would really appreciate it.

this was an RB20 auto car that's been DET/5spd swapped. i would like to figure out if the harness was never modified or replaced accordingly.

• black plug coming off the harness wrapping around the back of the intake manifold
• grey IACV plug
• random brown/grey plugs hanging out near the left strut tower
• the vacuum nipple coming off the turbo inlet pipe. i can't entirely tell what this connects to in the vacuum line diagram.

so neither my TPS nor my IACV are plugged in. what should be here?

yes, i've googled, yes, i've read the SAU threads and asked the FB groups. yes i've read the vacuum line diagrams and stuff.

[GTSBoy]

The nipple is easy. It's not the turbo inlet pipe. It is the outlet of the turbo and it is the source of the boost signal to drive the wastegate.

[thatjawncrazy]

14 minutes ago, GTSBoy said:

The nipple is easy. It's not the turbo inlet pipe. It is the outlet of the turbo and it is the source of the boost signal to drive the wastegate.

aha. thanks for that one!

[thatjawncrazy]

2 hours ago, GTSBoy said:

The nipple is easy. It's not the turbo inlet pipe. It is the outlet of the turbo and it is the source of the boost signal to drive the wastegate.

interestingly the wastegate vacuum hose is connected to some other tube below the intercooler piping.

car still acts like garbage when throttled. ugh. anyone have pics of their stock engine bay?

[GTSBoy]

6 hours ago, thatjawncrazy said:

interestingly the wastegate vacuum hose is connected to some other tube below the intercooler piping.

car still acts like garbage when throttled. ugh. anyone have pics of their stock engine bay?

That one is definitely wrong. Wastegate is connected to turbo inlet (via being connected to the BOV return pipe, which is connected to the turbo inlet). That is bad, bad, bad. You are driving that without a boost reference. It should be overboosting.

[Yeetus]

Hey mate, the brown and grey plugs would usually go to the TPS, brown one goes on the bottom connector and grey is the top but isn't used unless the car is auto, your TPS looks different to mine though I've got a series 2 engine.

[thatjawncrazy]

On 08/07/2025 at 7:27 PM, GTSBoy said:

That one is definitely wrong. Wastegate is connected to turbo inlet (via being connected to the BOV return pipe, which is connected to the turbo inlet). That is bad, bad, bad. You are driving that without a boost reference. It should be overboosting.

Yup. I actually just blew my turbo. LOL. 

should have checked this thread before today.

i also have no boost solenoid. so it should be...one line from wastegate to turbo inlet, and a T somewhere in that line that connects to the wastegate return pipe?

[GTSBoy]

1 hour ago, thatjawncrazy said:

i also have no boost solenoid. so it should be...one line from wastegate to turbo inlet, and a T somewhere in that line that connects to the wastegate return pipe?

No. The simplest wastegate hookup, with no solenoid or other form of "boost control" (ie, control over your boost control, if you know what I mean) is a single hose, direct from the turbo outlet/hot pipe, straight and only to the wastegate actuator. It is that pressure signal that drives the wastegate to open, providing the boost control (and by "boost control" here, I mean, limiting how high it can go, which is essentially the spring pressure of the actuator).

You only end up with tee pieces and alternate flow paths once you start adding things to the boost control system to allow you to determine how much of that boost signal makes it to the actuator. There are so many ways to do that that there is no single way to run the hoses and tees and the like.

If you have a stock boost solenoid, then all it does is either allow all the boost signal to go to the actuator, or open up to allow some of it to bleed off. There needs to be a restriction in that bleeder to allow only a small amount to bleed off. And in a stock system, that would then be plumbed back to the turbo inlet (for "emissions control" reasons). That is actually what that nipple on your BOV return pipe could/would be for.

If you have an aftermarket boost controller and solenoid, then the above is mostly true, but there is no need for a restrictor in the bleed, because the solenoid is pulse width modulated to create a variable bleed off. The air that escapes from the bleed can either be vented, or also returned to the turbo inlet. For emissions reasons it should be returned to inlet, but the amount of air being vented is so small that it really doesn't matter (either from an emissions perspective, or from an air-fuel ratio affecting perspective).

[thatjawncrazy]

GTSboy, thank you again.

i’ve been reading all the other SAU RB25 vacuum line threads and you have the utmost patience for people asking “where do my vacuum lines go”. it’s appreciated.

i did also enlist a few other RB guys local to me. 

i would like to run 5 to 4. turbo pressure side to wastegate actuator.

i would like to run 3 to 1. power steering air valve line to intercooler outlet. (this is how it was previously).

i was told 2 is the bleed port for the boost solenoid - which i’m not running - so should be capped.

 

anyone see any problems with that?  

 

[GTSBoy]

4 to 5 is fine. It will be slow, but that's better than blowing turbos.

I don't have a PS pump idle up solenoid on my car, because... I think when we put the Neo in we retained my R32 lines. But....

From inspecton of the R34 vacuum hose diagram, you can see that the solenoid needs to be connected to the turbo inlet as source of clean air) and the plenum (as source of vacuum - which is the place for the air to flow to to cause the idle to increase).

So 3 to 1 is VERY WRONG. 3 should go to either the turbo inlet, or the plenum. Follow the other hose from the PS solenoid and if it goes to the plenum, then your 3 goes to 2

2 would also serve as a bleed port for a boost solenoid.

[thatjawncrazy]

On 18/07/2025 at 4:28 AM, GTSBoy said:

4 to 5 is fine. It will be slow, but that's better than blowing turbos.

I don't have a PS pump idle up solenoid on my car, because... I think when we put the Neo in we retained my R32 lines. But....

From inspecton of the R34 vacuum hose diagram, you can see that the solenoid needs to be connected to the turbo inlet as source of clean air) and the plenum (as source of vacuum - which is the place for the air to flow to to cause the idle to increase).

So 3 to 1 is VERY WRONG. 3 should go to either the turbo inlet, or the plenum. Follow the other hose from the PS solenoid and if it goes to the plenum, then your 3 goes to 2

2 would also serve as a bleed port for a boost solenoid.

thanks, will conduct myself accordingly.

i unfortunately put about 150 miles on the car before reading this thread again, however no PS issues, and definitely no turbo issues. it spools normally and still feels fast to me, probably because my previous car was an N/A auto sedan...gonna go fix the PS vacuum line though lol.

safe to say, got what i came to this thread for. thanks