Boost cut after swap(RB25DET NEO) R34

[Kapr]

Hi..
Just another problem.
So maybe you can help.
I(my mechanic) done swap from my RB20DE NEO to RB25DET NEO.
Everything is OK but we have a boost cut.
Coils/Plugs are ok...AFM is ok. ECU is ok.
I have Walbro 255 but it "changed" sound few months ago(you can hear it ouside) you did not hear that when it was new(maybe faulty?)

Sooo...what now? What can cause this boost cut? It is even when you standing still...

[Duncan]

Can you clarify what you mean by boost cut, do you mean it misfires both when under load (driving) and when stationary and out of gear? Or does the ECU pull power when it reaches a specific boost level that is too high (boost control issue)?

Does it occur at idle with no throttle?

When you say "the ECU is OK", what ECU is it and why do you think it is OK? Have you used the NEO ECU, and if so do you have a MAP sensor attached?

Same for the AFM, why do you think it is OK?

Do you have any way to put a fuel pressure gauge on it (even just a mechanical one between the fuel filter and fuel rail)?

[Kapr]

Hi. Sry iam not a mechanic and iam not at the car atm so i dont know 100% but they told they measure those and even try to change those.
AFM they have two.
Coils are new a they have my old one too.
Plugs too.
ECU...we have 25 NEO stock and Nistune 25 NEO.

 

But i dont know if any one those could be the problem and why/if/what can cause this,
Only thing they did not check is fuel...but that walbro 255 is new(like 1,5 years)...

That fuel pressure gauge idk...but i let them know

Any suggestions?

 

EDIT: how can they know if it is like you say he ECU pull power when it reaches a specific boost level that is too high?
Tha car has boost controler

Edited August 1 by Kapr

[GTSBoy]

1 hour ago, Kapr said:

EDIT: how can they know if it is like you say he ECU pull power when it reaches a specific boost level that is too high?
Tha car has boost controler

Don't even try to run it on the stock ECU if you're going to have the boost controller bring boost above ~10 psi. I've already told you that.

If you use the Nistune ECU, you will need to CAREFULLY read the available documentation for Neo tuning, and read some threads on the Nistune forums, to discover the various things you have to do to prevent the ECU from going bananas when the boost is too high. The is a table associated with th boost sensor that must be modified to prevent it from shitting the bed.

This is just one of the things that you will need to do to the tune in Nistune, because the Neo turbo ECU will be expecting to see a number of things (such as the TCS) that are not there, and you have to block the DTCs on those.

It is totally not surprising to me that you are having the problems that you are, but the solutions to these problems have been known for >15 years. So just get it done.

[Kapr]

So stock ECU does not like anything above 10 psi? 
That Nistune one is just for "try" if it will be any different, I know it need to be tune for that.

I know but YOU may know about these problem but i/we dont. They few little Skylines here let alone people who know anything about tham so that is why iam asking here  

[Kinkstaah]

Stock ECU (or more accurately stock tune) absolutely refuses to go over 10psi and behaves like you have seen.

The Nistune is the same if it is the stock tune. If the Nistune chip has been tuned, the resulting tune could be literally anything for any combination of parts.

The Nistune just makes the stock ECU Tunable.

[Kapr]

Hi. Ok thanks for the info. I/we do not know what tune is on that Nistune ECU so it is why there is stock ECU and that Nistune is for later when the car is done. I have guy who tunes Nistune and other ECU so i will then take it to him to see and properly tune for the car  

And for the update.
On 99,9% it is bad fuel pump.
Which is wierd cuz it is like 2 years old(brand new walbro 255) and that filter (also brand new) was dirty. 
I was alwaya pumped premium(100) on very good petrol station so i dont know how is that possible...
 

[Kinkstaah]

I used to have Walbro 255's when they were a pretty common update. (or were they 044? 040?)

In any case I had a dirty tank. I thought the tank was good, but I would find the filter would clog in maybe ~50km of driving. If the issue persists, well, your fuel system or tank has crap in it that the pump is sucking up.

[Kapr]

Yeah they are pretty common even in here so it was nice uprage and it was cheaper then OEM.

I thought about that, If the new one still clog after a while it is a new or clean tank

[Kapr]

So sadly the fuel pump etc was not it. The pump and filter is new and it is still cutting around 4000 rpm.

A read something about ABS/TCS (engine is from stagea and ECU too so no TCS) and i dont know for 100% that my car has ABS.
It has ABS "cube" in engine bay but i dont think it works(or at least from my braking experience).

Can be something like that? 
I will be driving the car next week to Nistune tuner to properly see what is wrong...
 

[GTSBoy]

The ABS and/or TCS being missing/broken will not cause the engine to misbehave. It just casues CEL to come on and annoy you. The CEL is useless if it is always on, so you have to do the things you have to do to get rid of it, so it can be useful.

Being a Stag ECU, then yes, it will not expect TCS to be present. But it will expect ABS to be present and working. You will need to either make the ABS CU talk to the ECU (don't ask me what that will take on an NA R34), and make sure the hardware is working....or, you just need to blank it out in Nistune.

Do not persist with the stock ECU. You will just have problems.

Gte it Nistuned. Start from there.

DO YOU HAVE A BOOST SENSOR? The ECU's boost sensor that it. It is connected to the loom at the rear of the coil cover. Usually rides on the firewall on an R34, but is usually bolted down to a bracket along with all the other crap at the back of the coil cover when a Neo is dropped into another car. If you do not have it, the ECU will shit the bed.

So, do you have one?

[Kapr]

Ok so ABS/TCS is "not" the problem.
Yeah i know now about the ABS with Nistune(i told that to the Nistune guy) but iam not 100% why or why i dont have ABS.
ABS cube is in the engine (always has been) but i had braked couple of times rly hard and brakes just locked and scream.
Does all Skylines have ABS cube?

That boost sensor
https://www.sau.com.au/forums/topic/367633-wtb-r34-gtt-turbo-pressure-sensor-pics-inside/

Is this it? I need to ask but i have a picture of engine bay and i do not see that (100% is not in that place)

If i do not have this the car can do what mine is?(cutting around 4000 rpm)

Here is the pic of my engine bay(only one i have)


BUT i think i dont have that cuz loom was used from my RB20DE NEO and i know (obviously) that it does not have boost control or any wire to that...) so maybe that is the problem? (i dont know for 100% if that car where the engine is from had that)...but i dont have the loom from that

Edited August 7 by Kapr

[GTSBoy]

Yes. You will need to get the original boost sensor and wire it in. That is.... if you plan to use a stock ECU - even a Nistuned ECU. The Nissan ECU will not operate correctly without it connected. And yes, it will do annoying stuff like you are experiencing.

You will almost certainly be better off with some aftermarket ECU, than trying to hunt down rare sensors.

[Kapr]

Ok thanks..that was my second question if i can somehow bypass this even with Nistune(that is all i have right now...aparat from stock ECU.

So with some aftemarket it can by "bypassed" or make it work? 
I know there is Link or Haltech (mainly used).
We have ECUmaster here so i will gather some info about it.

[GTSBoy]

On 07/08/2025 at 3:26 PM, Kapr said:

So with some aftemarket it can by "bypassed" or make it work? 
I know there is Link or Haltech (mainly used).
We have ECUmaster here so i will gather some info about it.

Aftermarket ECUs just don't need that sensor.

[Kapr]

Ok. And Nistune? I read (here too) mainly on Stagea threads that it can by "turn off" with Nistune. But they say they still had the sensor plug in.
Next week iam going to the Nistune(and other ECUs) tuner so i can finally make it work.

I drove the car home and apart from the cut it drove great. No misfire no smoke no hesitation or rough start...so hoping it is this sensor that is missing  

[GTSBoy]

Yes it can be adjusted so that it does not ruin the party in Nistune. But you must have the sensor connected or the ECU will be bitchier than a teenage daughter.

[Kapr]

Ok so i will NEED to have this sensor anyway even with Nistune (or standalone ECU)

https://justjap.com/products/genuine-nissan-boost-pressure-sensor-evap-control-system-fits-nissan-r34-skyline-c34-nm35-stagea-pnt30-x-trail-rb25det-vq25det-sr20vet?currency=AUD&srsltid=AfmBOoqfxX48bW9bEwH62orcNhtBfp7ekAL0C9Ca89ySFGUiBzXfXeze

Is this the correct one? And this is only thing i need? No other "things" connected to the sensor or something? I do have the wiring.

Edited August 9 by Kapr

[GTSBoy]

On 10/08/2025 at 2:39 AM, Kapr said:

k so i will NEED to have this sensor anyway even ... standalone ECU

No. Only for a standard ECU with/without Nistune.

Standalone has no need for it. That's like the 3rd time I've said so.

 

And as to that sensor linked - yes, it does look like it would be the right one. I've never bothered to see how available they were - just assumed that they'd be hard to find. I think I'd be planning to go Link ECU rather than buy sensors I don't actually want to make Nistune work, if I did not already have Nistune. As you do not already have Nistune, you should realy consider your options.

[Kapr]

Hi. Yeah thanks.
Car is already at tuners and it staying there to resolve "many" issues they do not like. So it is in a good hands.
Getting a good tune on that Nistune ECU too so the car would be good. Just needs a little tune up and love.